c
PREDATOR-PREY RELATIONSHIPS AND REPRODUCTION
OF THE BARN 0V7L IN SOUTHERN TEXAS
by
LEE CHARLES OTTENI, B.S.
A THESIS
IN
RANGE SCIENCE
Submitted to the Graduate Faculty
of Texas Tech University in
Partial Fulfillment of
the Degree of
MASTER OF SCIENCE
Approved
August, 1971
/V:
r J
ACKNOWLEDGMENTS
I am indebted to Dr. Eric Bolen, my academic and
project advisor, and my committee, including Drs. Bill
Dahl, Don Klebenow, and Robert Packard.
The Rob and Bessie Welder Wildlife Foundation provided
the major portion of the financial support and equipment
for this study.
Additional aid was supplied by the Depart-
ment of Range and Wildlife Management, Texas Tech
University.
Texas A & M permitted use of their radio-
tracking computer program.
Dr. Clarence Cottam, Director of the Welder Foundation,
was most helpful in all aspects of this study.
His field
work on barn owl on the Welder Refuge provided barn owl
nest data from 1965 to 1969.
Dr. Cottam also assisted in
identification of avian remains in the owl pellets.
Mr. W.
Caleb Glazener, Assistant Director of the Welder Refuge
procured necessary materials.
Dr. Marshall VThite allowed
use of unpublished data concerning food habits of barn owls
on the Welder Refuge from 1965 through 1968.
11
TABLE OF CONTENTS
ACKNOWLEDGMENTS
ii
LIST OF TABLES
vi
LIST OF ILLUSTRATIONS
I.
II.
viii
THE STUDY
1
The Study Area
1
Methods
2
Capture of adults
2
Handling of juveniles
3
Determination of sex
3
Analysis of food habits
3
Census of prey populations
4
Radio tracking and instrumentation
5
Additional sources of data
7
FOOD HABITS
8
Pellet Analysis
8
Fluctuation, over Seven Years, of Prey in
Pellets
9
Mice
10
Rats
12
Shrews
13
Rabbits
14
Gophers
14
Birds that roost over water
14
111
III.
Birds that roost over land
15
Invertebrates
16
Seasonal Variations in Diet
16
Biomass Contribution
17
POPUU^TION DYNAMICS OF THE PREY POPULATIONS
Rodent Populations
22
Vegetation cover on the trapping grids
22
Habitat preference
23
Population fluctuations
25
Variation in species composition
29
Blackbird Populations
IV.
22
BREEDING BIOLOGY AND CHRONOLOGY
29
32
Nest Sites
32
Nest Material
32
Nest Initiation
33
First nests of the year
33
Length and magnitude of the nesting season
33
Number of broods per season
33
Nesting Behavior
35
Laying Rate
35
Incubation
36
Nest attentiveness during incubation
36
Clutch Size
37
Nesting Success
39
iv
V.
VI.
VII.
RELATIONSHIP OF PREDATOR AND PREY
41
Evaluation of Prey Biomass and Barn Owl
Reproduction
41
Analysis of Predator-Prey Relationships
47
RADIO TRACKING
49
Diurnal Location of Owls
49
Activity Patterns of Young
49
Activity Patterns of the Females
50
Activity Patterns of the Males
52
SUMMARY
54
REFERENCE BIBLIOGRAPHY
57
APPENDIX
59
A.
INDEX AND SCIENTIFIC NAMES OF PLANTS
60
B.
BIRDS TAKEN BY BARN OWLS ON THE WELDER
WILDLIFE REFUGE, 1965-1971
61
V
LIST OF TABLES
Table
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Frequency Occurrence of Food Items in the
Diet of Barn Owls on the Welder Wildlife
Refuge, 1965-1971
11
Seasonal Variation in Frequency of Food Taken
by the Barn Owls at Welder Wildlife Refuge,
1969-1971
18
Numerical and Biomass Composition of the Diet
of the Barn Owls as Determined by Pellet
Analysis, on the Welder Wildlife Refuge,
from 1965 to 1971
20
Distribution of Captures of Rodents, on Three
Different Plant Communities, in Relation to
the Distribution of the Traps According to
the Density of Bunchgrass
24
Distribution of Traps and Captures of Rodents
in Eight Plant Communities of the Welder
Wildlife Refuge from February, 1970, to
May, 1971
26
Total Live-Trap Captures of Rodents on the
Welder Study Areas from July, 1969, to
May, 1971
27
Seasonal Variation in Species Composition of
Live and Snap Trap Trapped Mammals from
July, 1969, to May, 1971, on the Welder
Refuge
30
Periods of Nest Initiation for the Barn Owl
on Welder Wildlife Refuge, 1965-1971
34
Calculation of Mean Clutch Size for Nests
Judged the Completed Clutch of a Single
Barn Owl, Welder Wildlife Refuge, 19651971
38
Comparison of Nest and Egg Success Between
Years for the Barn Owl, Welder Wildlife
Refuge, 1965-1971
40
VI
Table
11.
12.
13.
Influence of Prey Availability on Breeding
Success of Barn Owls, Welder Wildlife
Refuge, 1965-1971
42
Breeding History for the Barn Owl on the
Welder Wildlife Refuge, 1965-1971
45
Seasonal Variation in Numbers and Biomass of
Trapped Mammals on the Welder Refuge, July,
1969, to May, 1971
46
Vll
LIST OF ILLUSTRATIONS
Figure
1.
2.
Rodent population dynamics on eight major
vegetation types of the Welder Wildlife
Refuge
28
Reproductive success of the barn owl, relative
to the ratio of mammal versus bird biomass
in the diet of the owls
43
Vlll
CHAPTER I
THE STUDY
This study was designed to reveal various ecological
aspects of the barn owl (Tyto alba) in southern Texas
from 1969 to 1971.
tion of:
Specific aspects included determina-
(1) food habits; (2) relative abundance of the
owls' prey populations; (3) breeding of the owls correlated with the abundance of the prey; (4) nesting activities,
including clutch size and nesting success; and (5) nocturnal activities by telemetry.
The Study Area
This study was conducted on the Rob and Bessie Welder
Wildlife Foundation (hereafter referred to as the Welder
Refuge), eight miles northeast of Sinton, Texas, in San
Patricio County.
It consists of 7,800 acres and is bor-
dered on the north by the Aransas River.
A subhumid climate exists in this area, characterized
by hot summers and mild, short winters.
The growing
season is 11 months with 31 inches annual rainfall varying
from 14.9 in 1956 to 48.8 in 1960.
Prevailing winds blow from the Gulf all months of
the year.
Hurricane winds occur every few years.
Severe
rains and subsequent flooding usually come with the high
winds of hurricanes.
The soil on the Refuge has developed from differing
sources.
Box and Chamrad (1966) recognize these sources
as the Beaumont and Lissie geologic formations and deposits
from the Aransas River.
General vegetation of the Refuge has been classified
as a tall-grass prairie by Thomas (1962).
Box (1961)
classified it as a coastal grassland climax.
