AN ABSTRACT OF THE DISSERTATION OF
Jerome P. Leonard for the degree of Doctor of Philosophy in Wildlife Science presented
on February 27, 1998. Title: Nesting and Foraging Ecology of Band-tailed Pigeons in
Western Oregon.
Redacted for Privacy
Abstract approved:
L. Jarvis
Pacific Coast band-tailed pigeons (Columba fasciata) have declined markedly
during the past several decades. Despite implementation of increasingly restrictive
hunting regulations, populations have shown minimal signs of recovery. I radio-marked
127 adult band-tailed pigeons in the central Oregon Coast Range in April-May of 1993-95
to document nesting chronology and productivity, and characterize nesting and feeding
habitats. One-hundred and thirty-seven nests initiated by 65 different birds were
monitored. Multiple brooding was predominant; 45 birds initiated 2-3 nests each with 7
birds producing 3 successful nests each in one nesting season. All nests had a clutch size
of 1. The reproductive period started in late-April and went through mid- to late-October.
Nesting attempts peaked mid- to late-June, and 21% of all young fledged after September
15. Nest re-use was rare, but 78% of the consecutive nests I observed by individual birds
eliminated nesting intervals by overlapping nesting cycles. Nesting overlap averaged 7
days. Nest survival probabilities were consistent between years with a pooled estimate of
0.689 (95% C.I = 0.613 0.775). Band-tails nested in a variety of tree and shrub species,
with 70% of the nests observed in Douglas-fir (Pseudotsuga menzesii). Only 17% of all
nests were in deciduous trees or shrubs. Nest height was highly variable and averaged
10.3 m. They also used a variety of stand conditions (open and closed) and
seral/community types (sapling-pole to old-growth in deciduous and conifer
communities). However, the majority of nests (77%) were in stands classified as conifer
community with 55% in the closed-canopy, sapling-pole seral condition. Feed areas were
diverse in their physical and vegetative characteristics and were located in both riparian
and upland zones. The principal food component within most of these sites was either
Pacific red elder (Sambucas racemosa) or cascara buckthorn (Rhamnus purshianus).
Band-tailed pigeons were highly mobile throughout their nesting season. They traveled
an average of 5.02 km to feed areas and had a mean home range size of 11,121 ha. Due
to their mobility, and diverse habitat use, band-tailed pigeons would be an important
species for consideration in large watershed management designs.
Nesting and Foraging Ecology of
Band-tailed Pigeons in Western Oregon
by
Jerome P. Leonard
A DISSERTATION
submitted to
Oregon State University
in partial fulfillment of
the requirements for the
degree of
Doctor of Philosophy
Presented February 27, 1998
Commencement June 1998
Doctor of Philosophy dissertation of Jerome P. Leonard presented on February 27. 1998
APPROVED:
Redacted for Privacy
Major Professoi, representirg Wi
Redacted for Privacy
Head of Department of Fisheries
d Wildlife
Redacted for Privacy
Dean of Graduate
I understand that my dissertation will become part of the permanent collection of Oregon
State University libraries. My signature below authorizes release of my thesis to any
reader upon request.
Redacted for Privacy
ACKNOWLEDGMENTS
This project would not have been possible without the assistance and support of
many individuals. It was truly a joint effort, and my deepest thanks go to those who gave
much time and effort to bring this research project to completion. First, I wish to thank
my advisor and friend Dr. Robert L. Jarvis whose guidance and patience proved
invaluable throughout this research. I really enjoyed my doctorate program and
considered it a privilege to work and learn under him. I would also like to thank my
graduate committee--Drs. Diane J. Carroll, William C. McComb, E. Charles Meslow,
John C. Tappeiner, and Christine Maguire. Their advice, insight and interest in this
project were extremely helpful and encouraging. I am grateful to all the individuals that
helped collect field data including: Marianne Brooks, Heidi Brunkal, Grant Casady,
Peggy Gunter, Jeff Henshaw, Kendra Ibarguen, Doug Koenig, Matthew Koenig, Mark
McRae, Angela Mehall-Niswander, Becky Saxton, and Tim Sensabaugh. Many thanks
go to Howard Bruner--my respected colleague and friend--who provided excellent
insight, a great listening ear, good advice and much laughter in the lab and field.
Special thanks are due to George Weaver who provided hours of statistical
assistance, and Todd Sanders for sharing his data, experiences and insights about pigeons.
I am especially thankful for Laurie LaPointe who provided invaluable editorial comments
and suggestions on the thesis. Many thanks go to my fellow graduate students who had a
major influence on seeing that goals were met while providing fun and excitement.
The U.S.D.I. Fish and Wildlife Service, U.S.D.A. Forest Service, U.S.D.I. Bureau
of Land Management, Oregon Department of Fish and Wildlife, and the California
Department of Fish and Game provided funding, equipment and personnel contributions.
Special thanks go to Jan W. Skotnicki who provided funds to boost field crew moral.
To Jerry and Sandra Kwast and Virgil and Cara Lynn Gosser, I would like to
extend my deepest gratitude, respect and friendship. They continually modeled wholeheartedness, grace, and thankfulness, and showed me the real reason why I came to
Oregon. I would also like to give many thanks to my Northwest Hills Church family for
all their prayers, and continued support, interest, and encouragement.
I would like to thank my family--Dad, Mom, Tony, Jin-Soo, Jack, Sheralee,
Theresa, Pat and Rod--for their years of support, prayers, patience, and love. Great
appreciation is extended to my other family, the Wongs and Tongs, for their continued
prayers, encouragement, financial support, grace, patience, and love. Finally, to my wife,
friend, and co-laborer Cynthia (an example of Proverbs 31). You are the best.
TABLE OF CONTENTS
Page
INTRODUCTION....................................................... 1
......................................................... 7
........................................................... 10
STUDYAREA
METHODS
CAPTURE, INSTRUMENTATION AND RADIO-TELEMETRY
.......... 10
............................................ 12
REPRODUCTIVE BIOLOGY ...................................... 13
DATA ANALYSES ............................................... 15
HABITAT SAMPLING
RESULTS............................................................ 17
REPRODUCTIVE BIOLOGY
...................................... 19
Nesting................................................... 19
......................................... 20
............................................ 20
............................................ 23
............................................. 23
HABITAT CHARACTERISTICS .................................... 26
Nest and Nest Tree Characteristics ............................. 26
Nest Site Characteristics ..................................... 30
Nesting Chronology
Nesting Interval
Nesting Success
Adult Survival
Feed Site Characteristics
..................................... 35
........................ 40
MOVEMENTS .................................................. 47
Flight Distances ............................................ 47
Breeding Home Ranges ...................................... 50
NEST SUCCESS-HABITAT RELATIONSHIPS
Philopatry................................................. 50
TABLE OF CONTENTS (Continued)
Page
DISCUSSION
.
53
...................................... 53
Nesting Chronology ......................................... 53
Multiple Brooding and Nesting Interval ......................... 55
Nesting Rate ............................................... 58
Nesting Success ............................................ 59
Adult Survival ............................................. 64
HABITAT ASSOCIATIONS ....................................... 65
Nest and Nest-Tree Characteristics ............................. 66
Nest Site Characteristics ..................................... 70
Feed Areas ................................................ 72
MOVEMENTS .................................................. 76
MANAGEMENT AND RESEARCH IMPLICATIONS .................. 81
BIBLIOGRAPHY ...................................................... 86
REPRODUCTIVE BIOLOGY
LIST OF FIGURES
Figure
Page
1.
Band-tailed pigeon breeding ecology study area, central Coast Range of
western Oregon, 1993-95
2.
Number of radio-marked band-tailed pigeon nests and young that fledged
throughout the nesting season in the central Coast Range of Oregon
1993-95
............................................ 8
......................................................... 21
3.
Nesting chronology of radio-marked band-tailed pigeons and the number
of nests initiated during the nesting season in the central Coast Range of
Oregon, 1993-95
.................................................. 22
4.
The number of successful and unsuccessful radio-marked band-tailed
pigeon nests ( = 137) initiated in coniferous and deciduous tree or shrub
species in the central Coast Range of Oregon, 1993-95
.................... 28
5.
Nest and nest tree characteristics of coniferous and deciduous trees or
shrubs used by radio-marked band-tailed pigeons in the central Coast
Range of Oregon, 1993-95
.......................................... 29
6.
Frequency of radio-marked band-tailed pigeon nests (= 138)
corresponding to nest aspects (one of eight 450 segments) relative to main
stem of nest tree in the central Coast Range of Oregon, 1993-95
............. 31
7.
The total number of successful and unsuccessful radio-marked band-tailed
pigeon nests ( = 138) in various stand seral conditions and community
types (based on Brown 1985) in the central Coast Range of Oregon, 199395
8.
.............................................................. 32
Frequency of nests initiated by radio-marked band-tailed pigeons in
relation to the average DBH (cm) of the nest site in the central Coast
Range of Oregon, 1993-95
.......................................... 33
9.
Frequency of nests initiated by radio-marked band-tailed pigeon in relation
to the density of trees (ha) in the nest site in the central Coast Range of
Oregon, 1993-95
................................................. 34
10.
Frequency of radio-marked band-tailed pigeon nests (= 138)
corresponding to nest site slope aspect (one of eight 45° segments) in the
central Coast Range of Oregon, 1993-95
............................... 36
LIST OF FIGURES (Continued)
Figure
Page
11.
Percent frequency of band-tailed pigeon nests (p, = 136) in relation to
mountain topography in the central Coast Range of Oregon, 1993-95
12.
General landscape characteristics andJor stand conditions of feed areas
( = 25) used by radio-marked band-tailed pigeons in the central Coast
Range of Oregon, 1993-95
......... 37
.......................................... 38
13.
General site and vegetative characteristics of feed areas ( = 25) used by
radio-marked band-tailed pigeons in the central Coast Range of Oregon,
1993-95
......................................................... 39
14.
Frequency of nests initiated by radio-marked band-tailed pigeons in
relation to percent canopy closure of the nest site (0.2-ha plot centered on
nest tree) in the central Coast Range of Oregon, 1993-95
.................. 41
15.
Nest success probability of radio-marked band-tailed pigeon nests in
relation to percent canopy closure of the nest site (0.2-ha plot centered on
nest tree) in the central Coast Range of Oregon, 1993-95
.................. 42
16.
Frequency of nests initiated by radio-marked band-tailed pigeons in
relation to nest site aspect (after conducting a variable selection procedure
reducing aspect from eight 450 segments into two 180° categories) in the
central Coast Range of Oregon, 1993-95
............................... 43
17.
Nest success probability of radio-marked band-tailed pigeon nests in
relation to site aspect (after conducting a variable selection procedure
reducing aspect from eight 45° segments into two 180° categories) in the
central Coast Range of Oregon, 1993-95
............................... 44
18.
Predicted probability of nest success by band-tailed pigeons in the central
Oregon Coast Range 1993-95, as a function of site aspect (SW: 157.6°
337.5° = 1; NE: 337.6° 157.5° = 0) and various levels of percent canopy
closure (from 30-100%)
............................................ 48
LIST OF TABLES
Table
1.
Page
Number of radios attached to band-tailed pigeons by attachment technique,
radio type, and sex (or unknown yearling), in the central Oregon Coast
Range, 1993-95
.................................................. 18
2.
Mayfield survival rates of radio-marked band-tailed pigeon nests during
incubation (weeks 1-3) and post-hatch (weeks 4-7) in the central Oregon
Coast Range, 1993-95
............................................. 24
3.
Radio-marked band-tailed pigeon nest survival probabilities in relation to
initiation date in the central Oregon Coast Range, 1993-95
4.
Survival of radio-marked adult band-tailed pigeons during the breeding
season (May September) in the central Oregon Coast Range, 1993-95
5.
Drop-in deviance F-test for the vegetative and physical habitat
characteristics of successful band-tailed pigeon nests in the central Oregon
Coast Range, 1993-95
................ 25
...... 27
............................................. 45
6.
Final logistic regression model for the vegetative and physical habitat
characteristics of successful band-tailed pigeon nests in the central Oregon
Coast Range, 1993-95
............................................. 46
7.
Aerial distances (km) traveled by radio-marked band-tailed pigeons
between nest sites and feed areas in which they were observed in the
central Coast Range of Oregon, 1993-95
............................... 49
8.
Aerial distances (m) between successive nests of radio-marked band-tailed
pigeons in the central Oregon Coast Range, 1993-95
9.
Breeding home ranges of nesting radio-marked band-tailed pigeons based
on minimum convex polygons (ha), in the central Coast Range of Oregon,
1993-95
..................... 51
......................................................... 52
ToHim
"Who is able to do immeasurably
more than all we ask or imagine"
and
Mom and Dad
who gave me the needed encouragement
to go back to school
NESTING AND FORAGING ECOLOGY OF BAND-TAILED PIGEONS
IN WESTERN OREGON
INTRODUCTION
Pacific Coast band-tailed pigeons (Columba fasciata) range from British
Columbia, Canada, through Washington, Oregon, extreme western Nevada, and
California, into northern Baja California in Mexico. Band-tailed pigeons provide many
recreational opportunities; they are highly prized by hunters, photographers and bird
watchers. They are swift and powerful fliers, and reside in rugged coastal forested
habitats (Jeffrey et al. 1977). Unfortunately, these distinguished birds have greatly
declined in abundance over the last several decades and they are now at their lowest
levels since monitoring began in the early 1950's (Tomlinson and Bartonek 1989, Braun
1994), though at one time, their numbers rivaled those of passenger pigeons [(Bendire
1892) as cited by Neff 1947]. Ti agreement, Jarvis and Passmore (1992) reported that the
population index in the late 1980's was only 30-50% of that in the 1960's. The causes of
the population decline are largely conjectural. From a modeling exercise, Jarvis and
Passmore (1992) concluded that excessive mortality was more likely the cause of the
decline than inadequate recruitment; however, band-tailed pigeon populations have
shown little signs of recovery despite implementation of increasingly restrictive hunting
regulations. Biologists and managers hypothesize over this continual decline, suggesting
habitat loss and/or degradation, inadequate recruitment, over-harvest, and/or disease, but
lack any empirical evidence. One reason for this lack is that reliable population
monitoring techniques for band-tails have not yet been developed. Another contributing
2
factor is the scarcity of knowledge regarding their breeding ecology. For example, the
specific habitat needs of breeding band-tailed pigeonsnesting and feeding habitats, and
mineral sitesare largely unknown, and very little is known about their reproductive
performance, making annual predictions of population status and trends difficult.
