17: 165-174. Copenhagen1986
ORNISSCANDINAVICA
Patch utilization by migrating birds: resource oriented?
Thomas E. Martin and James R. Karr
Martin T. E. and Karr, J. R. 1986. Patch utilization by migrating birds: resource oriented? - Ornis Scand. 17: 165-174.
Use of gap (created by tree falls) and non-gap forest understory sites by migrating
birds in central Illinois was studied during spring and autumn for three years (19781980). Fruit and understory foliage were concentrated in gaps. Birds that relied on
these resources (foliage-gleaning insectivores, frugivores in autumn) used gaps more
than non-gaps. Birds that fed on food other than fruit and foliage insects ("frugivores" in spring, other insectivores) did not use gaps more than non-gaps.
Bird abundance varied markedly among gap and non-gap sites, potentially reflecting
differences in site preferences. Site selection, as determined by bird abundances, was
consistent (correlated) between years for birds that fed on items that were concentrated in gaps but not for birds that did not rely on these patchy resources. Foliage
density is a measure of foraging substrates for foliage-gleaning birds to search. Abundance of foliage-gleaning insectivores was highly correlated with foliage density in
both spring and autumn. Frugivore abundance was highly correlated with fruiting foliage density during autumn when they are frugivorous, but not during spring when
they are insectivorous. Insectivores not relying on foliage insects or fruit were uncorrelated with either index of resource availability. These same relationships hold
even when examining gap sites only. Thus, migrants can be consistent in their selection of foraging sites and this consistency appears to exist when resource densities
are markedly different among sites (patchy) but not when resources are more dispersed.
T. E. Martin, Dept of Zoology, Arizona State University, Tempe, Arizona 85287,
U.S.A. J. R. Karr, Smithsonian Tropical Research Institute, Box 2072, Balboa, Republic of Panama.
1. Introduction
Migratory birds use considerable amounts of energy for
migration; they lose 1-4% of their gross body weight
per hour of flying (Graber and Graber 1962, Hussell
1969). As a result, migrants require periodical replenishment of lost fat stores at stop-over sites to allow successful completion of migration (Nisbet and Medway
1972, Berthold 1975). Individuals able to maximize
their foraging efficiency (rate of food intake) at these
stop-over sites increase their rate of fat deposition and
their chances of successful migration. One means of enhancing foraging efficiency is to migrate when food is
most abundant. Indeed, spring arrival of migrant warblers in southern Illinois coincides with eruptions of
their primary food (lepidopteran larvae); warbler num-
bers peak at the same time as their food (Graber and
Graber 1983).
When resources are patchy, foraging efficiency also is
enhanced by selecting habitat patches with more abundant food resources (Charnov 1976, Krebs et al. 1978,
Cowie and Krebs 1979, Martin 1985a). However, patch
selection by migrating birds is rarely studied. Instead,
analyses of habitat selection during migration typically
have been restricted to general habitat patterns (e.g.,
Parnell 1969, Laursen 1976, Rappole and Warner 1976,
Martin 1980). Only Willson et al. (1982) examined fine
scale habitat selection patterns during migration.
Willson et al. (1982) showed that migrants were more
abundant in understory of light gaps than in undisturbed
forest understory. Light gaps are created by tree falls.
More light reaches the forest floor in light gaps, causing
Received7 January1985
Accepted 20 June 1985
?
ORNIS SCANDINAVICA
11 ORNIS SCANDINAVICA 17:2 (1986)
165
increasedgrowth and colonizationby understoryvegetation (Hartshorn 1978, Thompson 1980, Runkle
1982). As a result, understoryfruitsand foliage insects
are more concentratedin light gap patchesthan in nongaps. Thus, preferencesfor light gaps may reflect selection of patcheswith abundantfood by migrants.
Yet, the density of plants, and associated food resources, differsamonggap sites (Runkle 1982). If birds
are truly selecting sites based on availabilityof resources,then two resultsare predicted:(1) Birdsshould
be more abundantsat sites with greateramountsof the
food types they eat. (2) Birdsshouldbe more abundant
at the same sites each year, assuming differences in
qualityamongsites varies only a little amongyears. To
examine these predictions, we established permanent
gap and non-gapsites and monitoreduse of the understory of these sites by migratingbirdsduringspringand
autumnfor three years. We focus on three questions.