The proximity of the area to the Gulf of Mexico
results in a diverse flora of over 1,300 plant species
and more than 500 animal species.
Methods
Capture of adults
Most adult barn owls were caught in nest boxes set
up for cavity-nesting birds.
The method of capturing the
owls was to block the entrance hole, trapping the owl
inside the box.
After handling, the bird was returned to
the box and the entrance was again blocked.
minutes elapsed, the block was removed.
VThen five
This brief
period of detention within the box was usually sufficient
in calming it so that it did not immediately leave the box.
Snares were used on top of next boxes to capture
adult birds if they could not be captured inside the nest
box.
The snares were made from 60-pound test monofilament
line which was tied to 1-by 2-inch mesh welded wire base.
12 by 12 inches.
were:
The steps used to construct a snare
(1) cut the monofilament line into 18-inch lengths,
(2) tie an overhand knot to form a loop in one end of the
monofilament and tie the other end to the crosswire of
the base.
Sixteen nooses were tied to each unit.
Handling of juveniles
Juvenile barn owls were observed in the nest boxes
until their departure, which was approximately 70 days
after hatching.
in the nest box.
No attempt was made to tether the young
Juvenile barn owls, between five and
seven weeks of age, were banded with U.S. Fish and Wildlife Service bands.
Determination of sex
Sex was determined during the breeding season by
cloacal examination.
The female develops an incubation
patch and has a red, enlarged vent if she recently laid
an egg (Hamerstrom, pers. comm.).
No'observed sexual
dimorphism was noted in either young or adult owls after
the breeding season.
Analysis of food habits
Barn owl food habits were determined from examination
of pellets.
Pellets were collected on a monthly basis
from nesting boxes and roost sites located on the Refuge.
Information recorded in each collection included date and
location of collection, percentage frequency, and the
number of prey species in the pellets.
Census of prey populations
The census of small mammal populations was conducted
on 15 areas in eight of the 16 major plant communities.
These eight areas were:
Bunchgrass-annual forb community,
Mesquite-Buffalograss community, Chaparral-Bristlegrass
community, Prickly pear-shortgrass community, Paspalumaquatic V'/eed community, Live oak-chaparral community,
areas of annual forbs (disclimax due to brush control),
and pure stands of Texas wintergrass (Stipa leucotricha).
A complete list of scientific names of plants appears in
Appendix A.
Full descriptions of the plant communities
are given by Box and Chamrad (1966).
Within the areas sampled, composition of mammals
and relative abundance were determined using the markand-recapture method (Davis, 1959).
Trapping stations
were established on a grid at 25-foot intervals, forming
a rectangle of eight by six trap lines.
During each
3-day trapping period, 48 traps were set per night.
Individual mammals were marked by ear tags.
As each mammal
was captured for the first time, it was tagged and sex,
age, and location of capture on the plot was recorded.
The mammal was immediately released at the point of
capture.
Census of the blackbird population, consisting of
red-winged blackbirds (Agelaius phoeniceus), boat-tailed
grackles (Cassidix mexicanus), brown-headed cowbirds
(Molothrus ater), and bronzed cowbirds (Tangavius aeneus),
was also made by the mark-and-release method.
The birds
were captured in two decoy traps which measured 16 by 18
by 5 feet.
These traps were poultry-wire enclosures
containing live decoy birds, food, and water.
Birds
entered the traps through an opening in the top.
Trapped
birds were marked with U.S. Fish and Wildlife Service
bands.
Radio tracking and instrumentation
Traditional methods of banding, tagging, and dyeing
birds for home range determination all require visual
observation for identification (Cottam, 1956), and thus
were of limited value in studying a nocturnal species
such as the barn owl.
Therefore, radio transmitters
attached to the owls were used to facilitate locating
and identifying individuals.
Radio transmitters were secured to the owls, usually
at the site of capture.
Securing a transmitter to a barn
owl required placing the transmitter on the back between
the wings of the bird, then tying the harness wires into
a body loop and a neck loop.
The wires were crimped with
a keeper ring to prevent the knots from working loose.
The following data were recorded for each radiomarked barn owl:
age, sex, if the bird was an adult;
capture location; band number; and transmitter frequency.
The bird was released at the capture site after a check
had been made to insure that the transmitter was functioning properly.
Data were also recorded during each tracking session.
This information designated roost location prior to flight
in the evening, time of flight, location in the pastures,
and duration of feeding activities.
Transmitters:
Seven barn owls were harnessed with
transmitters similar to those described by Cochran and
Lord (1963).
The transmitters, designed for an operating
life of 100 days, weighed approximately 16 grams.
The
frequency range was 150.85 to 151.12 Mc, with transmitters
spaced at 25 Kc intervals.
The combination of 25 Kc
spacing and a distinctive pulse rate and tone insured
positive identification of an individual signal.
Effec-
tive field range of transmitters varied with receiving
antennae as follows:
(1) to a double 8-element beam
antenna, 2-2^ miles, and (2) to a 3-element beam antenna,
X-Xk
miles.
Receivers;
Twelve-channel transistorized and battery-
operated receivers used in this study were constructed by
W. W. Cochran.
Antennae:
Three directional antennae were used to
determine the locations of transmitter-carrying barn owls.
Two permanent, dual-rotating eight-element beam antennae
on eight-foot masts were placed on top of observation
towers.
The distance from antennae to ground level was
approximately 50 feet.
One portable three-element antenna
was used to locate owls during the day.
Additional sources of data
Prior to the initiation of this study, Welder Refuge
personnel collected data on the barn owls from 1965 to
1968.
Data prior to 1969 concerning nesting success,
breeding chronology, and banding of barn owls was taken
from records of Dr. Clarence Cottam.
Food habit data of
barn owls on the Welder Refuge from 1965 to 1968 were
provided by Dr. Marshall White.
CHAPTER II
FOOD HABITS
Pellet Analysis
Barn owls swallow small-sized prey whole; otherwise,
flesh is picked from the bones of animals too large to
swallow.
Varying quantities of bone, feathers, and fur
are eaten with the meat but are not digested.
Instead,
these materials are formed into pellets which are regurgitated.
Reed and Reed (1928) have shown that the 1-mm.
diameter of the pyloric opening between the stomach and
duodenum (in the great horned owl. Bubo virginianus) acts
as a mechanical barrier against the passage of coarse
material to the duodenum.
My own examination of the barn
owl stomach revealed its pyloric opening to be similar
to that of the great horned owl.
Wallace (1948) reported
that the absence of free acidity in the stomach, as shown
by fluoroscopic studies, prohibits any appreciable
corrosive action, so that the bones, fur, and feathers
are typically ejected undigested.
Hence, examination of pellets is probably the best
method of determining food habits of the barn owl, but
incomplete data may sometimes result from this method.
There are at least two sources of error.
8
Two pellets are
usually formed daily.
Guerin (in Wallace, 1948: 25)
reported one small pellet is regurgitated during the
night while on the foraging ground and later a larger
pellet is ejected at the diurnal roost.