Concern for band-tailed pigeons has increased because so little is known about
abundance, habitat requirements, and reproductive potential of this important game bird
of western North America.
Band-tail pigeons have a delicate relationship between reproduction and mortality.
Although adult band-tails are thought to have a high survival capacity (60-70%), their
reproduction potential is very low. They lay one egg per clutch, but they are sequential
nesters (Grinnell 1913, Fitzhugh 1970) with some birds nesting at least 3 times in one
nesting season (MacGregor and Smith 1955). In Oregon, band-tails have sufficient time
to nest twice per year (Jarvis and Passmore 1992). Any circumstance or event that results
in unusually high mortality could have immediate and long-lasting detrimental effects on
the population.
The nesting behavior of band-tailed pigeons is typical of Columbids. They are
monogamous (Delacour 1980) and build fragile, platform nests, which take 1-3 days to
build (Goodwin 1983, Neff 1947). Both parents partake in incubation, brooding and
feeding the young. Incubation is continuous, with the exchange between parents
occurring in the morning and late afternoon; males attend nests from midmorning until
midafternoon and females from midafternoon until the following midmorning (Neff
1947, MacGregor and Smith 1955, Peeters 1962). The incubation period is
3
approximately 19-20 days (Neff 1947, MacGregor and Smith 1955) with the nestling
phase lasting approximately 20-28 days (Neff and Niedrach 1946, MacGregor and Smith
1955). Taken togethernest building, incubation, nestling phasethey constitute a 4050 day nesting cycle. Band-tailed pigeons may decrease the intervals between nestings by
overlapping nesting cycles, i.e., simultaneously caring for two sets of offspring at
different stages of development, and/or shorten the nesting cycle by reusing old nests
(Fitzhugh 1970, Westmoreland et al. 1986). Parents feed their young crop milk, but,
during the latter stages of nestling development, the young receive mostly regurgitated
externally acquired foods (Braun 1994). Fledging of young begins in late June, peaking
in late August and continuing through September (Jarvis and Passmore 1992).
Productivity of band-tailed pigeons is poorly understood. Jarvis and Passmore
(1992) suggested that recruitment of young varies little from year to year, and their
population model yielded a fall population of about 23% young. Also, hunter shot
samples contained only 16-20% young. However, these estimates of reproductive
performance are based on indirect evidence (crop gland cycles) and average values (age
structure), both of which require simplifying assumptions. Consequently, these results
are best viewed as defining the capabilities of the population rather than as estimates of
actual performance. Nesting records of individual pairs throughout the breeding season
are needed to estimate actual performance and to understand the influences of
environmental variables on annual recruitment.
Band-tails build a fragile platform nest in a variety of sites in what can broadly be
defined as forest environments (Neff 1947, MacGregor and Smith 1955). Typically, nests
are built in dense stands of shrubs or trees, often in steep mountainous terrain. Nests are
generally dispersed and searching for nests is usually futile. The only reported nesting
study of band-tails occurred in central coastal California in a semi-suburban area
(MacGregor and Smith 1955). Consequently, very little is known of population
productivity from direct observation of nests. Additionally, habitats used by nesting
band-tails have not been quantified; only general qualitative descriptions of nesting
habitat are available.
Throughout the Northwest, band-tails concentrate at mineral sites to consume
mineral during the nesting season (March and Sadleir 1972). Production of eggs and crop
milk requires large amounts of mineral, principally calcium (March and Sadleir 1975).
However, the principal food of nesting band-tailed pigeons in the Northwest are berries of
elder (Sambucus spp.) and Pacific cascara buckthorn (Rhamnus purshiana), which
contain very little mineral (Jarvis and Passmore 1992). Consequently, band-tails need to
supplement their mineral-poor diet while nesting in the Northwest. Mineral sites may be
the scarcest resource needed by band-tailed pigeons for reproduction and the distribution
and abundance of mineral sites may affect reproductive performance of the population by
constraining the distribution of suitable nesting sites. Band-tails are also very sensitive to
disturbance and distribution of trees around mineral sites. However, little information
exists on distribution of nests relative to mineral sites and how forest management
activities adjacent to mineral sites impact nest distribution. This deficit of information
occurs because very few band-tailed pigeon nests have ever been located.
5
Abundance of food apparently regulates the initiation and duration of nesting
(Gutierrez et al. 1975, Jarvis and Passmore 1992). In Oregon, and probably in much of
the Northwest, several types of berries, primarily elder and cascara buckthorn, provide the
necessary food during nesting (Jarvis and Passmore 1992). These berries first become
available in early- to mid-June and are present through September. Although containing
little mineral, berries are a rich source of protein and carbohydrate, both of which are
important in reproduction. Consequently, abundance, seasonal availability, and
distribution of these few types of berries may significantly influence reproductive
performance. In addition, modern forest management practices may have deleterious
effects on food supply, e.g., herbicide treatment of clearcut areas and/or increased harvest
of cascara bark, reducing food-bearing shrubs, and further constraining reproductive
performance.
Forests are intensely utilized in the Pacific Northwest for a variety of resources, of
which lumber is predominant (Highsmith and Kimerling 1979). Such uses have altered
the forest environment, and the forest management practices employed may have
important consequences for band-tail pigeons. For instance, band-tails are sensitive to
disturbance and to distribution of trees around mineral sites (Jarvis and Passmore 1992).
Regulation of forest management activities around mineral sites to minimize disturbance
during the nesting season and to leave forest cover adjacent to mineral sites is beneficial
to band-tails. Unfortunately, other than mineral site use, very little is known about how
band-tailed pigeons use forest resources. Until more is learned of what constitutes
foraging and nesting habitats and how band-tailed pigeons use those habitats, the impact
of forest management practices on the welfare of band-tailed pigeon populations cannot
be assessed.
This project was designed to investigate and provide information on population
productivity and habitat use of breeding band-tailed pigeons in the central Coast Range of
Oregon. The specific objectives of the study were: 1) to determine basic reproductive
parameters, including nesting chronology, frequency of nesting, nesting rates, and nest
and fledging success; 2) to describe habitat characteristics of nesting and foraging areas;
and 3) to characterize foraging/movement patterns, including location of feed and mineral
areas, frequency and duration of visits, and seasonal changes in use of these areas.
Secondary objectives were to examine adult survival, philopatry, and home range sizes of
nesting band-tailed pigeons.
7
STUDY AREA
The study was conducted in the western half of Oregon's Willamette Valley and
the Coast Range Mountains, centering around Corvallis. The northern boundary is
approximately from Salem (440 57' N, 123° 05' W) to Lincoln City (44° 57' N,
124° 01' W); the southern boundary extends from Eugene (440 05' N, 123° 05' W) to
Florence (43° 58' N, 124° 06' W) with an approximate area of 4,050 km2 (Fig. 1).
This region has a maritime climate with typically mild, wet winters (Oct-Jun).
Snow is infrequent and ephemeral, generally lasting for a few days, except on the higher
peaks and ridges. Summers (Jul-Sep) are usually cool and dry with fog occurring often in
the river valleys (Hemstrom and Logan 1986). Annual precipitation ranges from 150 to
300 cm and occurs primarily during the winter months in the form of rain; temperatures
range from 0°C in winter to 24°C in summer (Franklin and Dyrness 1988).
Western Oregon is characterized by dense coniferous forests in mountainous
regions and diversified agriculture in the Willamette and coastal valleys. Specifically, the
Coast Range is distinguished by steep slopes and deeply-cut river and creek drainages
with elevations ranging from 15 m to 1,250 m. The natural forest overstory is dominated
by Douglas-fir (Pseudotsuga menziesii), western hemlock (Tsuga heterophyalla), and red
alder (Alnus rubra); western redcedar (Thuja plicata) and bigleaf maple (Acer
macrophyllum) are also common. Understory vegetation is variable in composition and
patchy in distribution; common species include salmonberry (Rubus spectabilis), vine
maple (Acer circinatum), salal (Gaultheria shallon), Oregon grape (Berberis spp.),
8
ljncoln City
Salem
I1
2
OREGON
Newport
rJ
Legend
I
/
L
Albany
20
*
+
* trap site
**.
nest site
*
*
-'=--
Corvallis
Eugene
Florence
Figure 1. Band-tailed pigeon breeding ecology study area, central Coast Range of
western Oregon, 1993-95.
huckleberry (Vaccinium spp.), and hazelnut (Corylus cornuta) (Franklin and Dyrness
Ii,1:f:]
Most forest areas are managed for timber production of Douglas-fir. Clear-cutting
is the usual method of harvest, therefore forests are a patch work of different but even-
aged stands. Early successional communities following harvest and/or fire generally
develop into dense shrub communities, unless eliminated through intensive management.
Many of the dominant shrub species are prolific mast producers, e.g., red and blue elder,
and cascara (Franklin and Dyrness 1988). Agricultural crops in the Willamette Valley
include fruit and nut orchards, row crops (beans, corn, peas, carrots), wheat and ryegrass.
In coastal valleys, subsistence farming is the principal agricultural activity (Highsmith
and Kimerling 1979, Jarvis and Passmore 1992).
Trapping occurred at four different locations within the study area. Two bait sites
were in the Coast Range foothills; one site was approximately 10 km west of Corvallis
(44° 37' N, 123° 20' W), and the other was 20 km southwest of Corvallis (44° 26' N, 123°
23' W). The 3rd and 4th sites were on the western edge of the Willamette Valley, one
23 km south (44° 21' N, 123° 18' W) and the other 8 km southwest (440 28' N,
123° 21' W) of Corvallis. These latter 2 sites were at livestock farms where birds fed on
corn silage (Fig. 1). All sites had deciduous and coniferous trees nearby that were used as
perches both before and after consuming grain.
F[i
METHODS
CAPTURE, INSTRUMENTATION AND RADIO-TELEMETRY
Band-tailed pigeons were captured and equipped with radio transmitters from
April through June during the springs of 1993 through 1995. Two similar capture devices
were used; both consisted of a self-propelled (elastic cords) trap net (one measured
5x8 m, with 2.5 cm square mesh; the other measured 1x2 m, with mesh cloth netting).
The larger trap was triggered by an electronic remote release while the other used a
manual release. Traps were baited with cracked and whole corn. At the livestock trap
sites, the smaller trap net was utilized by burying it in the corn silage where birds readily
fed. Trapping primarily occurred in the morning, sunrise to 1130 hours. An attempt was
made to limit capture to approximately 10 birds per capture attempt. This restriction
reduced handling and holding time, reducing stress and feather loss. Bird-handling
procedures followed guidelines set forth by Oring et al. (1988).
Sex (Passmore and Jarvis 1979) and age class (immature or adult) (Silovsky et
al. 1968, White and Braun 1978) were determined by plumage characteristics. All bandtailed pigeons were fitted with a metal U. S. Fish and Wildlife Service leg band. Batterypowered and solar-assisted battery-powered transmitters, weighing approximately 6.0-6.5
grams, with a 3- to 5-km range, and operating in the 150, 152-153 MHZ range, were
placed on captured pigeons using a backpack harness. Transmitter life-expectancies
ranged from 165 days for the battery-powered transmitters to 2-3 years for the solar-
assisted battery-powered transmitters. Also, during the first year of the study, some
smaller battery-powered transmitters (approximately 3-4 grams) were tied and glued with
11
cyanoacrylate to the under surface of the central two tail feathers. These were used to
evaluate and test for possible behavioral effects of harnesses, because studies have shown
deleterious effects of harnesses (e.g., Perry et al. 1981, Gessaman etal. 1991).
Radio monitoring of marked birds began soon after capture in spring and
continued until mid- to late-September and infrequently into early-October, when the
birds migrated out of the study area. Surveys to establish approximate locations of bandtails were made from the air via fixed-wing aircraft (Gilmer et al. 1981) using a scanning
receiver. These flights occurred 1-2 times per week (weather permitting) and consisted of
6-10, 80-km-long north-south transects spaced approximately 8-10 km apart from the
Willamette River west. Detected signals were recorded with an onboard LORAN C unit.
Bird locations were transformed into Universal Transverse Mercator (UTM) coordinates
and then plotted on USGS topographic maps. Pigeons were relocated by driving and
using a non-directional whip antenna. A handheld Yagi antenna was used to approach
birds on foot for visual observations to ascertain status, e.g., determine if birds were
nesting, feeding, or acquiring mineral. Attempts were made to locate each bird at least
once a week, and periodic flights were made outside the normal flight zone to look for
missing birds.
When radio-marked birds were discovered on nests (usually at a distance < 8 m
from nest tree or shrub), attempts were made to determine an approximate age of the nest
(i.e., egg or nestling present) with minimal disturbance (i.e., trying not to flush birds).
Band-tailed pigeons rarely flushed from nests while being observed. The status of active
nests was monitored from a distance (usually> 10 m) approximately once a week, and
12
near fledging nest visits were more frequent in order to determine when young fledged.
Band-tailed pigeons continuously attend the nest until the squab is approximately a week
old, so the presence of an adult was taken as evidence that the nest was still active. In the
latter half of the nestling stage, often the squab could be seen directly. Successful nests
were those in which a young was fledged or, when active nests could not be inspected
neai the day of fledging, a young was known to be present in the nest within 4 days of the
expected date of fledging.
HABITAT SAMPLING
Habitat characteristics were measured at sites where radio-equipped band-tailed
pigeons were visually observed nesting or feeding. UTM grid coordinates were recorded
for each site for later determination of distances traveled between nest and feed sites.
After a nest failed or the young fledged, the species, diameter at breast height (DBH),
crown base height, height of the nest tree or shrub, and the nest height were recorded.
Microhabitat features were measured for each nest: 1) compass direction of the nest
relative to the position of the main stem; 2) lateral distance of the nest from the main stem
of tree or shrub containing the nest; 3) diameter of the largest woody stem supporting the
nest; and 4) distance to overhead cover from the nest.
Vegetative characteristics were measured at each nest site within a circular plot of
0.2 ha (25-m radius), centered at the nest tree. Percent canopy closure was measured with
a moosehorn densiometer (Vales and Bunnell 1985) at 3 stations along each of four 25-m
transects in the 4 cardinal directions. The average of the 48 measurements (3 x 4 x 4)
was used as an estimate of canopy closure within the 0.2-ha plot.