First,do migratingbirdsof differingforaginghabitsdiffer in their abundanceat gap versusnon-gapunderstory
sites? Second, are birdsconsistentin their site selection
(i.e., more abundantin the same sites) each year? Finally, are differences in bird abundancesamong sites
correlatedwith differences in indexed resource abundance?
1978-1980. Autumn was divided into early (before 15
September) and late (after 15 September) subseasons.
Nets (30 or 36 mm mesh, 4 shelves, 12 m long) were
paired between gap and non-gap sites in the forest un-
derstory. Gap nets were placed at the edge of gaps
rather then in the centres to minimize net visibility as a
bias on capture rates. Similar methods have been used
by Schemske and Brokaw (1981) and Willson et al.
(1982) to examine gap and non-gap use by birds. A core
of 10 (5 pairs) nets was placed in the same locationsin
all seasons and years to examine consistency of site preferences by birds. The gaps used for these locations were
all old and well established with dense shrub under-
story. In each season an additional gap:non-gapnet
pairing was placed in a new gap (< 1 year old) that had
not developed a shrub understory.
A total of 5,212 mist net hours (MNH) was accrued
over the six seasons with 458, 476, and 765 MNH during
springs, and 1205, 1344, and 964 MNH for autumns of
1978, 1979, and 1980, respectively. Nets were opened 30
min before to 30 min after sunrise and left open for 4-6
hours in all seasons.
Vegetation cover was sampled during autumn 1979,
when plants were fruiting, using the point sample
method of Karr (1971) in which presence/absence of
vegetation was noted at the following height intervals:
G (ground), < 0.25, 0.25-0.5, 0.5-1, 1-2, 2-3, 3-5, 52. Study area and methods
7.5, 7.5-10, 10-12.5, 12.5-15, 15-20, and > 20 m. NumThe study site was Trelease Woods, a 22-ha woodlot lo- ber of points in which vegetation was present relative to
cated northeast of Urbana, Illinois, USA. Woodlots and total number of points sampled provided percent vegotherforestislandsare importantsourcesof foresthabi- etation cover in each height interval. A total of 30
tat for migratingand breedingbirdsin the mid-western points was sampled on three transects at each net. Two
U.S. (Martin 1980, 1981, Blake 1983). Principaltree transects ran parallel to the net at a distance of 3-4 m
species in TreleaseWoods includedoaks Quercusspp.,
from the net on each side with points taken every 1 m.
sugar maple Acer saccharum, elm Ulmus, white ash
Fraxinus americana, basswood Tilia americana, and
hackberry Celtis occidentalis. The understory included
paw paw Asimina triloba, spicebush Lindera benzoin,
grape Vitis spp., moonseed Menispermum canadensis,
pokeweed Phytolacca americana, Virginia creeper Parthenocissus, and poison ivy Toxicodendron radicans.
The third transect ran perpendicular to and bisected the
net with points taken every 2 m. Foliage of individual
plants that actually bore bird-dispersed fruits (males
and some female individuals of some plants species did
not bear fruit) was tabulated separately to provide foliage cover of fruiting plants. Each site was ranked relative to total foliage cover and fruiting foliage cover in
Most censusproceduresused in studiesof birdswere the understory (< 3 m). Analyses based on ranks are
designedfor use duringbreedingseasonwhen most spe- preferred because true availabilities of resources to anicies are relativelysedentaryand vocal. Studies during mals are difficult to measure accurately (Johnson 1980).
other seasons and in regions such as tropical forest
Abundance of birds at each net site were ranked
where assumptionsof more classicalproceduresare not based on capture rates (birds/100 MNH) and compared
met requiredifferentmethods (Karr1979, 1981). Birds among seasons and years and with foliage and fruit
during migration are highly transient and often are not cover rankings using a Spearman rank correlation.
vocal, especially in autumn. Thus, we used mist-nets to Comparisons among seasons and years allow examinmeasure avian use of patches because: (1) We were in- ation of whether birds are consistently more abundant
terested in avian use of forest understory patches from 0 at the same net sites. Comparisons with fruit and foliage
to 3 m above the ground and mist-nets sample the un- cover rankings allow examination of whether birds are
derstory,(2) mist-netsdo not depend on vocalizations more abundant at sites with more fruit or foliage cover.
or sedentary birds, and (3) mist-nets allow examination
Correlations were based only on the 10 sites netted in all
of avian use of small areas, which is a centralfocus of 6 seasons.
this study.