Possibly only
the larger pellet is collected from the diurnal roost
(i.e. the nesting boxes in this study) for the adult
birds.
Secondly, bias may be introduced by misidentifi-
cation or nondetection of animal remains within the
pellets.
Soft-bodied invertebrates, cartilaginous
materials, and perhaps nestling birds could escape detection (Glading et al., 1943).
Nondetection is most
prevalent in pellets of the very young owlets; due to the
high calcium requirement of owlets, many bones of prey
animals are completely dissolved for the calcium
(Errington, 1932).
Pellet materials collected from nest boxes and barn
owl roosts were taken to the laboratory for analysis.
In June, 1969, at the start of this study, the nest boxes
were cleaned of all previous material.
pellets were made each month thereafter.
Collections of
Monthly collec-
tions were analyzed in bulk due to trampling of most
pellets in the boxes by owls.
Fluctuation, over Seven Years,
of Prey in Pellets
Predation by barn owls upon the most available kinds
10
of prey is logical in principle, but factors conditioning
the degree of availability of prey species are complex.
Predation upon animals that aestivate, birds that migrate,
and young of various species will be greatest at times
when they are most available.
On a year-to-year basis,
environmental factors such as condition of habitat, floods,
and density of predators determines the pressure exerted
on a prey population (Craighead and Craighead, 1956:147).
Variations in fundamental predator-prey interactions due
to environmental factors thus complicate efforts of longterm analysis of the barn owls' food habits.
The following analysis of predation by the barn owls
from 1965 to 1971 is based solely on pellet analysis due
to the lack of information on the availability of prey
prior to the initiation of this study in 1969.
However,
since food habit data determined by pellet analysis are
available from 1965 to 1968, they are included in Table 1.
Mice
Of 11,625 identified individuals in pellets, mouse
remains comprised 4,715 individuals, or 40 percent.
The
species included pigmy mice (Baiomys taylori), fulvous
harvest mice (Reithrodontomys fulvescens), white-footed
mice (Peromyscus leucopus), and hispid pocket mice
(Peroqnathus hispidus).
11
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Hispid pocket mice, fulvous harvest mice, and pigmy
mice in pellets were subject to pronounced fluctuations
over the seven-year period.
In 1965, hispid pocket mice
composed 31 percent of the owls' diet whereas the following year only five percent.
From 1967 until 1971, this
mouse made up less than one percent of the owls' diet
(Table 1 ) .
Fulvous harvest mice reached a peak in 1966 following the hispid pocket mouse decline.
Harvest mice rose
from 13 percent in 1965 to 31 percent in 1966.
Following
their peak, they declined drastically to less than two
percent for the years 1967-1971.
Pigmy mice were a staple food item in the diet of
the owls in 1966 (28 percent) and 1967 (24 percent), but
in 1967, according to the pellet analyses, their population began to decrease.
Hurricane Beulah inundated
a major portion of the Refuge in 1967^ and pigmy mice
declined to a low of 3 percent of the total diet in
1968.
Pigmy mice rose to 15 percent of the owl diet in
1969 and reached a peak of 30 percent of the diet in
1970.
Rats
This group was represented by 2,430 individuals,
or 21 percent of the diet.
This group included the gray
13
wood rat (Neotoma micropus), rice rat (Oryzomys palustris),
and cotton rat (Sigmodon hispidus).
Cotton rat remains were detected from 1966 to 1970
in about the same proportion, between 9 and 21 percent.
The other rats, however, appeared to have been preyed
upon more following the 1967 flood, perhaps because of a
subsequent change in vulnerability.
Gray wood rats were not found in the pellets of barn
owls in 1965, nor were they present to any extent in
1966.
Their number began to increase in 1967 (12 percent
of diet), and following the flooding in October of 1967,
they were a major food item (26 percent of diet) in 1968.
The princioal habitat of the wood rat is on clav soils,
which were not flooded after Hurricane Beulah.
Rice rats were a trace item in the pellets until
1968.
In 1968, they comprised 6 percent of the diet,
and in 1969, they increased to 18 percent.
Shrews
Remains of 1,861 least shrews (Cryptotis parva)
comprised 16 percent of the total number of identified
individuals.
Numerically least shrews were a stable food
item in the barn owls' diet between 1965 and 1971,
ranging from 9 to 34 percent.
14
Rabbits
The remains of 318 cottontail rabbits (Sylvilagus
floridana) were removed from pellets--3 percent of the
total prey individuals identified.
Of the 16 rabbit
skulls examined from 1969 to 1971, all were sub-adult
individuals.
Adult rabbits are seemingly less vulnerable
because of their size in relation to the barn owl.
Rabbits were represented in the greatest abundance in
1967 (8 percent), the year mice dropped from 62 percent
to 19 percent.
Gophers
Pocket gophers (Geomys bursarius) comprised 4 percent
of the owls' diet from 1965 to 1971.
Total individuals
were 458.
Birds that roost over water
Birds roosting in association with aquatic communities at the Welder Refuge include members of the families
Rallidae, Icteridae, Ardeidae, and Recurvirostridae.
Remains of these birds were detected 1,066 times, or
9 percent of the total number of items identified.
Among the remains of 1,007 icterids were 182 grackles,
390 brown-headed cowbirds, and 435 red-winged blackbirds.
The owls apparently used the abundant blackbird populations as a buffer group when the availability of rodents
15
was low (Table 1).
With the decline of mice in 1967,
detection of icterids in pellets increased from less than
1 percent in 1966 to 14 percent in 1967.
In 1968, after
many of the rodents were presumably lost in the flood,
the blackbirds represented 17 percent of the owls' diet.
Declines in the rodent population in 1970 and 1971, indicated by declines in live trapping success and by low
representation in pellet analysis, were compensated by
an increased frequency of blackbirds in the owl diet.
The frequency rose to 16 percent in 1970 and 17 percent
in 1971.
Birds that roost over land
This group includes five orders of birds:
Falconiformes, Galliformes, Columbiformes, Cuculiformes,
and Passeriformes.
Remains of these birds were identi-
fied 403 times (3 percent) in the pellets.
Faliconiformes were represented twice by the sparrow
hawk (Falco sparverius).
Galliformes were detected 81
times--all bobwhites (Colinus virginianus).
Remains of
Columbiformes were found 11 times--9 mourning doves
(Zenaidura macroura) and 2 Inca doves (Scardafella inca).
Cuculiformes appeared once with the presence of one
yellow-billed cuckoo (Coccyzus americanus).
Except for
the dickcissel (Spiza americana), which existed in pellet
16
findings 111 times, the other species in the order
Passeriformes occurred in the pellets in minute numbers.
Remains of other passerine and undetermined small birds
made up the remaining 197 prey items identified as birds.
A complete listing of identified bird species of
both groups and the number of times they were found in
the pellets appears in Appendix B.
Invertebrates
Remains of invertebrate animals were rarely found
in pellets during the years 1967-1971.