13
Species, diameter (DBH > 13 cm), and height class of each tree within the 0.2ha plot were recorded. General site characteristics included aspect, slope (%), elevation,
position on slope (valley, bottom, mid, top, ridge top), and layers in canopy. Presence or
absence of supercanopies were noted. (A supercanopy is a sparse overstory consisting of
a large tree or trees in a forest of young and smaller trees, where a remnant tree(s) remains
from the previous stand.) Nest sites were classified into one of three plant communities
(deciduous, conifer/deciduous, conifer) and one of four stand seral conditions (saplingpole, small or large sawtimber, old-growth) with open or closed canopies (Brown 1985,
McGarigal 1993). Elevation and distance to nearest surface water were determined with
USGS topographic maps.
Feed sites were noted when radio-marked birds (often with many other band-
tailed pigeons) were observed feeding. The species of forage vegetation and distribution
(e.g., solid stand, scattered patches, sparse) and any other potential food sources present at
the site were recorded. General site type (riparian or upland) and characteristics of those
sites (creek, moist bottomland, moist or dry hillside, clearcut, etc.) were recorded. Other
general site characteristics included slope (%), aspect, average height of food source, and
presence of supercanopies. Attempts were made to return to feed sites periodically and
estimate bird use and food species phenology.
REPRODUCTIVE BIOLOGY
Initiation dates were determined by back-dating 45 days from fledging (40 to 49day nesting cycle, White and Braun 1978, Zeigler 1971), and clutch sizes were recorded
after visual observation (i.e., climbing trees when adults were flushed from their nest or
14
away on an incubation break). After nest failure was confirmed, the date of abandonment
or destruction was estimated as 40% of the interval between observations (Miller and
Johnson 1978, Pollock et al. 1989). Nest survival rates were based on the Mayfield
method (Mayfield 1975, Miller and Johnson 1978) using the computer program
MICROMORT (Heisey and Fuller 1985). MICROMORT was used to identify portions
of the nesting period that had homogeneous survival rates (i.e., constant daily mortality is
an assumption of the Mayfield method) and the data were then grouped for further
analysis. Survival estimates were made for each of the 7 week-long intervals (i.e., 49-day
nesting cycle), and likelihood ratio tests (LRT) were used to test the null hypothesis that a
reduced model with combined weekly intervals provided an adequate fit to the data.
Where no difference was detected (P> 0.05), adjacent intervals were combined and a
survival estimate for the combined interval was calculated. LRT were repeated until all
adjacent homogeneous intervals had been combined.
Nesting survival probabilities for early-nesting attempts and late-nesting attempts
were tested, using LRT, for differences in relation to initiation date. A calendar midpoint
of the yearly initiation interval was used to separate early- versus late-nesting attempts.
Breeding season (May September) survivorship of adult band-tailed pigeons was
calculated using MICROMORT (Heisey and Fuller 1985, Pollock et al. 1989); estimates
were determined similarly to that of nest survival as described above. For adults that
moved out of the study area, and/or their radio-transmitter failed, their remaining days of
the breeding season were censored from the analyses.
15
DATA ANALYSES
Summary statistics were calculated for feed areas, and nest, nest tree or shrub, and
nest site characteristics. Compass direction of nests relative to mainstem of tree or shrub
and nest site slopes were analyzed by categorizing the nest or site as being north (337.622.5°), northeast (22.6° 67.5°) east (67.6° 112.5°), southeast (112.6°- 157.5°), south
(157.6° 202.5°), southwest (202.6° 247.5°), west (247.6° 292.5°), or northwest
(292.6° 337.5°). Compass directions were tested for a uniform distribution among the 8
divisions using a circular distribution Goodness-of-fit test (Zar 1974).
Logistic regression (PROC LOGISTIC; SAS Institute. Inc. 1987) was used to
assess the relationship(s) between nest and nest site characteristics and nest fate. Nest
fate (the response variable) was treated as a binary random variable with a value of 1
when the nest was successful or 0 when the nest failed. The primary goal of this analysis
was addressed retrospectively by looking at: 1) how the odds of predicting nest fate were
dependent upon vegetative and physical habitat characteristics, and 2) how the odds were
affected by changes in the explanatory variables. The analysis was performed using dropin deviance F-tests to determine variable combinations for answering the above questions.
Also, some categorical explanatory variables were reduced to a single variable that
produced the greatest deviance reduction while lowering the degrees of freedom.
Vegetative and physical habitat characteristics of successful and failed nests were
compared to null models with no explanatory variables. Each habitat variable that
resulted in the largest significant drop in deviance was included in the final model, i.e.,
drop-in deviance and drop-in degrees of freedom were compared to a chi-square
16
distribution to determine significance. The additive model was then compared to each of
the remaining variables, adding those variables sequentially that resulted in a further
significant drop in deviance. This process was continued until no further variables
resulted in a significant drop in deviance. Whenever a variable was marginally
significant (
0.05), the Wald Type ifi test () was referenced to determine if the
variable should be included in the final model. The final model goodness-of-fit was
assessed with the Hosmer-Lemeshow test. Odds ratios were calculated for percent
canopy closure and site aspect to describe the strength of the association and allow a
direct interpretation of the influence of each variable upon nest success.
The minimum, straight-line distance was measured between locations for each
bird to determine breeding movement patterns. Differences in mean distances traveled
(i.e., to feed areas from nest sites) between nesting attempts (i.e., first, second, and third
nest attempt) were tested using a 2 sample t-test (Steel and Torrie 1980). Also, distances
between successive nests were compared among nest attempt and nest fate; a j < 0.05
level of significance was used for these analyses.
Breeding home range sizes, based on minimum convex polygons (Mohr 1947),
were estimated for birds that were known to be nesting, with the computer program
CALHOME (refer to Larkin and Halkin 1994). Correlation analysis was used to compare
number of locations (per bird) to polygon size to ascertain reliability of these areas as
home-range estimates. Breeding range sizes were then tested for differences, using a 2
sample t-test (Steel and Torrie 1980), between adult male and female radio-marked birds.
17
RESULTS
In the springs of 1993 through 1995, 127 band-tailed pigeons were captured and
equipped with radio transmitters (Table 1). Sixty-two percent of these birds were adult
males, 32% were adult females, and 6% were yearlings.
Tn 1993, nine birds were equipped with tail-mount transmitters, and 44 were
equipped with a harnessed backpack solar-assisted transmitter. Several technical
problems occurred with the solar-assisted transmitters. Approximately 75% of these
transmitters failed, i.e., had intermittent signals (no signal for weeks at a time) or quit
altogether. These failures reduced the sample size to 20-22 marked birds, that could be
followed consistently. Of the 9 tail-mounted transmitters, 6 were lost within 60 days of
marking, when the two central tail feathers pulled out. Only one tail-mount transmitter
stayed attached for the duration of the nesting season. No measurable differences
between birds with tail-mounted transmitters and birds with harnessed backpack
transmitters were observed in first nest initiation dates ( = June 26 vs.
= June 21) and
apparent nest success (67% vs. 73%). Thus, in 1994 and 1995, all transmitters were
attached using the harness backpack method.
Tn the spring of 1994, 38 adult band-tailed pigeons were equipped with either a
solar-assisted (n = 14) (an improved design over the 1993 model) or battery-powered
radio-transmitter ( = 24). Ten of those birds left the study area and/or had radio-
transmitter failure. However, signals from 15 (14 male and 1 female) adult band-tailed
pigeons radio-marked with solar-assisted transmitters in 1993 were heard in 1994. Only
Table 1. Number of radios attached to band-tailed pigeons by attachment technique,
radio type, and sex (or unknown yearling), in the central Oregon Coast Range 1993-95.
1994
1993
Attachment
Radio type
c?
tail-mount
small battery
7
harness
solar assisted
28
1995
d'
ci'
unkY
unkY
2
15
4
11
3
5
small battery
3
7
8
3
5
large battery
11
3
6
3
2
25
13
19
6
11
Total/sex
Total/year
35
1
1
53
17
38
36
19
7 of those 15 returning birds were followed with some consistency, since the solarassisted radios transmitted an intermittent signal making it difficult to locate birds.
In 1995, 30 adult band-tailed pigeons and 6 yearlings were equipped with
transmitters. Twelve adults and all 6 yearlings moved outside the study area andlor their
radio-transmitter failed. However, an additional 6 birds equipped in 1993 (2) and 1994
(4) with solar-assisted transmitters were followed in 1995.
REPRODUCTIVE BIOLOGY
Nesting
A total of 137 nests initiated by 65 different radio-marked band-tailed pigeons (45
males, 20 females, and only 1 pair had both individuals with transmitters) were observed
during the spring and summers 1993 through 1995. Even with the high rate of radio
failures in 1993, 29 nests initiated by 19 different birds were located. Nine of those birds
initiated 2-3 nests; 4 birds produced 2 successful nests each. Nesting information on
most individual birds for 1993 was not determined due to intermittent signals and radio
failure. Fifty-eight nests initiated by 32 different birds and 50 nests initiated by 22
different birds were located during 1994 and 1995, respectively. Sixty-five percent
= 35) of those birds initiated 2-3 nests; 8 birds produced 3 successful nests. Only 7
birds marked with solar-assisted transmitters were found nesting in more than 1 year
(primarily due to transmitter failure); 1 bird was observed initiating 8 different nests in 3
years. Nests usually consisted of a small platform made from small twigs, built with
seemingly modest effort. All nests had a clutch size of one.
20
Nesting Chronology
The observed reproductive period for band-tailed pigeons, was approximately 6
6.5 months from late-April through mid- to late-October (Fig. 2). The earliest observed
nest was initiated April 29, and for the 3 years of the study, nest initiation numbers
consistently peaked during the last two weeks in June. The last recorded nest initiation
occurred September 12. Thirty-eight percent of all young fledged on or after September
1; 21% of all young fledged on or after September 15. Initiation dates were extended 2-3
weeks in late-summer in 1994, compared to 1993 and 1995.
First nest initiation dates for 1994 1995 were highly variable among birds,
ranging from early-May to mid- to late-July (Fig. 3). Renesting attempts (after a bird lost
its previous nest) were initiated throughout the breeding season, mid-June through early-
September. Twelve marked birds were not adequately monitored prior to their first
observed nesting attempt, and thus, it could not be determined if they initiated an earlier
nest (Fig 3).
Nesting Interval
After a nesting failure, the period until a new nest was initiated averaged 12 days
(range = 2 32 days, SD = 8 days). However, band-tailed pigeons (78% of the time)
eliminated nesting intervals altogether by overlapping their nesting cycles, i.e.,
simultaneously caring for 2 sets of offspring at different stages of development. Nest
overlap averaged 7 days (range = 1
15 days, SD = 3 days). Nesting cycles that did not
(22% of the time), averaged 10 days (range = 1
cycles.
22 days, SD = 8 days) between the
21
C11
U initiation
J fledge
30
C
0)
25
U-
-o
C
20
(I)
4-
(I)
a)
15
0
a)
o
E
10
z
5
ri
16-30
April
1-15
16-31
May
1-1516-30
June
1-15
16-31
July
1-15
16-31
Aug
1-15
16-30
Sept
1-15
16-31
Oct
Initiation and Fledge Date
Figure 2. Number of radio-marked band-tailed pigeon nests and young that fledged
throughout the nesting season in the central Coast Range of Oregon, 1993-95.
22
25
20
U)
U)
ci)
15
9-
0
C)
-o
E
10
z
5
1-1516-31
May
1-15
1
1-15
0
16-31
1-15
July
June
16-31
Aug
1-15
Sept
Initiation Date
1st nest
2nd nest
LI
renest
3rd nest
Unknown
Figure 3. Nesting chronology of radio-marked band-tailed pigeons and the number of
nests initiated during the nesting season in the central Coast Range of Oregon, 1994-95.
(renest classified when the bird's previous nest failed; unknown represents birds that were
not adequately monitored prior to their first observed nesting attempt, and thus, it could
not be determined if they initiated an earlier nest)
23
By re-using old nests, band-tails could reduce the nest cycle length by eliminating
the time required for nest building. However, nest re-use was rare. Only one incident
was observed; a nest platform was re-used for a pair's first and third nest in 1994. Birds
marked in previous years, that returned the following year, did not re-use old nest
platforms or nest in the same trees.
Nesting Success
Of the 137 nests, 94 fledged a young and 40 were unsuccessful; the fate of 3 nests
was undetermined. Nesting data were represented most parsimoniously when the
7-
week nesting cycle was divided into 2 periods, incubation and post-hatch. Based on the
LRT, the early period had an interval length of 21 days (the approximate length of
incubation) (Table 2). Nest survival probabilities varied by 2.2% between 1993-95; the
overall nest survival probability was 0.689, SE = 0.004 (Table 2).
The midpoint of the initiation interval ranged from June 26 July 12 during 199395. This division placed about 44% of the nests in the early initiation class. Band-tailed
pigeon nest survival probabilities were significantly lower for nests initiated in the early
period than nests initiated in the late period, in 1993 (LRT, P <0.05). However, there
were no differences detected in relation to initiation dates for 1994 and 1995 (Table 3),
and the pooled data for the 3 years.
Adult Survival
A total of 134 radio-marked adult band-tailed pigeons (birds that returned the next
year were counted again) provided the repeat observations necessary to calculate adult
Table 2. Mayfield survival rates of radio-marked band-tailed pigeon nests during incubation (weeks 1-3) and post-hatch (weeks 4-7)
in the central Oregon Coast Range, 1993-95.