Of these, the
grasshopper, family Acrididae, was the only species
represented in significant numbers; 190 times or 2 perC ^N V^ 4 -
e x i t . .
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,
v,— ^N V-.* ^^ y-\ /-^ ^-». v-s +- ^^ /^
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X<=:00
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II
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percent of the items identified.
Seasonal Variations in Diet
The seasonal food-habits picture is least representative in late summer and early fall, when few pellets
were collected.
During this time of the year, the young
were expanding their activity range, and the adults used
established roosts less than before.
Pronounced changes in the percentage of prey species
in pellets occurred during the two years of this study.
Some of these appeared to be of seasonal origin (i.e.
migration); others appeared to be related to predation
17
upon mouse populations (Table 2).
The proportion of rats taken by barn owls varied
inversely with mice in the diet.
The greatest number of
rats taken in the summer of 1970 corresponded to the
lowest catch of mice for that period.
In the fall of
1969, when the greatest number of mice were consumed,
occurrence of rats in pellets was lowest.
The frequency of shrews in the diet remained relatively constant.
In seven of the eight seasons in which
pellets were collected, shrews contributed more than 25
percent of the total diet.
The occurrence of least shrews
in barn owl pellets dropped to 17 percent in the spring
of 1970.
Predation on birds, predominantly those that roost
over water, was characterized by a seasonal variation in
the frequency of occurrence.
In the winter, predation on
birds was directed toward adult migratory blackbirds.
Late spring and early summer predation was directed
toward nestling populations of grackles in 1970.
Pellet contents thus reflected opportunistic hunting
in different habitats by the owls, as seasonally characterized by certain species and their availability as prey.
Biomass Contribution
Pellet analysis provides a satisfactory index to the
18
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19
number of food items taken, and if average weights of
individual prey species are known, an index to biomass
can be developed.
Biomass is important because small
species, such as the pigmy mouse and least shrew—while
numerically abundant in the pellets—provide relatively
little biomass, whereas, larger species like the cotton
rat and wood rat, which are taken less frequently,
contribute greater amounts of biomass to the diet of the
owl.
Weights of specimens collected in live traps provided
a basis for determining biomass in the owl diet for a
seven-year period.
These weights are listed in Table 3.
Mouse populations which numerically contributed 41
percent of the owls' diet, comprised only 16 percent of
the biomass.
In contrast, the rat population, which
numerically was 21 percent of the owls' diet, contributed
50 percent of biomass in the owls' diet.
Shrews, the
third most important group in number, were ranked last
among the mammals in importance by biomass, contributing
only 1 percent of the food.
There was a strong relationship between the percentage frequency of species and the percentage biomass among
mammals, birds, and insects.
Mammals comprised 85 percent
of the individuals preyed upon by owls and 87 percent by
20
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21
weight of the total biomass.
Birds made up 13 percent
of the total individuals taken and 13 percent of the
total biomass.
Insects were represented numerically by
2 percent and comprised less than 1 percent of the biomass.
CHAPTER III
POPULATION DYNAMICS OF THE
PREY POPULATIONS
Intensive studies of the rodent populations were
conducted in eight major plant communities.
Studies of
bird populations were limited to the icterids which
roost over water.
Census of the insect populations was
not conducted because of their insignificant contribution
to the biomass of the barn owl diet.
Rodent Populations
Greatest emphasis was placed on the study of rodent
populations, since they formed--in both quantity and
biomass--the chief food item of barn owls (Wallace, 1948).
Vegetation cover on the
trapping grids
Major differences in vegetation occur on the Welder
Refuge among the various soil types (Box, 1961).
The
vegetation of the clay soils is characterized by lowgrowing woody vegetation, with an understory of dense
grass.
The sandy soils support a prairie vegetation of
bunchgrasses and annual forbs, with little woody vegetation.
Adjacent to the Aransas River and oxbow lakes,
local communities of dense woodland occur.
22
Several local
23
annual forb communities occur on disturbed areas throughout the Refuge.
Habitat preference
Trap success within the areas sampled showed that
rodents were not randomly distributed.
McCarley (1958)
showed microhabitat preference can be determined where
emphasis is placed on the percentage of total captures
in a given microhabitat type.
Analysis by this method
of three different plant communities--Bunchgrass-annual
forb, Chaparral-bristlegrass, and Mesquite buffalograss-indicated that bunchgrass density was a primary factor
in rodent distributions (Table 4).
These grasses were
primarily bluestems, switchgrass (Panicum virgatum) and
Pan American balsamscale (Elyonurus tripsacoides).
Bunchgrass on each trap square (625 square feet)
was arbitrarily classified as dense, intermediate,
or sparse.
Forty-four of the 144 traps were located
in areas surrounded by a dense stand of bunchgrass
(Table 4).
These 44 traps caught 46 percent of the
captures, averaging 8 percent captures per trap.
The 43
traps located in areas with intermediate stands of bunchgrass had 28 percent of the captures.
The 57 traps in
sparse bunchgrass had 26 percent of the total captures.
24
(N
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25
These differences are statistically significant
(x^ = 51.7, P > 0.05).
A comparison of trap success of all eight community
types indicated that greatest numbers of rodents occurred
in the Bunchgrass-annual forb communities and the disturbed areas supporting an annual forb complex (Table 5).
Therefore, as far as the distribution and abundance of
rodents as revealed by trapping are concerned, barn owls
are likely to encounter rodents at a higher density in
the grassland and annual forb communities.
Presumably
barn owls are more successful at capturing rodents in
these areas as compared to areas with a woody overstory.
Population fluctuations
Because of nonrandom sampling (Krebs, 1966), no
attempt was made to estimate population densities.
The
objective in this study was to measure population
changes and trends on the Welder Refuge, not to intensively study rodent population dynamics.
Trap success
provided an adequate reference figure for population
fluctuations (Table 6 ) .
Live trapping on the Refuge suggested rodent population levels were declining from summer, 1969 to winter,
1969-70 (Fig. 1 ) . This decline was momentarily reversed
during the winter of 1969-70.
The number of rodents again
26
01
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(Stipa 1
1
Prickly pe.
shortgra
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Mesquitebuf falog]
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> i
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1
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(disturb
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§
27
TABLE 6
TOTAL LIVE-TRAP CAPTURES OF RODENTS ON THE
WELDER STUDY AREAS FROM JULY, 1969,
TO MAY, 1971
Number
Percent
trap
success
Date
traps
set
traps
entered
July, 1969
540
154
28.5
January, 1970
288
37
12.8
February, 1970
192
24
12.5
March, 1970
288
57
19.8
April, 1970
288
26
9.0
M^y, 1970
864
17
1.9
June, 1970
2,160
83
3.8
July, 1970
1,296
84
6.5
August, 1970
2,160
178
8.2
288
139
48.3
2,160
388
19.3
432
83
19.2
March, 1971
1,584
351
22.2
May, 1971
1,584
299
18.9
14,124
1,920
X = 13.6
November, 1970
January, 1971
February, 1971
Total
declined noticeably during early spring of 1970, falling
to the lowest level (2 percent trap success) in May,
1970.
Rodent numbers gradually increased during the
28
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0
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4J
fd
4->
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tn
0
X
<
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rH
fa r^
b
en
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1969
b
o
IH Qi
C 0
0 fa T?
•H
0
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fd MH P
rH -H +J
prH a
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in
0
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0
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fd
0 rH U
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4J
Fig. 1.—Rodent
type s of the Welder W
perc entage of rodents
X
g
fd
C.
>i
T!
29
summer, 1970, and by fall of 1970, the highest rodent
population level was reached (48 percent trap success).
Late fall, 1970, was a period of rapid increase and
subsequent decline; however, the decline was not excessively prolonged or severe.
Numbers of rodents were
relatively stationary during winter and spring of 1971.
Variation in species composition
In 1969, prior to live trapping, I snap trapped
rodents in the major plant communities, to gain a cursory
view of the rodent populations.
Trapping at this time
yielded the highest percentage of rats of the two years
(Table 7 ) .
The rat populations declined, following the summer
of 1969 to less than 1 percent of the catch when census
of the mammals was terminated in the spring of 1971.
Mice
dominated the catch in all seasons of the study, ranging
from a low of 78 percent in the summer of 1969, to a
high of 99 percent in the spring of 1971.
These data
suggest that the population decline was not a systematic
reduction of numbers in all rodent populations, but
rather an extensive reduction in the rat populations.
Blackbird Populations
About 50,000 resident and migratory blackbirds roosted
among the tules (Scirpus californicus) in the Welder Refuge
30
tn
T-i
fd
H P
fd TJ
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fa
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31
lakes during the winter months.
These areas were utilized
as roost sites by resident blackbirds during the rest of
the year.
Red-wings were the most numerous of the
icturids, but there were also many cowbirds and grackles.
These roosting locations attracted birds from the Refuge
and also from adjacent areas to the north and west.
Grackles and red-wings used these areas in the spring
for nesting sites.
An attempt was made, following the methods used by
Southern (1955), to measure the rate of feeding by the
owls upon the population of blackbirds.
A total of
2,413 birds were banded--l,042 red-wings, 718 boat-tailed
grackles, and 653 cowbirds.
Birds were trapped between
January and July of 1970, in an area near the lakes.
All subsequent pellet collections were examined for
the metal bands.
It was not possible to estimate
directly the proportion of blackbirds caught by the owls
because no tags were found.
However, the availability
of blackbirds was never limited; the spring and summer
population was estimated at 15,000 birds.
CHAPTER IV
BREEDING BIOLOGY AND CHRONOLOGY
Nest Sites
The barn owl is primarily a cavity nester using
hollow trees, holes in river banks, and crevices in
cliffs as nesting sites.
Its use of man-made structures
such as towers, barns, and other buildings as additional
nesting sites has been documented (Pickwell, 1948; Wallace,
1948).
Barn owls have utilized nesting boxes set up to
accommodate cavity-nesting ducks on the Welder Refuge
since 1965.
Thirty-six of these boxes were placed around lakes
and in oak, mesquite, and anaqua (Ehretia anacua) tree
communities.
Of the 112 nests found during this study,
109 were in the nest boxes.
The remaining three were
located in a water-control gate house located on the
Welder Refuge.
Nest Material
Female barn owls trample their pellets in the confines of the nest boxes.
resembles a nest.
This layer of material underfoot
Wallace (1948) reported that barn owls
did not construct nests before egg laying, but during
incubation a female crushed many of her regurgitated
32
33
pellets in a presumed attempt to form a nest.
Nest Initiation
First nests of the year
Nest initiation began in January four years, in
February two years, and in March one year (Table 8 ) .
These differences suggest that the onset of nesting may
be regulated by the availability of food more so than by
other environmental requirements, such as day length.
Length and magnitude of
i
the nesting season
.
The length of the nesting season was determined
from the beginning dates of each year's first and last
|
.
i
nests. The last nests were begun in September of 1965,
October of 1966, April of 1967, May of 1968, June of
1969, May of 1970, and April of 1971.
nesting season lasted 5.3 months.
The average
Nesting attempts
increased rapidly as each season began, reaching the
highest level in March (Table 8 ) .
Number of broods per season
Past records at Welder Refuge indicated that a female
usually produced only one brood per year.
There was only
one record (1965) of a barn owl having two broods in the
same year on the Refuge.
»
34
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35
The male feeds the female while she incubates.
Both male and female parents tend the flightless young
and remain with the juvenile birds after flight is
attained.
Hence, this pattern of events does not permit
time for a second clutch during the year unless the male
were to divide his feeding obligations between the renesting female and the owlets of the first brood.
Nesting Behavior
The pair of barn owls roost together during the day
and hunt together at night prior to initiation of nesting.
This behavior has been substantiated by radio tracking.
Nest site selection occurs at night since the owls
are nocturnal.
Both male and female have been observed
in the boxes prior to nest initiation.
The male usually
roosts in a tree in the vicinity of the nesting female
when a box is selected and the female begins to lay.
Occasionally the male was observed in the box with the
incubating female.
Laying Rate
Bent (1938) reported that the laying rate of eggs
for barn owls occurred at intervals of two or three days.
Wallace (1948) found the laying rate in two nests to be
every other day.
Attempts to determine the laying rate
<
e
36
for the owls on the Welder Refuge resulted in nest
desertion.
Incubation
The incubation period for owls and other birds of
prey begins soon after the first egg is laid (Bent, 1938).
Length of the incubation period of nests in this study
was approximately 30 days.
Accurate incubation periods are difficult to determine from wild nests.
During the first weeks of incuba-
tion, the females are prone to desert their nest if
disturbed.
The incubation period according to Bent
(1938:142) is between 21 and 24 days; Wallace (1948)
reported the incubation period as 30 days.
Nest attentiveness during incubation
Observation records of the barn owl population on
the Welder Refuge indicated that all incubating birds
were females.
However, conflicting evidence exists in
the literature regarding the matter of both sexes
assisting in incubation.
Niethammer (in Kendeigh, 1952:
214) stated that only the female incubates.
Howell (1964)
found that only the female developed an incubation patch.
However, Bendire (1892) reported that both sexes assist
in incubation.
Forbush (in Kendeigh, 1952:214) observed:
t
J
37
. . . the female while incubating is steadily
supplied with food by the male, but occasionally
she leaves the nest, and he takes her place thereon, and probably in some cases he assumes some
part of the duties of incubation, as both male
and female have been seen sitting side by side on
the eggs.
It may well be that the reported cases of males incubating may be based on visits of the male to the nest with
food for his mate.
Male barn owls were observed in this study 15
times (4 percent of the observations on active nests)
in the nest boxes with females.
No observations were
made of only the male in a nest box with eggs.
Clutch Size
Of 112 separate clutches, 91 were considered to be
completed sets.