Nesting Cycle
Incubation
(weeks 1-3)
Post-hatch
(weeks 4-7)
Daily survival
Year
Exposure
days
A
Daily survival
SE
Exposure
days
S
SE
Nesting cycle
survival
probability
A
SE
95%
Confidence limits
1993
27
520
0.988
0.001
503
0.996
0.00 1
0.70 1
0.0 17
0.547 - 0.899
1994
57
1093
0.989
<0.001
1018
0.995
<0.001
0.685
0.008
0.578-0.826
1995
50
985
0.989
<0.001
743
0.994
<0.001
0.679
0.010
0.554 - 0.832
Pooled
134
1613
0.989
<0.001
2264
0.995
<0.001
0.689
0.004
0.613-0.775
25
Table 3. Radio-marked band-tailed pigeon nest survival probabilities in relation to
initiation date in the central Oregon Coast Range, 1993-95.
n
Nesting cycle
survival
probability
SE
95%
Confidence limits
Early
12
0.536*
0.039
0.326 0.883
Late
15
0.852*
0.025
0.682 - 1.000l
Early
26
0.761
0.017
0.610-0.948
Late
31
0.632
0.016
0.479-0.834
Early
21
0.703
0.023
0.526 0.940
Late
29
0.662
0.02 1
0.50 1
0.876
Early
59
0.69 1
0.008
0.58 1
0.822
Late
75
0.687
0.006
0.586 - 0.807
Initiation
Year
1993
1994
1995
Pooled
a
13
*
datea
Calendar midpoint of the yearly initiation interval was used to separate early versus late
nests.
Upper confidence limit was truncated at 1.
Significantly different rates (Likelihood Ratio Test, 2 df, P <0.05)
survival during the breeding season (May September). There were 3 confirmed
mortalities (2 in 1993, 1 in 1994); all 3 were killed on nests. Only radio transmitters, a
few larger bones, and some flight feathers were recovered from these mortality sites.
Adult survival probability for the combined breeding seasons of 1993-95 was 0.963,
SE = 0.002 (Table 4).
HABITAT CHARACTERISTICS
Nest and Nest Tree Characteristics
Band-tailed pigeons nested in a variety of tree and shrub species, with 70% of the
nests observed in Douglas-fir. Only 17% of all nests were in deciduous trees or shrubs;
6% were in vine maple (Fig. 4). Seventy-one percent of the nests initiated in conifers
were successful compared to 62% when nests were initiated in deciduous trees or shrubs.
Height of nests was highly variable and averaged 10.3 m (range = 1.8 36.3 m,
SD = 7.5 m) in coniferous, 10.2 m (range = 3.4 - 26.5 m, SD = 6.4 m) in deciduous, and
4.4 m (range = 2.3 5.5 m, SD
varied greatly (coniferous
range = 7.6 -31.1 m; shrub
0.9 m) in shrub species. Nest tree or shrub height also
= 19.0 m, range = 6.1 44.8 m; deciduous
15.2 m,
= 5.6 m, range = 3.7-8.2 m), with a combined
(coniferous, deciduous, and shrub) mean DBH of 28.7 cm (range = 4.1
83.1 cm,
SD = 17.3 cm). Height of the canopy base for nest trees or shrubs averaged 6.0 m
(range = 0 33.8 m) for coniferous, 6.5 m (range = 0.3 - 18.9 m) for deciduous, and 2.9 m
(range = 0.9 5.5 m) for shrub species (Fig. 5). Nests were placed with an average of
65%, 71%, and 42% of the canopy above the nest in coniferous, deciduous and shrub
species, respectively. Nest platform distances from the main stem of the tree or shrub
27
Table 4. Survival of radio-marked adult band-tailed pigeons during the breeding season
(May - September) in the central Oregon Coast Range, 1993-95.
Daily survival
Year
n
S
SE
5 month
survival
probability
1993
46
0.9994
<0.0001
0.913
0.009
0.806
1994
50
0.9998
<0.0001
0.970
0.004
0.9141.0O0*
1995
38
1.0000
0.0000
1.000
0.000
1.000
Pooled
134
0.9997
<0.0001
0.963
0.002
0.922 - 1.0O0
A
Upper confidence limit was truncated at 1.
SE
95%
Confidence limits
1.000*
1.000*
success
failure
(%) percent of nests within tree or shrub species
Coniferous
Abgr
(3%)
(<1%)
Pipo
Psme
(70%)
(2%)
Tabr
(<1%)
Thpl
(6%)
Tshe
Deciduous
Acma
(1%)
]
(<1%)
AIru
Conu
(3%)
Quga
(3%)
Shrub
Acci
(6%)
Coca
(2%)
]
Hodi
(<1%)
Sasc
(<1%)
I
0
Species
10
20
30
I
40
50
60
70
80
90
100
Number of Nests
Figure 4. The number of successful and unsuccessful radio-marked band-tailed pigeon
nests (n = 137) initiated in coniferous and deciduous tree or shrub species in the central
Coast Range of Oregon, 1993-95. (Abgr grand fir, Abies grandis; Pipo ponderosa pine,
Pinus ponderosa; Psme Douglas-fir, Pseudutsuga menziesii; Tabr - Pacific yew, Taxus
brevifolia; Thpl western red cedar, Thuj a plicata; Tshe - western hemlock, Tsuga
heterophylla; Acci - vine maple,
circinatum; Acma big leaf maple, Acer
macrophyllum; Alru red alder, Alnus rubra; Coco - hazel, Corylus cornuta; Conu
Pacific dogwood, Cornus nuttallii; Hodi oceanspray, Holodiscus discolor; Quga Oregon white oak, Ouercus garryana; Sasc mountain willow, Salix scouleriana)
29
Coniferous
(n= 112)
Deciduous
Shrub
(n = 12)
(n = 13)
30
25
25
20
20
15
10
IH
t_
5
0
I
I
I
012345
012345
N
0IIIIIIIIII
012345
Distance (m)
H = Tree or shrub height
C = Crown base height
N = Nest height and distance from main stem
Figure 5. Nest and nest tree characteristics of coniferous and deciduous trees or shrubs
used by radio-marked band-tailed pigeons in the central Coast Range of Oregon, 1993-95
(bars indicate ± SD).
30
varied considerably, ranging from contact with the main stem to over 7 m away
(coniferous
5
= 1.1 m, range =0 5.5 m; deciduous
= 1.9 m, range =0- 6.4 m; shrub
=0.8,range=0-7.1 m)(Fig. 5).
Distances were variable to the closest cover/structure above the nest platform and
averaged 44 cm (range = 2.5 - 300 cm, SD = 40) for all nests. The average diameter of
the main supporting limb of nest platforms was relatively small (. = 4.5 cm,
SD = 2.9 cm). Nests were not uniformly distributed around the main stem; a higher
proportion of the nests (40%) were positioned south or southwest of the main stem of the
tree or shrub ( = 0.003; circular distribution Goodness-of-fit test) (Fig. 6).
Nest Site Characteristics
Band-tailed pigeons nested in a wide range of stand conditions (open and closed)
and community types (sapling-pole to old-growth in deciduous and conifer communities).
However, the majority of nests (77%) were in stands classified as conifer community with
55% in the closed-canopy, sapling-pole seral condition, and only 6% of the nests were
initiated in the deciduous community type. Very few nests (approximately 7%) were
initiated in open-canopied stands, but all of these nests were successful (Fig. 7).
Vegetation structure of forest stands surrounding nests had an average of 2 layers
(range = 1 4, SE = 0.85). Mean DBH of these stands was 28.3 cm (range = 12.7
73.7 cm, SD = 10.5 cm), and 69% of the stands surrounding nests had an average DBH
between 16 cm and 32 cm (Fig. 8). The density of trees at nesting sites varied
considerably (range = 15 1050 trees/ha), but had a relatively normal frequency
distribution around the mean of 387 trees/ha (SD = 182 trees/ha) (Fig. 9).
31
292.
;7.5
E
247.
12.5
I ) I .)
Figure 6. Frequency of radio-marked band-tailed pigeon nests ( = 138) corresponding to
nest aspects (one of eight 450 segments) relative to main stem of nest tree in the central
Coast Range of Oregon, 1993-95. (non-random distribution around the main stem;
P = 0.003, circular distribution Goodness-of-fit test)
32
Open
sapling-pole
(<1%)
mixed
conifer
fl failure
success
(4%)
(%) percent of nests within community type
small sawtimber
deciduous
(<1%)
conifer
(1%)
Closed
sapling-pole
deciduous
(4%)
(9%)
mixed
conifer
(55%)
small sawtimber
deciduous
(1%)
(7%)
mixed
(14%)
conifer
large sawtimber
conifer
old-growth
conifer
(1%)
Ui
0
Condition
Community
(2%
10
20
30
40
50
60
70
80
Number of Nests
Figure 7. The total number of successful and unsuccessful radio-marked band-tailed
pigeon nests (n = 138) in various stand seral conditions and community types (based on
Brown 1985) in the central Coast Range of Oregon, 1993-95.
33
35
Figure 8. Frequency of nests initiated by radio-marked band-tailed pigeons in relation to
the average tree DBH (cm) of the nest site in the central Coast Range of Oregon, 1993-95.
(numbers represent midpoint of the DBH class)
34
20
15
(I)
ci)
z
0
>
C-)
C
ci)
0
U)
U
Number of Trees (per hectare)
midpoint classes
Figure 9. Frequency of nests initiated by radio-marked band-tailed pigeon in relation to
the density of trees (ha) in the nest site in the central Coast Range of Oregon, 1993-95.
(numbers represent midpoint of the density class)
35
Slope (%) of all nesting sites ranged from 0 to 82% (mean = 33%, SD = 19.2%),
and nest sites relative to slope aspects did not differ significantly from a random
distribution (P = 0.26; circular distribution Goodness-of-fit test) (Fig. 10). A high
percentage (86%) of band-tailed pigeon nests were positioned in the mid- to top-region of
the elevational gradient (Fig. 11), and nest site elevation averaged 360 m (range = 85
975 m, SD = 166 m). Distance to nearest water (determined from USGS topographic
maps) averaged 316 m but was highly variable (range = 11 2000 m, SD = 298 m).
Feed Site Characteristics
A total of 25 feed sites used extensively by radio-marked and non-marked bandtailed pigeons throughout the breeding seasons (1993-95) were discovered. These feed
areas were quite diverse in their physical and vegetative characteristics and were located
in both riparian and upland zones. A majority (64%) of the sites were associated with
creeks or moist bottomlands. Feed sites located on uplands were usually associated in
areas with open or sparse over-stories, i.e., in clearcuts, large gaps, or young stands,
which allow the shrub species access to high levels of sunlight (Fig. 12). The principal
food component within most of these sites was either red elder (June through mid-
August) or cascara (mid-August through September). However, 2 sites were different;
one contained primarily black hawthorn (during mid-June through late-July) and the other
primarily blue elder (mid-August through September).
Eighty-four percent of the feed sites had a food source that was <9 m in height;
28% were < 3 m in height (Fig. 13). The food component within the sites was most
frequently (78%) in thick dense patches scattered throughout the site. One site consisted
3()
N
292.
7.5
E
247.
12.5
S
Figure 10. Frequency of radio-marked band-tailed pigeon nests (, = 138) corresponding
to site slope aspect (one of eight 45° segments) in the central Coast Range of Oregon,
1993-95.
37
3%
ridge top
Cl)
(I)
cl)
z
0
ci)
0
ci)
0
3C
valley
Coast Range Mountains
Figure 11. Percent frequency of band-tailed pigeon nests ( = 136) in relation to
mountain topography in the central Coast Range of Oregon, 1993-95.
38
Riparian
Zone
type
Number of Feed Areas
Figure 12. General landscape characteristics andlor stand conditions of feed areas
(n = 25) used by radio-marked band-tailed pigeons in the central Coast Range of Oregon,
1993-95. (%) = percent of feed areas
39
Average Food Height (m)
<3
3-9
9
12
> 12
Food Source within Area
sparse clump
scattered
thick patches
(64%)
solid stand
Slope of Feed Area (%)
<25
(64%)
25 - 55
56 - 100
> 100
Location of Super Canopy
adjacent
scattered within
surrounding site
0
5
10
15
Number of Feed Areas
Figure 13. General site and vegetative characteristics of feed areas ( = 25) used by
radio-marked band-tailed pigeons in the central Coast Range of Oregon, 1993-95.
(%) = percent of feed areas within each classification
20
of a single, large blue elder shrub on the edge of a clearcut. The majority (64%) of the
feed sites had a slope of < 25%; 2 sites had slopes> 100% (Fig. 13). All feed areas had
some form of super-canopy (taller trees or snags) near the food sources, allowing birds to
perch before and after flying down to feed (Fig. 13).
NEST SUCCESS-HABITAT RELATIONSHIPS
The frequency of nests initiated by radio-marked band-tailed pigeons increases as
percent canopy closure increases (Fig. 14). However, an inverse relationship occurs when
plotting the nest success probability against percent canopy closure (Fig. 15). When nest
site aspect is reduced (using the variable selection procedure) from eight 45° segments
into two 180° categories (classified as NE = 337.6° 157.5° or SW
157.6° 337.5°
sites), a higher proportion of the nests fall into the SW category (Fig. 16). Also, SW nest
sites had higher nest success probability (Fig. 17) than NE nest sites.
Only two of the 23 vegetative and physical habitat characteristics tested were
found to be significantly different between successful and unsuccessful band-tailed
pigeon nests (P <0.10 from drop-in deviance F-test) (Table 5). Logistic regression
indicated that site canopy closure was a good predictor of differential nest success/fate
(W = 4.80, P = 0.029), and that site aspect was an adequate predictor of differential nest
success/fate (W = 3.74,
= 0.053). Log odds ratios showed that band-tailed pigeon nests
were 0.68 times less likely to succeed with every 10% increase in site canopy closure.
However, nests placed on slopes facing the SW were 2.11 times more likely to succeed
than nests on NE facing slopes (Table 6). The fit of the predicted probability of nest
success model was adequate (p = 0.667; Hosmer and Lemeshow Goodness-of-Fit test)
41
70
50
U)
U)
a)
Z40
0
ci)
0
30
20
10
30-50
51-60
61-70
71-80
81-90
91-100
Percent Canopy Closure
Figure 14. Frequency of nests initiated by radio-marked band-tailed pigeons in relation to
percent canopy closure of the nest site (0.2-ha plot centered on nest tree) in the central
Coast Range of Oregon, 1993-95.
42
1
-Q
-o
0
UC')
Cl)
0
0
0
U)
(I)
0
z
0.4
0.2
30-50
51-60
61-70
71-80
81-90
91-100
Percent Canopy Closure
Figure 15. Nest success probability of radio-marked band-tailed pigeon nests in relation
to percent canopy closure of the nest site (0.2-ha plot centered on nest tree) in the central
Coast Range of Oregon, 1993-95.