The size of these clutches ranged from
3 to 8 eggs per nest, with a mean of 4.9 eggs per nest
(Table 9). A total of 21 nests, 11 with 1- egg and 10
with 2 eggs, were excluded from the analysis as incomplete clutches.
Lack (1947) found a tendency for the mean clutch
size of owls to increase with latitude as with an increased abundance of rodents.
Henny (1969) reported a
mean clutch size of 5.3 for barn owls in Switzerland,
Latitude 46°-47°N, and 4.9 for barn owls in Maryland,
Latitude 38°-43°N.
Clutch size of the 68 clutches in
38
TABLE 9
CALCULATION OF MEAN CLUTCH SIZE FOR NESTS
JUDGED THE COMPLETED CLUTCH OF A SINGLE
BARN OWL, WELDER WILDLIFE REFUGE,
1965-1971
*
Clutch size
No. Clutches
Total eggs
3
19
57
4
12
48
5
29
145
6
24
144
J
1
7
4
28
!
1
8
3
24
Totals
91
446
Mean clutch size
4.9
Standard deviation
1.3
Maryland and the 91 clutches in southern Texas, Latitude
28 N, thus yielded identical results.
Therefore, it
appears the number of eggs produced by barn owls is
correlated with the local density of available food
more than latitude.
Lack (1954:22) suggested that the number of eggs
laid by each species of birds has been established by
natural selection to correspond with the largest number
of young for which the parents can, on the average, find
1
39
and provide sufficient food.
His hypothesis, regarding
the clutch size of the barn owl, was supported here and
will be discussed in Chapter 5.
Nesting Success
Nesting success of the barn owl was 66 percent over
the 7-year period (Table 10). Losses occurred because
of abandonment with no known cause (10 percent), bee
swarm taking over the nest box (1 percent), and disturbances associated with nest inspections during incubation
(23 percent).
Of 446 eggs laid in these nests, 245 (55
percent) hatched.
40
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CHAPTER V
RELATIONSHIP OF PREDATOR AND PREY
Unlike most other birds, barn owls remain in the
care of their parents for more than two months after
fledging.
The prey populations must be sufficient to
provide the male with food to feed the female and himself
for seven weeks and to feed the young for approximately
18 weeks.
If a shortage of food occurs where the male
no longer can provide sufficient food to the female, she
will desert her nest and search for food.
If adequate
food cannot be provided for the owlets, they will die
from starvation.
Evaluation of Prey Biomass and
Barn Owl Reproduction
The most important measure of breeding success is
the average number of young raised per pair (Lack, 1966:
142).
There were 1.5 times as many young raised per pair
in years when prey populations were abundant than in years
when their availability was moderate to sparse (Table 11).
A comparison between the biomass of mammals in the
barn owl diet and the number of young raised per pair
indicated that mammals were the necessary staple food on
which successful reproduction depended.
41
A decline in
42
TABLE 11
INFLUENCE OF PREY AVAILABILITY ON
BREEDING SUCCESS OF BARN OWLS,
WELDER WILDLIFE REFUGE,
1965-1971
Barn Owls
Abundance of prey
(Scale 1-14)^
Abundant
Scarce
(3 years)
(4 years)
7-14
1-6
Number of pairs breeding
14.0
11.5 N.S.
Mean clutch size
5.0
4.4 N.S.
Young fledged/pair
2.5
1.0*
Abundance scale defined as biomass per year divided by
total biomass for all years, rounded to nearest whole
number.
*P < 0.05
biomass of rodents was correlated (r = 0.913) by a
decline in number of owlets fledged (Figure 2 ) . The
otherwise abundant bird population alone did not sustain
successful barn owl breeding.
The Refuge supported between 12 and 17 breeding barn
owl pairs through 1970.
During this time the rodent
population, determined from analysis of barn owl pellets,
remained at a high through the breeding season of 1969.
The numbers and biomass of rodents were rapidly declining
(Table 13) in the winter of 1969-1970.
43
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68
Years
69
70
71
Fig. 2.—Reproductive success of the barn owl,
relative to the ratio of mammal versus bird biomass in
the diet of the owls.
**P < 0.01
p
0
44
Twelve pairs of barn owls, carried over from the
successful breeding season of 1969, attempted to breed
as the rodent populations crashed in the winter of 19691970.
There was a 13 percent reduction in the number of
pairs attempting to nest between 1969 and 1970.
The
average clutch size declined by 8 percent between 1969
and 1970.
The greatest difference between years was in
the number of young fledged per pair.
In 1969, 92 per-
cent more young were fledged per pair than in 1970 (Table
12).
The availability of prey, particularly rodents,
appeared to be the factor limiting owl production during
the spring of 1970.
Apparently, the adults could not
supply a sufficient amount of food to many of the young,
and high juvenile mortality resulted.
The 3 owlets that fledged in 1970 were fed a diet of
91 percent blackbirds by the adults.
Two owlets of
another nest--abandoned when they were 5 weeks old—were
fed a diet of 72 percent blackbirds prior to abandonment.
These high proportions of birds provided further evidence
that rodents were not available in the spring and summer
of 1970.
Throughout 1970, available biomass of the rodent
population continued to decline (Table 13).
The number
of barn owls also declined as the rodent population
declined.
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47
In the breeding season of 1971, 58 percent fewer
pairs nested than in 1970 (5 vs. 1 2 ) , indicating that the
barn owl population adjusted to the lower population
level of rodents.
The mean clutch size was reduced 3
percent (3.8 to 3.7) and the number of completed clutches
was 50 percent lower than in 1970.
Rodent trapping and
barn owl pellet analyses indicated the lowest biomass of
rodents on the Welder Refuge in the winter and spring of
1971 (Tables 12 and 13).
Analysis of Predator-Prey Relationships
The barn owl population on the Welder Refuge was
adjusted to the carrying capacity of the area, and did
not change greatly until the crash of all small mammal
populations in 1970.
Some individual prey species
occurred at high population levels while others were
low, prior to 1970.
However, the overall prey population
of mammals during the barn owl breeding seasons appeared
sufficient to meet the requirements of the owls.
Also,
the total owl population maintained a constant rate of
food consumption during years preceeding the 1970 rodent
crash.
A year-to-year increase in barn owls was not
apparent because of the balance between the predator and
prey populations.
Thereafter, however, a decline in the
total owl population was preceeded by a decline in the
48
populations of rodents, the primary food of the barn
owls.
In conclusion, the increase or decrease in any
single small mammal species appeared to have had no
immediate effect on barn owl nesting density or success.
However, the productivity of the barn owl population was
greatly reduced when small mammals decreased in availability to a point where the barn owls depended on birds
for 32 percent or more of their food.
CHAPTER VI
RADIO TRACKING
Diurnal Location of Owls
Radio-marked owls were located during the day for
visual observation and for collection of pellets.
Loca-
tion fixes were taken periodically to detect shifts in
the location of the diurnal roost.
ally used the same diurnal roost.
Adult owls continuThe juvenile birds
expanded their range of roosting sites from the time they
fledged until tracking was discontinued.