43
70
U)
U)
C)
z
60
j
50
40
NE (337.6°- 157.5°)
SW (157.6°- 337.5°)
Site Aspect
Figure 16. Frequency of nests initiated by radio-marked band-tailed pigeons in relation to
nest site aspect (after conducting a variable selection procedure reducing aspect from
eight 45° segments into two 1800 categories) in the central Coast Range of Oregon, 199395.
1
0.8-f
-D
-D
0
S.-
0
U)
(I)
w
0
0
(I)
U)
0.4
0.2
I
NE (337.6°- 157.5°)
SW (157.6°Site Aspect
Figure 17. Nest success probability of radio-marked band-tailed pigeon nests in relation
to site aspect (after conducting a variable selection procedure reducing aspect from eight
45° segments into two 1800 categories) in the central Coast Range of Oregon, 1993-95.
45
Table 5. Drop-in deviance F-test for the vegetative and physical habitat characteristics of
successful band-tailed pigeon nests in the central Oregon Coast Range, 1993-95.
Degrees of Freedom
Model
Deviance
Drop-in
Deviance
Reduced
Model
Full
Model
Drop-in
df
P-value
Null Model
167.21
SiteCanopyClosure(%)
161.21
6.00
136
135
1
0.014
Site Aspect
157.42
3.79
135
134
1
0.052
Nest Position in Canopy
154.80
2.62
134
133
1
0.106
DensityofSaplingsatSite
155.01
2.41
134
133
1
0.121
NestTreeCrownBaseHeight(cm)
155.21
2.22
134
132
2
0.330
Elevation of Site (m)
156.68
0.75
134
133
1
0.387
Nest Tree Position on Slope
155.55
1.88
134
132
2
0.392
LayersinSiteCanopy
155.69
1.73
134
132
2
0.421
Site Slope (%)
156.78
0.64
134
133
1
0.423
DensityofSnagsatSite
156.83
0.59
134
133
1
0.443
Stand Typeb
155.89
1.53
134
132
2
0.466
NestHeightinTree(m)
155.94
1.48
134
132
2
0.477
Distance to Nearest Water (m)
156.93
0.49
134
133
1
0.485
NestTreeHeight(m)
156.01
1.41
1.34
132
2
0.495
NestfromMainStem(cm)
156.05
1.37
134
132
2
0.504
DistancetoOverheadCover(cm)
156.12
1.31
134
132
2
0.521
NestTreeDBH(cm)
156.13
1.30
134
132
2
0.523
Nest Tree Speciesc
157.04
0.38
134
133
1
0.536
AverageDBHatSite(cm)
157.17
0.25
134
133
1
0.617
NearestFeedArea(km)
156.72
0.70
134
132
2
0.705
NearestMineralSite(km)
156.88
0.54
134
132
2
0.763
NumberofTreeSpeciesatSite
157.36
0.06
134
133
1
0.802
DensityofTreesat5ite
157.41
0.01
134
133
1
0.912
136
anortheast (337.6° - 157.5°) and southwest sites (157.6° - 337.5°)
hconifer (>70% conifer), mixed (30-70% deciduous), and deciduous (>70% deciduous) forests
°Douglas-fir and other (all other species)
Table 6. Final logistic regression model for the vegetative and physical habitat
characteristics of successful band-tailed pigeon nests in the central Oregon Coast Range,
1993-95. Hosmer and Lemeshow Goodness-of-Fit Statistic 5.824 with 8 d.f.
(p = 0.667).
a
Parameter
Model
Coefficient
intercept
3.771
percent canopy closure
site aspectz
SE
Change in
Odds Ratio
P-value
95%
Confidence limits
-0.388
0.177
0.68
0.0285
0.48 -0.96
0.748
0.387
2.11
0.0530
0.99 - 4.51
southwest sites (157.6° - 337.5°) in relation to northeast sites (337.6° - 157.5°)
47
using nest success as the response variable and nest site canopy closure and nest site
aspect as explanatory variables (Fig. 18).
MOVEMENTS
Flight Distances
No differences were detected in linear distances traveled by band-tailed pigeons
between years 1993 -1995. Therefore, to maximize sample sizes, measurements of aerial
distances for all 3 years were used for analysis of movement patterns.
Breeding band-tailed pigeons were highly mobile throughout their nesting season.
They traveled an average of 5.02 km (SD = 7.43), with some birds flying> 20 km daily to
feed areas (Table 7). The longest detected distance traveled to a feed area was over
51 km. Trends in travel distances to feed areas decreased as the nesting season
progressed; travel distances were less for birds during their second and third nests
(Table 7).
Band-tailed pigeons visit mineral springs or seeps approximately once per week
during the nesting cycle, which made it difficult to determine which mineral sites the
radio-marked birds utilized. Twenty incidents of radio-marked birds present at 3 different
mineral sites were recorded. Each time, these birds utilized the nearest known mineral
site to their nest (T.A. Sanders, unpubl. data); distances averaged 8.4 km (range = 3.5
14.0 km, SE = 0.83). The average distance, for all nests (n = 138), to the nearest known
mineral site (T.A. Sanders, unpubl. data) was 9.97 km (SD = 4.57) and ranged from
2.0 km to 21.3 km away.
Logit (Y) = 3.771
1
O.038(% canopy closure) + O.748(site aspect)
U)
Cl)
a)
C)
C)
O6
NE
O.4
-o
0)
2
0
0
30
40
50
60
70
80
90
100
Percent Canopy Closure
Figure 18. Predicted probability of nest success by band-tailed pigeons in the central
Oregon Coast Range 1993-95, as a function of site aspect (SW: 157.6° 337.5° 1;
NE: 337.6° 157.5° 0) and various levels of percent canopy closure (from 30-100%).
(Hosmer and Lemeshow Goodness-of-Fit Statistic = 5.824 with 8 d.f.; p = 0.667)
Table 7. Aerial distances (km) traveled by radio-marked band-tailed pigeons between
nest sites and feed areas in which they were observed in the central Coast Range of
Oregon, 1993-95.
Nest Number
n
Mean
SD
Minimum
Maximum
nest
61
5.98
9.24
0.34
51.58
nest
36
3.84
4.04
0.29
17.76
nest
14
3.89
3.83
0.80
15.59
combined
111
5.02
7.43
3rd
50
Distances between successive nest attempts for radio-marked band-tailed pigeons
averaged 657 m (range
10 3712 m, SD = 754); no difference (> 0.05) was observed
between the 1st to 2nd and 2' to 3rd nest attempt distances (Table 8). Also, nest fate did
not seem to influence (> 0.05) the distance between their following nest from their
previous nest (successful nest:
= 555 m, SD = 610; failed nest:
5
= 824 m,
SD=951 m).
Breeding Home Ranges
Minimum convex polygons constructed to quantify 70 adult breeding range sizes
were based on sample sizes of 8 to 37 locations/bird. No relationship was detected
between number of locations/bird and polygon area, which indicated that sample sizes
were sufficient to reflect breeding movements. Polygon sizes were not different
(P> 0.05) between adult males and females; overall mean size was 11,121 ha
(range = 314
180,800 ha) (Table 9).
Philopatry
Seven birds were observed nesting for 2 or more consecutive summers: 6 birds for
2 years, and 1 bird for 3 years. All of these birds returned to nesting locations within
5.0 km of locations each nested at during the previous year; average distance between
nests for consecutive years was 2.1 km. Four of the 7 birds that returned, nested < 1 km
(range = 67 703 m) from their nest of the previous year. Although limited in sample
size, these data revealed that these birds exhibited philopatry to nesting areas.
51
Table 8. Aerial distances (m) between successive nests of radio-marked band-tailed
pigeons in the central Oregon Coast Range, 1993-95.
Nest Attempts
Mean
SD
Minimum
Maximum
and 2"' nests
44
604
759
10
3712
2 and 3rd nests
20
771
732
67
2382
combined
64
657
754
52
Table 9. Breeding home ranges of nesting radio-marked band-tailed pigeons based on
minimum convex polygons (ha), in the central Coast Range of Oregon, 1993-95.
Sex
n
Mean
SD
Minimum
Maximum
Female
21
11,666
12,367
538
50,000
Male
49
10,887
27,887
314
180,800
Total
70
11,121
24,298
53
DISCUSSION
REPRODUCTIVE BIOLOGY
Nesting Chronology
Long and extended breeding seasons have been documented for many Columbid
species around the world (Frith 1982, Goodwin 1983). One representative is the
mourning dove (Zenaida macroura) which has one of the longest breeding seasons of all
North American birds (Peters 1961). Nesting pairs of this species have been reported
breeding from March through October (Geissler et al. 1987). Also, Columbids in general
are mechanistic, breeding whenever food is plentiful. For example, common
woodpigeons (Columba palumbus) in England (Murton and Westwood 1977), eared
doves (Zenaida auriculata) in Brazil (Bucher 1982), and New Zealand pigeons
(Hemiphaga nocaeseelandiae) (Clout et al. 1995) have been shown to be highly dependant
on food for breeding and even shift their breeding season in accordance with changes in
the temporal pattern of food supply. Nest density of Columbids in Puerto Rico was
positively correlated with the seasonal changes in the abundance of fruits of key tree
species (Rivera-Milan 1996).
Band-tailed pigeons also exemplify these patterns. First, their breeding season is
lengthy; active nests of band-tailed pigeons have been reported in some portions of the
species' range throughout the entire year (Stephens 1913, Lamb 1926, Abbott 1927,
Vorhies 1928, Allen 1941, Neff 1947, MacGregor and Smith 1955). During this study,
the breeding season persisted 6 months, starting in late-April and going through early- to
mid-October. Secondly, availability of food seems to strongly influence reproduction of
54
band-tailed pigeons, especially in regard to timing of initiation of nesting (Jarvis and
Passmore 1992). For example, Jarvis and Passmore (1992) showed that pigeons
consumed a variety of foods during the early part of the nesting season when berries are
scarce. However, by mid-June, pigeons were consuming berries of Pacific red elder
(Sambucus racemosa) almost exclusively, and their diet for the remainder of the breeding
season was primarily berries [red and blue elder (s.. cerulea), and cascara (Rhamnus
purshianus)]. During this project, red elder was consistently very abundant by mid-June.
Similar observations were made by Jarvis and Passmore (1992). The relationship
between the availability of food and nest initiation seems valid when compared to the
peak in nest initiation (mid- to late-June).
Unlike most other areas outside the Pacific Northwest, the Oregon Coast Range
seems to provide an abundant, and stable/consistent food resource (berries) from year to
year, thus allowing band-tailed pigeons a consistently timed breeding season (as also
suggested by Jarvis and Passmore 1992). However, the annual changes in abundance of
mast production due to fluctuations in precipitation and/or temperature, and historical
changes in mast production due to changes in forestry management practices and the
effects they have (had) on band-tailed pigeon production are poorly understood.
Information regarding the relationships between fruiting/mast phenology of key species
and seasonal and annual changes in nesting chronology and nest density is needed to
better understand the dynamics of nesting Columbid populations in a variety of different
habitats (Rivera-Milan 1996).
55
The wide range of dates for first nesting attempts in the spring (Fig. 3) may also
reflect band-tailed pigeons' mechanistic behavior. In early spring, many pigeons readily
feed on corn silage available at livestock farms and cracked corn and seeds made
available at bird feeders. These food sources/feed sites are relatively dependable
throughout the spring and summer. Birds returning in the spring that locate these
dependable food sources respond by establishing a nesting territory and proceed to nest
early. Even though band-tails are highly mobile, the increase in energy and time
expended due to greater travel distances between feed areas and nesting territories may
limit or prevent birds from nesting early. For example, wood pigeons will begin
establishing territories in early spring even though food is not abundant. The pairs are
unable to spend much time in their territories because of the time-consuming
requirements of foraging. When food becomes abundant, the birds spend extensive time
in their territories and begin courtship and subsequent nesting (Murton 1965; Lofts and
Murton 1966; Lofts et al. 1966). In this study, a few of the radio-marked band-tailed
pigeons that fed on corn silage were recorded nesting as early as mid- to late-April, but
many others were not recorded nesting until they were observed feeding on berries (early-
to mid-June). However, the percentage of pigeons nesting early is still unclear. Birds
were difficult to locate due to weather conditions; fog and rain in the Coast Range made it
difficult to conduct aerial surveys in April and May.
Multiple Brooding and Nesting Interval
Multiple brooding seems relatively common for many of the Columbid species
that have been closely studied, e.g., the wood pigeon (Murton 1965), New Zealand pigeon
(Clout et al. 1995), white-crowned pigeon (Wiley and Wiley 1979), white-winged dove
(Cottam and Trefethen 1968), and the mourning dove (Swank 1955, Hanson and Kossack
1963). Earlier studies on band-tailed pigeons (March and Sadleir 1970, Gutierrez et al.
1975) indicated that they may attempt to nest 2 times in a season. When favorable
environmental conditions occur for extended periods, band-tailed pigeons may attempt 3
nests, e.g., MacGregor and Smith (1955) reported one pair raising 3 broods in one year.
Nevertheless, the production of 3 squabs or 3 complete nests in one season has generally
been considered unusual.
Multiple brooding during this study was predominant; 60% of the marked birds
initiated 2-3 nests, and 15% of all nesting birds during 1994-95 produced 3 fledgings.
This production of 3 squabs by breeding pairs may play a greater role in the recruitment
of band-tails than previously thought, especially for birds in the Northwest. With a 50day nesting cycle (defined as the period from nest building to fledging) (Zeigler 1971,
Braun 1994) and with an approximate 150-day nesting season (May through September),
band-tails would scarcely have enough time for 3 complete nesting cycles. However,
78% of the consecutive nests I observed by individual band-tailed pigeons were
overlapping. This overlap period ranged from 1-15 days, averaging about 7 days. All but
1 of the 8 birds that produced 3 squabs overlapped their nesting cycles. Most of these
birds initiated their first nest late-May and fledged their third squab in mid- to lateSeptember.
Overlapping nesting cycles have rarely been observed or reported in band-tailed
pigeons or, for that matter, any Columbids. Clout et al. (1988) observed one pair of New
57
Zealand pigeons that had overlapping clutches. Mourning doves have been known to
produce a second clutch in the same nest before their first nestlings have fledged (Sayre
and Silvy 1993). There is also circumstantial evidence for overlap in wild wood pigeons
and stock doves (Columba oenas) (Murton and Isaacson 1962). During this study, it was
difficult to determine precisely the fledging date, thus making it difficult to determine the
number of overlapping days between fledging and initiation of a new nest. However, I
am confident that when using a 45-day nesting cycle (instead of a 50-day), my nesting
overlap estimates are accurate or even conservative. In light of this, I may have underestimated the percentage of nesting band-tailed pigeons that produced 3 young per season,
due to missed early nesting attempts and the many incomplete nesting records of my
radio-marked band-tailed pigeons.
Another method for minimizing the time required for multiple nesting attempts is
by shortening the nesting cycle by re-using old nests, thus, eliminating the time required
for nest building. Nest construction for Columbids may take 2-4 days (Cowan 1952,
Wiley and Wiley 1979). Mourning doves have been known to re-use nests for 25-40% of
their nesting attempts (McClure 1950, Harris et al. 1963, Yahner 1983). However, this
behavior has not been commonly observed in pigeons. For example, both Gosper (1996)
and MacGregor and Smith (1955) observed one nest being re-used by crested pigeons
(Ocyphaps lophotes) and band-tailed pigeons, respectively. In each case, the nest was
given only a minor refurbishment before re-use. Only one incident of nest re-use by
band-tailed pigeons was observed during the 3 years of this study. Therefore, nest re-use
probably contributes little in minimizing the required time for multiple nesting attempts.
Nesting Rate
A high proportion (70%) of the radio-marked birds that were followed nested.
However, nesting propensity may have been under-estimated. Failure to detect nesting by
some birds may have occurred because of 1) transmitter failure and intermittent signals,
2) the difficulty of locating females on nests, because they are associated with nests for
only a small percentage of the daylight hours (early-morning and late-evening), and 3)
because they are highly mobile, traveling beyond the study area. However, 88% of those
birds that were not found nesting and that were followed with some consistency were
usually observed exhibiting typical breeding behaviors as if they were nesting (e.g.,
coming and going from feed areas at appropriate times for nesting males and females,
females observed leaving nesting cover at times of nest exchange). If these birds were
indeed nesting, then 96% of all radio-marked adult birds (ASY=after second year,
1+ years old) that remained in the study area reproduced during 1993-95.
Nesting rates seemed to vary little between years (range = 9 1-100%). Likewise,
Jarvis and Passmore (1992) concluded that recruitment of band-tailed pigeons in Oregon,
based on age ratios of harvested birds, varied little annually. However, in other areas, and
other species of pigeons, nesting has been described as highly variable. Clout et al.
(1995) reported that New Zealand pigeons had highly variable nesting rates, and in a year
when fruit production was extremely poor, the birds did not breed. Gutierrez et al. (1975)
discussed how band-tailed pigeons responded to an abundant acorn crop in southwestern
New Mexico by breeding in the fall. The consistent nesting effort and output observed in
59
western Oregon may reflect the consistency of food production in the central Oregon
Coast Range.
Although few in number (7), all of my radio-marked SY birds (SY=Pt calendar
year after hatching) did not nest. One bird used several feed areas throughout the
breeding season, always with a large number of birds. The remaining SY birds were
highly mobile, never settling in a given locale (i.e., a nesting territory) before moving out
of the study area. Jarvis and Passmore (1992) concluded that SY band-tailed pigeons
were about one-third as productive as ASY adults (based on a sparse set of active crop
gland data). Histological examination of the testes by Gutierrez et al. (1975) indicated
that SY band-tailed pigeons were capable of breeding, but they had no evidence on
nesting by SY birds in the wild. Age and breeding experience have been shown to affect
the production of young in ringed turtle-doves and captive mourning doves (Lehrman and
Wortis 1967, Armbruster 1983). I conclude that SY birds rarely nest and contribute little
to the production of band-tailed pigeons in Oregon.
Nesting Success
Researcher disturbance (i.e., nest visitation) has been shown to negatively affect
nesting success in mourning doves (Westmoreland and Best 1985). Soutiere and Bolen
(1976) and Howe and Flake (1989) observed that nest abandonment was the cause of 18%
and 14% of their nest failures, respectively. They indicated that at least half of these
abandonments/losses could be attributed to the activities of the investigators (especially to
initial disturbance by the investigators). Westmoreland and Best (1985) suggested that
the negative effects on nest success caused by investigators may be greater in areas where
avian predation is dominant, because birds rely heavily on sight to locate prey (i.e.,
exposed eggs when the adult flushes). In this study, it was difficult to determine cause of
nest failure, whether from abandonment or predation, especially for nests when band-
tailed pigeons were first found and flushed from their nests. For example, some of these
disturbed nests were unattended and empty upon the next visit; was the egg stolen from
an active nest or scavenged after abandonment? If researcher-induced nest failures
occurred, then the reported nesting success rates may be conservative.
The bulk of nest loss occurred during the incubation stages of the nesting cycle.
Daily nest survival probabilities were lower during the incubation stage (Table 2). Other
researchers have shown highest nest losses during the incubation period (Harris et al.
1963, Caidwell 1964, Schroeder 1970, Best and Staufer 1980). This may occur because
adults brooding young would be less likely to abandon the nest after a significant amount
of investment, i.e., the loss of an egg represents less wasted parental investment than the
loss of young.
Nest success rates reported from nesting studies of Columbids have varied greatly
and exhibit both annual and geographic variation: mourning doves 0.25 (Westmoreland
and Best 1985) to 0.75 (Howe and Flake 1989); Zenaida doves (Zenaida aurita) 0.26
(Nellis et al. 1984) to 0.49 (Rivera-Milan 1996); white-crowned pigeons 0.26 to 0.68
(Wiley and Wiley 1979); New Zealand pigeons 0.12 to 0.40 (James and Clout 1996).
However, previous reports on band-tailed pigeon nesting success have been relatively
consistent and high (although determined with small sample sizes) at 0.65 (MacGregor
and Smith 1955) to 0.73 (Glover 1953). In this study, the overall nesting success rate was
61
similar (0.69), which may indicate that there is little variation in nesting success among
geographic locations. Also, within this study, band-tailed pigeons exhibited minimal
seasonal variation (0.68 to 0.70). Together, these factors probably lead to a consistent
yearly recruitment rate (unlike most of the other Columbids). Jarvis and Passmore (1992)
also indicated that they felt that band-tailed pigeon recruitment within the central Oregon
Coast Range varied little from year to year. As with nesting rates and nesting chronology,
food availability may play some role in nesting success rates (e.g., poor mast production
may lead to poor nest success). Thus, the lack of variation observed suggests the need for
further examination of band-tailed pigeon nesting success in different habitats (e.g.,
geographic areas) and under varying conditions (e.g., during a year with poor mast
production).
There was no discernable effect of initiation date on nesting success. Harris et al.
(1963), Caidwell (1964), and Olson and Braun (1984) noted greater success in latter
months of mourning dove breeding seasons. Lowered nest success in the early season has
been attributed to weather conditions and a greater likelihood of nest desertion. In 1993,
we observed a higher percentage of nest desertion early in the season, when compared to
other years. Weather-related factors may have influenced nest success differently in
1993, but weather conditions were not recorded. In turn, Murton (1958) reported that
wood pigeons exhibited poorer success early in the season when food was less abundant
and less reliable. Food may have also influenced nest success differently in 1993, but it
was not quantitatively or qualitatively measured. Murton (1958), and Wiley and Wiley
(1979) observed an increase in nest predation as the nesting season progressed for wood
62
pigeons and white-crowned pigeons, respectively. They felt that predators started preying
on pigeon nests after an initial lag until nest numbers increased, and then became
accustomed to taking pigeon eggs. However, this relationship may be less important for
band-tailed pigeons because wood and white-crowned pigeons exhibit nest "clumping"
(similar to nesting colonies) which may help predators key on nests. Band-tailed pigeons
have only rarely been observed nesting in colonies (Neff 1947).
Several studies (McClure 1950, Schroeder 1970) have suggested that the autumn
hunting seasons have reduced mourning dove nesting success and discouraged further
nesting attempts. Zeigler (1971) estimated (based on active crop glands of harvested
birds) a 4-7% loss of productivity by band-tailed pigeons due to hunting (during that time
the hunting season started September 1). However, Geissler et al. (1987) concluded that
hunting under the current regulations had no substantial effect on mourning dove nest
success, and the reductions in nesting activity in September was a natural phenomenon
and not caused by hunting disturbance. During this study, 21% of all band-tailed pigeon
young fledged on or after September 15 (the current starting date of Oregon's band-tailed
pigeon hunting season); only 5% fledged in October. One nest fatality observed may
have been related to the hunting season: an approximately 3-day-old squab was
abandoned on the nest shortly after September 15. Zeigler (1971) showed that bandtailed pigeon squabs (in captivity) could not survive under the care of one parent if they
were
6 days old when the second parent was removed. However, fledging success
greatly increased when the young were older than 9 days before the parent was removed.
Similar results were shown for mourning doves (Haas 1980). Hunting under current
63
regulations probably has little effect on band-tailed pigeon late nesting activity and nest
success.
Nesting success in relation to habitat characteristics. --Site canopy closure and
site aspect yielded significant logistic regression models explaining success of nests by
band-tailed pigeons. The negative relationship between nest success and site canopy
closure was unexpected. Many species appear to select nest sites that provide better
concealment, and the risk of nest predation often decreases with increasing concealment
(Alonso et al. 1991, Tuomenpuro 1991, Martin 1992). For example, thick, dense stands
could reduce the ability of Accipiter hawks to search for prey. However, nest site choice
may often be a trade-off between the need for concealment and the need for individuals to
maintain some view of the surroundings of the nest (Gotmark et al. 1995). More open
nest sites may provide easier detection of nest predators, thus allowing the adult and/or
young to remain silent and delay any movements, and provide adults with avenues for
escape. Nesting band-tailed pigeons would remain motionless but always watching the
approaching observer. When the adult flushed, it would often perform distraction
displays along what appeared to be an intended route of escape. Site canopy closure may
not be an adequate or an appropriate measure of nest cover, but it does give some idea of
nest site concealment. The majority of band-tailed pigeons (84%) nested in stands with
high canopy closure. Perhaps the relationship between site canopy closure and nesting
success in this study is spurious due to the very low number of nests (16%) in the open
canopy classes.
A higher percentage of band-tailed pigeons nested on slopes facing southwest, and
they were more likely to succeed than those birds nesting on northeast facing slopes. The
reason for this site aspect influence on nest success is unclear (e.g., is it due to
microclimate differences or differences in inclement weather effects?). Temperatures on
south-facing slopes may tend to be higher and warm up more quickly in the morning
hours. Naslund et al. (1995) suggested that marbled murrelets (Brachyramphus
marmoratus) in Alaska choose nest stands with directional aspects that minimized
exposure to prevailing weather systems. Positioning nests to maximize heat retention (in
cold weather) or loss (in hot weather), or to minimize exposure to predators has been
documented for other species, and may vary with nesting experience (Marzluff 1988,
Zann and Rossetto 1991), but I draw no conclusion regarding band-tailed pigeon nest
aspect strategy.
Adult Survival
The only survival data currently available for adult band-tailed pigeons are annual
estimates derived from band returns; these estimates range from 0.57 - 0.73 (Smith 1968,
Wight et al. 1967, Kautz and Braun 1981, Jarvis and Passmore 1992, Schroeder and
Braun 1993). Although annual survival estimates are useful in assessing population
trends, period survival estimates are needed to evaluate the seasonal distribution of
mortality so that biologists can develop appropriate and effective management
prescriptions (Schulz et al. 1996).
Vulnerability of birds during their breeding season has been documented for many
species, and these losses of breeding adults can have detrimental effects on the
65
population. Very little is known of adult survival during the breeding season for
Columbids. Schulz et al. (1996) are some of the first investigators to estimate adult
mourning dove survival during the breeding period, April-August. Their estimate for this
period (0.7 16; 95% CI = 0.651 0.782) was relatively high when compared to estimates
of annual survival of adult mourning doves (0.39 1 0.527) (Hayne 1975, Dunks et al.
1982, Tomlinson et al. 1988). Thus, Schulz et al. (1996) suggested that spring/summer is
probably not a period of high mortality for adult mourning doves. Band-tailed pigeons
exhibit similar characteristics. Based on comparisons with estimates of annual adult
band-tailed pigeon survival in the central Oregon Coast Range during 1965-73, which
ranged from 0.445 (95% CI = 0.252 - 0.638) to 0.853 (95% CI = 0.6 19 - 1.000) (Jarvis
and Passmore 1992), the breeding period survival estimate during this study, 0.963 (95%
CI = 0.922 1.000), seems to indicate that the breeding season (mid-April through mid-
September) is a period of low adult mortality. Adult mortality during the breeding season
is not likely a major factor contributing to band-tailed pigeon population declines.
However, the breeding season survival estimates are derived from only a small
percentage of the breeding range and, therefore, cannot be broadly extrapolated to the
entire range. Additional band-tailed pigeon survival data from other areas (e.g., different
parts of the breeding range) and other seasons (e.g., migration and wintering) are needed
to adequately evaluate population dynamics.
HABITAT ASSOCIATIONS
The decline of band-tailed pigeons has led to speculation about the role of habitat
condition, i.e., what effect could the loss and/or degradation of prime breeding habitat
have on reproduction and survival? Sharp (1992) stated (without evidence) that bandtailed pigeons prefer and/or require old-growth/mature forest stands for breeding. Thus, it
is reasonable to suggest that the loss and reduction of old-growth acreage may be one of
the primary reasons for the decline of band-tailed pigeons in the Northwest. However,
band-tailed pigeons and many other Columbids have exhibited a remarkable ability to use
a wide range of available habitats within their breeding range. This noteworthy behavior
may possibly stem from how they select habitat; band-tailed pigeons may select forested
habitat based more on structural components than on species composition for nesting.