However,
roosting range of the juvenile birds never extended
beyond the roosting area of the adult birds.
Activity Patterns of Young
Two sibling owlets were radio-marked and returned to
their nest.
One was radio-marked June 25, 1970, the
second June 30, 1970.
Both owlets left the nest within
24 hours after being handled.
They roosted in anaqua
trees near the nest box during the day.
They were never
visually observed roosting in the same tree with the
adults, but were located in trees within the home range
of the parents.
During the first 3 weeks after fledging,
they remained within 250 yards of the parents' diurnal
roost.
They later ranged out as far as one-half mile
49
50
from the parental diurnal roost.
The young birds left their diurnal roost each
evening about one-half hour after sunset and perched in
the top branches of trees near the nest box.
Individual
variation on a daily basis was great and may have been
the result of environmental conditions (cloud cover,
rain, etc.) which were not measured.
The young called for food in the area of the nest
box during the nights until August 19, 1970—55 and 50
days after fledging.
They perched in the top branches
of a tree while calling for food.
by both parents.
Juveniles were fed
Nightly tracking of the young did not
indicate that they were learning to hunt.
They did not
return to the nest box after fledging.
Activity Patterns of the Females
The first brooding female was harnessed on March 27,
1970.
This owl was fed by her mate and remained with
her brood (then three weeks old) continuously for the
next three days.
When observed on April 26, 1970, the
female was located, during the day, approximately 1 mile
from the box, in an Oak-anaqua thicket; the young owlets
were still in the nest box.
the young at night.
Both she and the male fed
51
The next attempt to locate this female was May 6,
1970.
The two nestlings were found dead, but they had
been dead for only a few days.
attributed to starvation.
The cause of death was
Radio contact was not made
with the female on this date, nor was it ever reestablished.
Hence, it could not be determined whether the
adult birds had deserted the nest when their young were
three-fourths grown, or if the adults had been killed.
On June 11, 1970, a second brooding female owl was
captured in a snare placed on the top of the nest box
containing her young.
At this time she was radio-marked.
This transmitter operated only 10 days, but visual contact with this owl was continued until August 19, 1970.
On March 8, 1971, a third female owl and her mate
were captured in a nest box and radio-marked.
no indication of nesting.
There was
Radio tracking of these owls
indicated that they never returned to the nest box
after March 8, 1971.
Lack of feathers and pellets in
the box confirmed their permanent abandonment of this
particular nest box.
From March 9, until May 26, 1971, the third female
was located diurnally in an Oak-anaqua thicket 0.4 mile
from the point of initial capture.
The period of activity
from the time she left the diurnal roost until she became
52
inactive on the nocturnal roost averaged less than one
hour per night.
The range of her activities covered
less than 160 acres.
During the 80 hours of tracking,
she and her mate shared the same roosts and flew as a
pair.
This female never attempted to nest from the time
she was handled until May 30, 1971, when the study was
terminated.
Activity Patterns of the Males
The first adult male radio-marked (August 1, 1970)
was the parent of the previously mentioned radio-marked
owlets.
handled.
He left the Refuge for two days after being
After this period of absence, he returned and
again continued feeding of the young each night.
Nightly feeding activities began for this male and
his mate within one-half hour of the time (7:00 p.m. CST)
the young began calling for food.
He was active for the
first three hours following sunset, hunting in surrounding marsh areas.
His maximum flights covered 160 acres.
The second male radio-marked (March 8, 1971, with
female) never provided food for either his mate or young.
His activity periods differed sharply from those of the
previous male which fed owlets.
Results of the telemetry study defined the hunting
ranges of 3 owls--2 males and 1 female—as roughly 160
53
acres.
This technique substantiated estimates previously
made with visual observations by Evans and Emlen (1947).
They estimated the range of the barn owl as 165 acres.
CHAPTER VII
SUMMARY
1.
A two-year ecological study of barn owls was
initiated in 1969, on the Welder Wildlife Refuge.
Prior
studies of this owl by Refuge personnel had been conducted
since 1965.
The unpublished data from 1965 to 1969 have
been included in this thesis.
2.
Food habits of the barn owl were determined by
identification of prey remains in the pellets.
Pellet
material contained 11,408 food items, of which mammals
comprised 85 percent, birds 13 percent, and insects 2
percent frequency and the percent biomass of mammals,
birds, and insects.
3.
Pronounced changes in the percent of prey species
in pellets occurred during the two-year study.
Some of
these appeared to be of a seasonal origin; others
appeared to be of a longer cyclical nature.
4.
Census of the rodent populations was conducted
in eight major plant communities of the Refuge.
A com-
parison of trap success among communities indicated that
greatest numbers of rodents occurred in the Bunchgrassannual forb communities and the disturbed areas supporting
54
55
an annual forb grass complex.
5.
Seasonal fluctuations in the rodent populations
were severe, ranging from a high trap success of 48.3 to
a low of 1.9.
6.
The blackbird population fluctuated between
50,000 resident and migratory blackbirds during the winter
to 15,000 birds during the spring and summer.
The avail-
ability of blackbirds was never known to be limited.
7.
Census of insect populations was not conducted.
Their contribution to the biomass of the owls' diet was
insignificant.
8.
Nest initiation begins in the first three months
of the year, reaching a peak in March.
The duration of
nesting is irregular, lasting from two to ten months.
9.
The incubation period averaged 30 days.
the female was found incubating.
Only
The male fed his mate
during incubation.
10.
Clutches of eggs found in this study varied
from one to eight eggs per nest.
Of these, 91 were
judged complete and suitable for calculation of mean
clutch size.
Clutches of more than 2 eggs averaged 4.9
eggs per nest.
11.
Nesting success of the barn owl was 65.9 percent
over the 7-year period.
56
12.
Barn owl owlets remained in the nest seven to
eight weeks after hatching.
Then, they remained in the
care of the parent birds for approximately 8 to 12 weeks.
13.
Mortality at the nest included possible sibling
cannibalism, maternal cannibalism, and death from exposure
or starvation when the parents abandoned the nests.
14.
There were 1.5 times as many young raised per
pair in years when prey populations were abundant than
in years when prey populations were moderate to scarce.
15.
There was an apparent interaction between both
quantitative and qualitative nature of food as the otherwise abundant bird population alone did not sustain
successful barn owl breeding.
16.
The increase or decrease in any single small
mammal species appeared to have had no immediate effect
on barn owl nesting density or success.
However, when
small mammals decreased in availability to a point where
the barn owls relied on birds for at least 32 percent of
their diet, the productivity of the owl population was
greatly reduced.
17.
The hunting ranges of 3 radio-marked owls—2
males and 1 female—were determined to be roughly 160
acres.
REFERENCE BIBLIOGRAPHY
Bendire, C. E. 1892. Life histories of North American
birds. U.S. Nat. Mus. Spec. Bull. 1.
Bent, A. C. 1938. Life Histories of North American birds
of prey. Part II. Dover Publications, Inc., N.Y.
482 pp.