Nest and Nest-Tree Characteristics
Band-tailed pigeons, and Columbids in general, are known for their seeming lack
of investment in nest building. Nests are typically tenuous platforms of dead twigs of
loose construction (also refer to Neff 1947, MacGregor and Smith 1955, Curtis and Braun
1983). I observed that the amount of small twigs per nest varied greatly; in some nests,
the egg was readily visible from below because of so few twigs, while others were
substantial structures utilizing many twigs and other organic material (e.g., moss and fir
needles). Organic material, such as moss and lichens, is considered to provide an
important microclimate for marbled murrelet eggs and nestlings by providing insulation
against inclement weather and allowing only minimal accumulation of standing water
(Naslund et al. 1995). Schroeder (1970) found that mourning dove nests placed in
crotches of trees were subject to rain-soaking. Although moss and lichens were not a
common feature of most band-tailed pigeon nests, they appeared more often when nests
were placed near the main stem of the nest tree. The utilization of these secondary
67
materials probably provide additional support andlor concealment against predators
(Coon et al. 1981, Best and Stauffer 1980, Rivera-Milan 1996) rather than being utilized
for microclimate advantages. Rarely were feathers observed (occasionally one or two
breast feathers were noted) in nests. Columbids provide constant incubation, unlike most
birds, which take respites from incubation, usually to forage (Drent 1975). Constant
incubation probably eliminated the need for vascularized incubation patches in
Columbids (Westmoreland 1986) and the need for the nest to contain a significant
amount of down. As the squabs matured, their nests became encrusted with droppings
(also noted by Curtis and Braun 1983). This feature of accumulated droppings could be a
useful sign when identifying the fate of band-tailed pigeon nests.
Although band-tailed pigeon nests are less substantial than those of most other
birds and seem more vulnerable to destruction by weather and other disturbances, the
band-tailed pigeon enjoys an approximate 70% success rate. Their modest nests take
relatively little time to construct and may be one of the time-conserving adaptations of
pigeons (and doves) related to the reproductive strategy of multiple brooding
(Westmoreland et al. 1986).
Band-tailed pigeons have been shown to nest in a variety of both coniferous and
deciduous tree/shrub species throughout their breeding range. Conifers seem to be most
frequently used (Neff 1947, Glover 1953, MacGregor and Smith 1955, Peeters 1962,
Curtis and Braun 1983); band-tails were observed nesting 83% of the time in conifers
during this study. Nest tree selection by band-tailed pigeons may only be a reflection of
availability. Conifers (primarily Douglas-fir and some western hemlock) are the
dominant tree species in the central Oregon Coast Range (Franklin and Dyrness 1988).
However, further south in their breeding range, band-tails utilized a much higher
percentage of deciduous species (MacGregor and Smith 1955). Mourning doves have
been observed nesting more frequently in coniferous or other evergreen species during
early spring before leaves emerged on deciduous species (Moore and Pearson 1941). A
seasonal shift between coniferous and deciduous species by band-tailed pigeons was not
observed. This is likely because leaves of most deciduous species are fully emerged by
the time band-tailed pigeons are nesting.
Nest and nest tree/shrub height, location in the tree/shrub (e.g., nest distance from
main-stem, percent of crown above nest) and nest tree DBH have varied greatly within
the different nesting investigations of band-tailed pigeons. MacGregor and Smith (1955)
reported an average nest height (akin to this study) of 11 m with a range of 3.7 29.0 m.
Peeters (1962) and Curtis and Braun (1983) described similar variability of nest heights
ranging from 2.1
12.2 m above the ground. Nest distance from main stem varied
greatly; nests were up against the trunk/main-stem to over 7 m away (see also Peeters
1962). These nest and nest tree characteristics measured were not related to nesting
success of band-tailed pigeons. Yahner (1983) found no relationship between mourning
dove nesting success and nest-site variables similar to those measured in this study.
However, an important variable, density of cover (above) provided by the nest
trees/shrubs, was difficult to quantify and record for band-tailed pigeons. This overhead
cover has also been considered an important factor influencing selection of nest sites for
mourning doves (Harris et al. 1963). The morphology of each nesting tree was different,
but most nests were placed in the tree/shrub with the majority of canopy above the nest
(see also Peeters 1962). Thus, the height of nests and their placement in the nest tree may
be primarily a function of the type of vegetation (conifer or deciduous) that provides
adequate concealment (Murton
1965).
A significant proportion (independent of nesting
success) of the nests discovered in this study were positioned south or southwest of the
main stem of nesting substrata. It's difficult to determine if this is a concealment driven
response (e.g., foliage) or a microclimate driven response (e.g., protection against
inclement weather). However, Harris et al.
(1963)
and Yahner (1983) found similar
results for mourning doves; a significant proportion of nests were positioned south and
east of the main stems. They suggested that the birds may have been responding to the
summer storms in southern Minnesota, which usually have northerly or westerly winds.
Summer storms in the Oregon Coast Range are accompanied by south winds (cold
fronts). Although, on hot days (high pressure over the Willamette Valley), marine air
(cool) moves inland to replace the heated air that rises. This movement occurs in the
afternoon. The south or southwest aspects of nests would not provide protection from
cool, rainy weather, in fact it would increase exposure. However, on hot days an
exposure to the southwest may reduce afternoon heating because of the movement of
marine air late in the day.
Neff and Niedrach
(1946)
and Peeters (1962) reported that trees utilized for
nesting by band-tailed pigeons tended to be adjacent to openings/clearings (possibly
facilitating access to the nests) and above a small precipice or steep slope (providing
additional height when flying from the nests). These characteristics were noted for many
70
nest trees but were not observed with any consistency during this study. For example,
band-tailed pigeons were also observed nesting in areas with no acclivity (Fig. 11) and in
young, dense, even-aged stands with no gaps or clearings. Further comparison of nest
tree characteristics (e.g., age, height, species, canopy depth) and physiographic features of
nests and nest trees (e.g., aspect, exposure, concealment) from different geographic
locations may provide additional insights into band-tailed pigeon habitat requirements.
Nest Site Characteristics
It is difficult to characterize the important features of nest trees/shrubs selected by
band-tailed pigeons because of their diversity in choice. Even more challenging is the
identification and description of the essential qualities of their nesting stands or areas.
Many Columbids are considered "habitat generalists" when classifying nesting habitats;
different species can nest in a variety of forest conditions ranging from open forest edge
to forest interior. For example, Rivera-Milan (1996) described the Zenaida dove as a
habitat generalist because it could change its nesting preferences throughout the year in an
opportunistic manner; additionally, it was observed nesting in all available island habitats
(Burger et al. 1989). Nesting white-crowned pigeons exhibited considerable variation in
selecting forested habitats within different study areas as well as between major study
sites (Wiley and Wiley 1979). Wood pigeons have been observed nesting in habitats
ranging from thickets and hedgerows, to mature coniferous and deciduous woodlands
(Murton 1965, Saari 1984). Nesting band-tailed pigeons can also be characterized as
"forest habitat generalists." Nests have been found in a variety of areas with different
dominant tree species: oak (Ouercus spp.), pine (Pinus spp.), fir (Abies spp.), Douglas-fir,
71
redwood (Sequoia sempervirens), cedar (Thuja spp.), hemlock (Tsuga spp.), and alder
(Alnus spp.) (Neff 1947, Glover 1953, Peeters 1962, Jeffrey 1989). I also observed bandtails nesting in a variety of forest stand seral conditions (open and closed, sapling-pole to
old-growth) and different community types (deciduous to conifer-dominated
communities). Just about every type of forest community and stand condition, as
categorized by Brown (1985), were utilized for nesting, except the mountain shrubland
community (shrub class seral stage), most likely due to the fact that these shrub areas
provide minimal structure with little over-head cover or concealment (shrub classes are
usually <5 m tall). A majority of the nests found during this study were in the conifer,
closed sapling-pole forest stand classification (55%, Fig. 7). This habitat type is quite
common and may currently be the dominant forest type in the central Coast Range of
Oregon. Although the results do not represent "preference" of forest habitat types
(because spatial distribution and quantities of available habitat were not measured), bandtailed pigeons may be using the forested habitats in proportion to availability.
Glover (1953) reported that band-tailed pigeons utilized nest sites that were very
near a permanent water source. They suggested the combination of both dense cover and
proximity of available water as nesting habitat requirements. Band-tailed pigeons
exhibited a wide range of nest site distances in relation to permanent water
range = 11
(.
= 316 m,
2000 m) during this study. Their high mobility makes it seem unlikely they
would need a permanent water supply near their nests, unless this physical feature serves
some other purpose besides providing water for consumption. Creeks or streams are
common components in nesting habitat of marbled murrelets but primarily serve as
72
important flight corridors to nest sites (Hamer and Nelson 1995, Naslund et al. 1995).
Most locations in the central Oregon Coast Range are near (<1.5 km) permanent water,
anyway. The proximity of water may become more critical when selecting nest sites by
band-tailed pigeons in more southern or drier regions of their breeding range.
The loss and/or degradation of nesting habitat does not appear to be the central
factor in the decline of band-tailed pigeons. Because, in this study, band-tailed pigeons
exhibited high nesting rates, high nesting success rates, and used a wide range of forested
habitat types. Nesting habitat does not seem to be a limiting factor in the central Coast
Range of Oregon. Band-tailed pigeons may have preferences for some forest stand types
(e.g., closed-canopy sapling-pole) but seem to be highly adaptable and opportunistic in
relation to nesting habitat.
Feed Areas
The diet of band-tailed pigeons has been shown to vary seasonally and with
locale. In early spring, pigeons will utilize new buds and flowers of a variety of
deciduous trees and shrubs, most notably oak (Ouercus spp.), elder, cascara, madrone
(Arbutus spp.), and cherry (Prunus spp.), but as the fruit of these species develop and
mature they become the primary foods consumed during the breeding season (Neff 1947,
Glover 1953, Peeters 1962, Smith 1968, Jarvis and Passmore 1992). In the central Coast
Range of Oregon, band-tailed pigeons consume predominately Pacific red elder berries
from late-June to mid-August, but in late-August through September they feed
exclusively on cascara buckthorn and blue elder berries (Jarvis and Passmore 1992). The
abundance of these mast- and berry-producing species in shrub stand communities are of
73
significant importance for the production of band-tailed pigeons. The challenge,
however, has been understanding the extent and characteristics of these shrub
communities and the factors affecting the local distribution of these important fruitproducing species.
The feed areas discovered in this study were quite diverse in both their physical
and vegetative characteristics. Red elder and cascara buckthorn were the principal food
components within most of the feed areas, and the majority (64%) of these sites were
associated with riparian areas. The size of these feed areas were also highly variable and
difficult to quantify and characterize. Most areas did not have definitive boundaries
because fruit-bearing shrubs were usually scattered in thick, dense patches throughout
different forest community types and/or stand conditions. Additionally, some of these
feed sites would extend over large areas, e.g., an entire drainage or mountain side, thus
making it difficult to determine how much of the area the birds were actually utilizing.
Quantifying use by band-tailed pigeons of discovered sites was also challenging and
complex because the number of birds, at any given day, greatly fluctuated. These
fluctuations were most likely related to the phenology of mast production, which was not
monitored at individual sites, though other causes (e.g., predator harassment) may also
contribute.
Pacific red elder is found in coastal and sub-coastal areas from south-central
Alaska to the vicinity of San Francisco Bay, California (Kozloff 1976, Hitchcock et al.
1977, Ross and Chambers 1988). In Oregon, this species is well established in the Coast
Range and the Willamette Valley, occurring in open moist meadows, stream sides,
74
disturbed sites and roadsides, especially following fire or logging (Franklin and Dyrness
1988). Feeding band-tailed pigeons were mostly observed in areas where Pacific red
elder thrived, i.e., open canopied sites or areas with sparse over-stories. Small individual
clumps (in gaps or along road sides) to large dense patches (in clear-cuts or along
streams) of Pacific red elder with enormous amounts of berries were quite common and
readily observed in the Coast Range. Interestingly, I observed many similar sites that
appeared to receive little or no use by band-tailed pigeons.
Blue elder has a very similar range to that of red elder, but is less coastally
influenced and can be found frequently growing in inland valleys and hills (Kozloff 1976,
Hitchcock et al. 1977, Ross and Chambers 1988). This species is also very abundant in
the Coast Range of western Oregon, occurring on open slopes and valley bottoms and in
brush thickets and recovery forests, accompanied by species that grow in the open or after
disturbance. Blue elder and Pacific red elder occur in associations with many of the same
species. However, I did not observe or locate many band-tailed pigeons feeding on blue
elder. Foods within late summer (after mid-August) feed areas consisted primarily of
cascara.
Cascara buckthorn can be found from southern British Columbia south through
the western portions of Washington, Oregon and California (Starker and Wilcox 1931,
Kozloff 1976, Hitchcock et al. 1977, Ross and Chambers 1988). This species thrives in
areas with deep, rich, sandy or humus soils that are typically found in low river bottoms,
flats, valleys, and borders of streams. Cascara is generally found as scattered individuals,
but small patches and thick stands do occur. More often it is noted along old fence rows
75
and in brushy places where there is considerable side shade, but full overhead light
(Starker and Wilcox 1931). No regularity was observed as to its occurrence in the Coast
Range; some drainage basins had considerable quantity while adjoining areas of similar
characteristics had little or no cascara. Since the turn of the century, cascara has been
highly sought and harvested for medicinal purposes, e.g., approximately 200 products, in
Canada alone, contain cascara (Prescott-Allen and Prescott-Allen 1986). Starker and
Wilcox (1931) reported that at least an average of 2,500 tons of dry cascara bark were
harvested annually from western Oregon, Washington, and northern California between
1919 to 1930, the majority coming from western Oregon. They suggested that the supply
of cascara was being rapidly depleted and would soon be exhausted if the current harvest
rates continued. During that time much of the original cascara acreage was cleared for
agricultural purposes, thus lessening the possibility of re-growth. Rarely were large
individuals or patches of cascara trees discovered by chance during this study. However,
radio-marked birds were followed to previously unknown cascara sites that usually
contained substantial amounts of fruiting cascara. At these individual sites, the age
classes of cascara were usually diverse (from small seedlings/shrubs to large trees,
>25 cm DBH) and typically being used by many other band-tailed pigeons. Currently,
little is known about the amount of cascara in the Coast Range, how that amount and
distribution compares to historical records, and if there are detrimental affects caused by
cascara harvest on band-tailed pigeon production.
Band-tailed pigeons were readily attracted to bait and/or corn silage in the spring,
but after mid-June they were virtually uninterested in these foods. Due to their seasonal
disdain of grain baits and their extensive consumption of a few types of berries during
summer, Jarvis and Passmore (1992) suggested that this behavior indicated that food is
scarce in spring but abundant from mid-June through September. Food (particularly
Pacific red and blue elder and cascara buckthorn) appears plentiful in the central Coast
Range of Oregon and does not seem to be a limiting factor in the production of band-
tailed pigeons. However, a better understanding of factors affecting the local
distributions of cascara buckthorn and Pacific red elder are needed. These plant species
seem critical to the reproduction of band-tailed pigeons (possibly many other wildlife
species as well), they occur on sites subjected to intensive forestry, and they may be
sensitive to some silvicultural practices.
MOVEMENTS
Band-tailed pigeons have always been considered a highly mobile species during
the breeding season (Neff 1947, Braun 1972, Jarvis and Passmore 1992, Schroeder and
Braun 1993). In support of this, Curtis and Braun (1983) captured 3 band-tailed pigeons
in Colorado, 7.1
14.0 km away from their nests, feeding on bait (birds were captured
and marked with transmitters). Band-tailed pigeons during this study also exhibited high
mobility throughout the nesting season, making lengthy daily flights ( = 5.02 km) to
feed areas. Some of these birds were observed making extremely long flights (between
20 50 km) from nest sites to feed areas. As the season progressed, however, travel
distances to feed areas tended to decrease, especially after their first nesting attempt
(Table 7). Some of these longer travel distances were in early spring before Pacific red
elder was readily available in the Coast Range; some birds would fly great distances back
77
to the Willamette Valley to feed on waste grain/corn silage. Jarvis and Passmore (1992)
suggested that until Pacific red elder berries became readily available in mid-June, bandtailed pigeons were traveling considerably more in search of other available foods.
Also, white-crowned pigeons exhibited similar daily movement distances in Puerto Rico,
ranging from 1 to 8 km, along with seasonal changes in flight distances (Wiley 1979).
Wiley (1979) suggested that these movement patterns were probably a response to food
supply. Change in food supply (berries becoming plentiful) is most likely the primary
reason for the decrease in feeding travel distances observed during this study.
Many instances were noticed where band-tailed pigeons did not necessarily travel
to the nearest food source; large patches of red elder were observed much closer to nests
than some utilized feed areas. Typically, when birds were observed at feed areas, as well
as mineral sites, many other band-tailed pigeons were also noted, along with much cooing
and other intraspecific interactions. These gatherings of birds at feed and mineral areas
might have provided some additional social benefits or functions. Ward and Zahavi
(1973) proposed that assemblies of birds (e.g., roosting or breeding) may serve principally
as information centers, wherein knowledge of the location of food or of good feeding
sites may be obtained by individuals lacking such knowledge. Bucher (1992) suggested
that the movements of passenger pigeons (Ectopistes migratorius) to and from nesting
colonies and roost sites certainly must have helped solitary nesting pigeons detect food
sources simply by "local enhancement," if not by information transfer at the central place.
Thus, the assembly of band-tailed pigeons at feed areas and mineral sites may provide
opportunities for information transfer as well as other benefits, such as increased survival
due to enhanced predator detection, or dilution of predation risk (Munro and Bedard
1977, Nudds 1980), and provide prenuptial opportunities/activities for unpaired birds
(Peeters 1962). In addition, high quality feed areas and mineral sites might simply attract
more birds.
Although band-tailed pigeons were not often observed at mineral sites, this does
not diminish the importance of mineral deposits for breeding band-tailed pigeons. Jarvis
and Passmore (1992) stated that mineral deposits seem to be a key resource for bandtailed pigeons during the breeding season, and even suggested that these deposits may be
more important than feed or nesting sites in determining distribution of birds during the
nesting season. Known mineral sites traditionally used by band-tailed pigeons in western
Oregon (<80) are considered relatively scarce (Jarvis and Passmore 1992, T.A. Sanders,
unpubl. data). During this study, all records of marked birds utilizing mineral deposits
were at the nearest known site to the bird's nest (range = 3.5 - 14.0 km away). A bird 14
km away from its nest consuming mineral may suggest that mineral deposits are relatively
scarce, or the birds prefer to travel greater distances to take advantage of some group
benefits, as previously discussed. However, one mineral site utilized by a marked bird
was a previously unknown site; many band-tailed pigeons were observed at this site
consuming remnant mineral left in the soil from livestock minerallsalt blocks. On two
other occasions, a small group of unmarked band-tailed pigeons (only 2-3 in number)
were observed in two different areas (near a lake shore and the coastal shoreline)
exhibiting behaviors similar to those at known mineral deposits. In both instances, the
birds made their way to the ground and pecked the substrate and had departed all within
79
30-45 minutes. These accounts reveal the difficulty of locating, observing, and
identifying the birds behavior at different areas when only a small proportion of time is
spent at a particular site. Also, many other unknown mineral deposits may exist which
band-tailed pigeons frequent. In conclusion, the distribution of known mineral sites had
no effect on nest success rates (Table 5) and the lack of difference between distances
traveled and mineral deposit distribution/occurrence strongly suggests that mineral
deposits have little influence on band-tailed pigeon nest distribution and nesting success
in the central Oregon Coast Range.
The breeding home ranges of band-tailed pigeons observed in this study seem
quite large; overall mean size was 11,121 ha. Some could theorize that important
resources (e.g., quality nesting and feeding areas) are scarce or widely spaced and thus
require birds to travel great distances and/or forage over large areas. For example, habitat
quality has been shown to influence bird movement; because of poor habitat quality, wild
turkeys (Meleagris gallopavo) in northwest Alabama reportedly exhibited greater
movements than turkeys in the Alabama Piedmont (Everett et al. 1979). However,
habitats utilized by band-tailed pigeons seemed widely distributed and readily available,
preferred foods appeared plentiful and nesting success rates were high. The assumed
shape of the home-range may greatly affect calculations of size. Spherical shapes may be
much larger than the area actually used, because band-tailed pigeons may have disjunct
core-areas of use with narrow pathways connecting them. Losito and Mirarchi (1991) and
Sayre et al. (1980) reported highly variable home-range sizes of juvenile (50 6,150 ha)
and adult mourning doves (342 - 1,123 ha). Mourning doves will make a few flights of
considerable distance to satisfy their daily needs but appear to remain reasonably close to
their chosen nesting areas throughout the breeding season (Sayre et al. 1993). Thus,
home-range size may reflect habitat quality or may simply be an artifact of the fact that
band-tailed pigeons prefer to make some long distance flights during times when
resources are widely scattered (e.g., early spring) and/or are possibly seeking some
unknown intraspecific interactions. Changes in home-range sizes over the nesting season
were not assessed because of limited locations/observations of marked birds.
During this study, band-tailed pigeons exhibited strong site fidelity to nesting
areas, i.e., during both the breeding season and from year to year. Earlier banding studies
have shown similar evidence of fidelity to the same general breeding areas (primarily feed
areas and mineral deposits) from year to year (e.g., Jarvis and Passmore 1992, Schroeder
and Braun 1993). Even though band-tailed pigeons are highly mobile throughout the
year, they nested relatively close to their previous nest ( = 657 m). Birds that were
followed over several years nested within 5.0 km of a previous year's nest (57 % nested
within 703 m), and they were also observed in feed areas used the previous year. Other
Columbids have exhibited similar homing tendencies. For example, New Zealand
pigeons have been shown to use approximately the same home ranges for 2 years, nesting
and feeding in the same general areas (Clout et al. 1991). Strong and Bancroft (1994)
retrapped adult white-crowned pigeons banded as nestlings on their natal keys, in Florida
Bay; thus, the postfledging dispersal period may allow birds to familiarize themselves
with feeding sites that can be used in subsequent nesting seasons (see also Brewer &
Harrison 1975, Morton et al. 1991). Little is known about the postfledging dispersal
patterns of band-tailed pigeons and their homing rates to natal areas. It appeared during
this study that some parental contact was maintained following initial fledging.
so,
then adult birds may be leading young to feeding sites and establishing traditional
breeding areas (Strong and Bancroft 1994). Also, little knowledge exists about the effects
of forestry practices, such as clear-cutting a nesting stand, on seasonal home-range
patterns of band-tailed pigeons. A loss of a nesting stand or significant feed area could
cause seasonal and/or yearly repercussions on production for individual birds.
MANAGEMENT AND RESEARCH IMPLICATIONS
Despite the recent amount of interest and available information on band-tailed
pigeons, many management questions remain unanswered. The primary need is for the
understanding of causes for the population trend decline experienced in the Northwest
(Jarvis and Passmore 1992, Braun 1994). Without first filling several other significant
gaps in our knowledge of band-tailed pigeon biology (particularly adult and juvenile
survival), the reasons for decline will remain conjectural and future management efforts
cannot be fully effective. However, results from this study (e.g., high nesting rates and
success, and diverse use of nesting habitats) do suggest that protection and proper
management of breeding habitat (nesting and feeding areas, and mineral sites in western
Oregon) by itself will not ensure the adequate protection needed (i.e., reversal of
population trends) for the species.
Earlier investigators recommended radio-telemetry as a potential means for
investigating band-tailed pigeon breeding ecology and population parameters (Jarvis and
Passmore 1992). Telemetry would be a useful tool and may be the only practical way of
estimating seasonal survival rates (e.g., migration, hunting season, wintering) (Schulz et.
al. 1996). However, telemetry equipment and methodology for tracking large numbers of
birds that use extensive areas needs to be refined. Transmitters with a longevity of at
least 12 months (preferably 2-3 years) and with high reliability will need to be developed.
The solar-assisted battery transmitters used in this study exhibited high failure rates
(especially during the first year of the study). After revisions were incorporated into the
solar-assisted package, the transmitters worked relatively well during the breeding season
but return rates the following year were very low. These low return rates were most likely
due to radio failure during winter but may have been partially caused by harness failure
(transmitters falling off) or adults experiencing unusually high mortality rates (deleterious
effects from transmitters) after leaving the breeding grounds. Visual observations, and
nesting and movement patterns indicate that radio-marked band-tailed pigeons were not
negatively affected by the transmitters. More research is needed, however, into the
physiological and pathological effects of attaching radio-transmitters to Columbids
(Gessaman et al. 1991), and for the development of new and/or improved methods of
transmitter attachment (Schulz et al. 1996).
Although the abundance of mast- and berry-producing species in the shrub stand
condition attracts band-tailed pigeons and a wide variety of wildlife, little is known about
distribution and abundance of these feed areas. As the amount of forest available for
harvest has been reduced in recent decades, intensive silvicultural practices have become
increasingly important. Intensive reforestation practices are intended to reduce the time
of, or completely by-pass, the early, highly competitive shrub stage. Reforested
plantations quickly produce stands with impoverished species diversity and a simplified
vertical structure that ultimately reduces diversity of the forest landscape (Hunter 1990).
Renewed interest in maintaining biodiversity on forest landscapes has also removed
attention from the shrub stand community, focusing primarily on balancing the amount of
mature/old-growth forest with other age classes of forest. Thus, the shrub stand
community has received little attention, and little information exists on the factors
affecting the distribution of mast- and/or berry-producing species (e.g., Pacific red and
blue elder and cascara).
Because of the mobility exhibited and the sizes of home-ranges used by bandtailed pigeons, design and implementation of habitat management plans may face some
difficulty due to land ownership distributions. Proper management will require not only
the provision of suitable nesting cover, but an adequate temporal and spatial distribution
of food resources. Even though band-tailed pigeons appear adaptable to habitat
disturbance and/or alteration, a regional, landscape-based approach will be needed in
order to assure that an adequate network of the suitable habitats are maintained. Bandtailed pigeons may also play a significant role as a seed disperser in the Northwest,
similar to white-crowned pigeons in south Florida's ecosystems (Strong and Bancroft
1994). Due to their mobility, large population size, diet diversity, and tendency to pass
seeds intact, pigeons may provide a mechanism for dispersal, gene flow, and
recolonization of some plant species, especially after disturbances such as clear cutting
and/or fire. As such, band-tailed pigeons would be an important species for consideration
in large watershed and/or reserve management designs (e.g., Late Successional Reserves)
(Howe 1984).
Nesting records of band-tailed pigeons presented herein indicate that recruitment
rates could produce a fall population containing about 40 45% young (assuming that the
breeding population is composed of at least 84% ASY adults, and that most breeding
pairs nest twice successfully). Jarvis and Passmore (1992) reported recruitment at
approximately 23% young, which was inadequate to maintain even a stable population
with current adult survival estimates. However, their recruitment estimate may be
slightly low due to the high number of fledgings that occur after September 1 (Fig. 2).
Recruitment seems to vary little from year to year, but may be relatively near their biotic
potential. Also, adult survival during the breeding season (mid-April through midSeptember) was much higher (96%) than expected; nesting is usually considered a
vulnerable time for breeding adults. Thus, the bulk of adult mortality must occur during
migration and/or on the wintering grounds. With this information in mind, management
options should focus on adult survival, especially after the start of the hunting season
(September 15). Band-tailed pigeons are also periodically subject to large losses due to
hunting and/or outbreaks of trichomoniasis. These catastrophic losses take place
irregularly, but may have a major influence on population trends for many years (Jarvis
and Passmore 1992). Thus, after population declines from heavy losses (primarily
harvest), even low harvest rates (i.e., additive mortality responses) may sustain
populations at low levels for years because band-tailed pigeons have such a low
reproductive potential. Therefore, management options/guidelines must not disregard
large episodic losses, and managers who model band-tailed pigeon populations need to
seriously evaluate the factors affecting adult survival and better understand the impacts of
different adult mortality/harvest rates on band-tailed pigeon population dynamics.
During this study it was difficult to determine actual departure dates for fall
migration. Band-tailed pigeons appeared to engage in long pre-migratory flights
(primarily to heavily used feed areas), and departures were most likely related to nesting
chronology. Some birds were last observedllocated in late-August and early-September
but a majority of birds nested through mid-September. The September 15 starting date
for the hunting season is biologically acceptable and appears to be the most appropriate
date, i.e, most nests are finishing up and a majority of birds have not yet begun their fall
migration.
Another key unknown in band-tailed pigeon biology which needs further study is
the juvenile/post-fledging component of the population, e.g., survival, movements and
dispersal, and the age of first breeding (Jarvis and Passmore 1992). Studies of bandtailed pigeons in migrational and winter use areas would be useful in understanding
additiOnal habitat needs and survival rates. The development of a reliable population
monitoring technique throughout the range of band-tailed pigeons is also urgently needed
(Braun 1994).
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