Box, T. W. 1961. Relationships between plants and soils
of four range plant communities in South Texas.
Ecology 42:794-810.
and A. D. Chamrad. 1966. Plant communities
of the Welder Wildlife Refuge. Welder Wildl. Foundation. Contrib. 5, Series B. 28 pp.
Cochran, W. W. and R. D, Lord, Jr. 1963. A radio-tracking
system for wild animals. J. Wildl. Mgmt. 27(l):9-23.
Cottam, C. 1956. Uses of marking animals in ecological
studies; marking birds for scientific purposes.
Ecology 37(4) :675-6Sl.
Craighead, J. J. and F. C. Craighead. 1956. Hawks,
owls and wildlife. Wildlife Management Institute,
Washington, D.C. 443 pp.
Davis, D. E. 1959. Manual for analysis of rodent populations. School of Hygiene and Public Health. The
John Hopkins Univ., Baltimore 5, Maryland. 82 pp.
Errington, P. L. 19 32.
study. Condor 34:
Technique of raptor food habits
75-86.
Evans, F. C. and Emlen, J. T. Jr., 1947.
on the prey selected by a barn owl.
3-9.
Ecological notes
Condor 49(1):
Glading, B., B. F. Tillotson, and D. M. Selleck. 1943.
Raptor pellets as indicators of food habits.
California Fish and Game 29:92-1921.
57
58
Henny, C. J. 1959. Geographical variation in mortality
rates and production requirements of the barn owl
(Tyto alba). Bird-Banding 40(4):277-290.
Howell, T. R. 1964. Notes on incubation and nestling
temperatures and behavior of captive owls. Wilson
Bull. 76(1) :28-35.
Ingram, C. 1959. The importance of juvenile cannibalism
in the breeding biology of certain birds of prey.
Auk 76(2):218-225.
Kendeigh, S. C. 1952. Parental Care and its evolution
in birds. Illinois Biol. Monogr. 22(1-3):358pp.
Krebs, C. J. 1966. Demographic changes in fluctuating
populations of Microtus californicus. Ecol. Monogr.
36:239-273.
Lack, D. 1947.
302-350.
Significance of clutch size.
Ibis 89:
. 1954. The natural regulation of animal
numbers. Clarendon Press, Oxford. 343 pp.
. 1966. Population studies of birds.
Clarendon Press, Oxford, v + 341 pp.
McCarley, W. H. 1958. Ecology, behavior, and population
dynamics of Peromyscus nuttalli in eastern Texas.
Texas J. Sci. 10:147-171.
Pickwell, G. 1948. Barn owl growth and behaviorisms.
Auk 65(3) :359-73.
Reed, C. I. and B. P. Reed. 1928. The mechanism of
pellet formation in the great horned owl. Science
68:259-260.
Southern, H. N. 1955.
193(4):88-98.
Nocturnal animals.
Sci. Amer.,
Thomas, G. W. 196 2. Texas plants, an ecological summary.
In Texas Agr. Expt. Sta. MP-585.
Wallace, G. J. 1948. The barn owl in Michigan; its
distribution, natural history and food habits.
Michigan Agr. Exp. Sta., Tech. Bull. No. 208. 61 pp.
APPENDIX
A.
Index and scientific names of plants
B.
Birds taken by barn owls on the Welder Wildlife
Refuge, 1965-1971
59
60
APPENDIX A :
INDEX AND S C I E N T I F I C NAMES OF PLANTS
Common Name
Scientific Name*
GRASSES:
Buffalograss
Buchloe dactyloides
Bristlegrass
Setaria leucopila
Switchgrass
Panicum virgatum
Texas Wintergrass
Stipa leucotricha
Pan American balsamscale
Elyonurus tripsacoides
TREES:
Live Oak
Quercus virginiana
Anaqua
Ehretia anacua
Mesquite
Prosopis glandulosa
RUSHES:
Tules
Scripus californicus
CACTUS:
Prickly-pear
Opuntia linheimeri
Gould, F. W. and T. W. Box. 1965. Grasses of the Texas
Coastal Bend. Texas A & M University Press, College
Station.
Vines, R. A. 1960. Trees, Shrubs and Woody Vines of
the Southwest. University of Texas Press, Austin.
61
APPENDIX B:
BIRDS TAKEN BY BARN OWLS ON THE
WELDER WILDLIFE REFUGE, 1965-1971
Number
of Individuals
Common Name
Scientific Name*
Least Bittern
Ixobrychus exilis
8
Sparrow Hawk
Falco sparverius
2
Bobwhite
Colinus virginianus
81
Virginia Rail
Rallus limicola
38
Yellow Rail
Coturnicops
noveboracensis
Common Snipe
Capella gallinago
1
Killdeer
Charadrius vociferus
3
Black-Necked Stilt
Himantopus mexicanus
1
Mourning Dove
Zenaidura macroura
9
Inca Dove
Scardafella inca
2
Yellow-Billed Cuckoo
Coccyzus americanus
1
Barn Owl
Tyto alba .
2
Scissor-Tailed Flycatcher
Muscivora forficata
3
Great Crested Flycatcher
Myiarchus crinitus
1
Ash-Throated Flycatcher
Myiarchus cinerascens
1
Barn Swallow
Hirundo rustica
6
Swallows
Petrochelidon spp.
8
Blue Jay
Cyanocitta cristata
1
Long-Billed March Wren
Telmatodytes palustris
4
62
APPENDIX B (Continued)
Common Name
Scientific Name
Number
of Individuals
Mockingbird
Mimus polyqlottos
7
Catbird
Dumetella carolinensis
1
Curve-Billed Thrasher
Toxostoma curvirostre
4
Sprague's Pipit
Anthus spragueii
1
Starling
Sturnus vulgaris
1
Palm V7arbler
Dendroica palmarum
1
Eastern Meadowlark
Sturnella magna
6
Red-Winged Blackbird
Agelaius phoeniceus
Orchard Oriole
Icterus spurius
Boat-Tailed Crackle
Cassidix mexicanus
182
Brown-Headed Cowbird
Molothrus ater
390
Bronzed Cowbird
Tangavius aeneus
Cardinal
Richmondena cardinalis
10
Pyrrhuloxia
Pyrrhuloxia sinuata
16
Indigo Bunting
Passerina cyanea
1
Painted Bunting
Passerina ciris
6
Dickcissel
Spiza americana
111
American Goldfinch
Spinus tristis
Savannah Sparrow
Passerculus
sandwichensis
Grasshopper Sparrow
Ammodramus savannarum
Vesper Sparrow
Pooecetes gramineus
435
2
1
2
11
63
APPENDIX B (Continued)
Common Name
Scientific Name
Number
of Individuals
Lark Sparrow
Chondestes grammacus
1
Cassin Sparrow
Aimophila cassinni
3
Lincoln's Sparrow
Melospiza lincolnii
3
Swamp Sparrow
Melospiza georgiana
1
^Nomenclature of AOU. 1957. Check-list of North American Birds. 5th ed. Baltimore.