Comparisons of capture rates among net sites, microPatchuse by birdswas sampledduringspring(15 Ap- habitats, seasons and years were made using the Fisher
ril - 29 May) and autumn (24 August - 18 October) of binomial probability test when sample size was small (n
166
ORNIS SCANDINAVICA 17:2 (1986)
< 35) and by X2analysis for large samples. Food habit
assignments followed Willson et al. (1982) except for
Yellow-rumped Warbler and American Redstart.
Yellow-rumped Warblers (scientific names of all birds
presented in Appendices A,B) were classified as foliage-gleaning insectivores rather than frugivores because Yellow-rumped Warblers were observed feeding
on fruit less than 1% of the time during the periods of
this study (n > 750 observations, unpublished data).
Yellow-rumps appeared to be more frugivorous after
mid-October (TEM, pers. obs.). American Redstarts
were also classified as foliage-gleaning insectivores because hawking (flycatching) represented less than 25%
of their foraging maneuvers (n > 500 observations, unpublished data, also Sherry 1979).
Tab. 1. Capturerates (birds/100mist-nethours)for birdscaptured in light gaps and undisturbed(non-gap) understoryin
spring,early(24 August- 14 September)and late (15 September - 18 October)autumn1978-1980.
Autumn
Gap
1978
1978
1979
1979
37.0
40.1
77.7
55.7
**
Early 1980 97.4
Late 1980 63.1
b) Frugivores (12 species)
Early 1978 6.8
Late 1978 17.2
3. Results
Gap Non-Gap
Non-Gap
a) All birds (70 species)
Early
Late
Early
Late
Spring
14.7*
1978 53.6
21.9*
42.9*
22.3*
1979 93.3
35.7*
71.6*
27.6*
1980 68.2
34.8*
20.5*
**
**
4.1
6.2*
1978
5.2
4.3
6.2
6.6*
1979 19.7
13.4
7.0
7.9*
1980 10.2
8.1
**
Early 1979 9.9
Late 1979 21.1
3.1. Vegetation
Light gaps had more (p < 0.001) foliage from ground to
3 m than non-gap sites (Fig. 1), while non-gap sites had
more foliage above 3 m. In addition, gaps had more (p
< 0.01, t-test) fruit foliage (x = 32.1%, SE = 10.6) below 3 m than non-gap sites (x = 4.6%, SE = 6.7) due to
a greater density of fruit plants in light gaps than nongaps.
3.2. Use of gaps versusnon-gaps
Early 1980 12.9
Late 1980 24.6
**
c) Foliage-gleaning insectivores (30 species)
Early 1978 15.8
Late 1978 6.8
2.6*
3.3*
1978 33.9
12.4*
Early 1979 28.5
Late 1979 7.8
10.2*
3.2*
1979 49.2
6.7*
Early 1980 35.3
Late 1980 7.9
20.7*
3.9
1980 32.9
8.1*
1978
8.5
2.9*
1979 10.9
4.6*
1980 13.3
8.4*
1978
2.4*
**
Significantly more birds were captured in gaps than in
non-gaps for all seasons, subseasons, and years (Tab.
Vegetation
**
Profiles
**
**
**
d) Other insectivores (16 species)
Early
Late
Early
Late
1978 9.4
1978 13.9
1979 36.8
1979 23.6
**
Early 1980 44.8
Late 1980 19.7
7.5
7.7*
25.7*
10.1*
**
44.8
8.9*
**
'NO N-GAP
e) Granivores (12 species)
-.4
I
I
LJ
I
Early
Late
Early
Late
Early
Late
1978
1978
1979
1979
1980
1980
4.9
2.1
2.6
3.1
2.2
5.4
**
0.4*
3.3
0.8*
2.5
0.0*
3.4
6.0
1979 13.4
10.9
1980 11.8
10.2
**
*
gap/non-gapcomparison:p < 0.05;
** early/lateautumncomparison:p < 0.05.
0
10 20 30 40 50 60 70 80 90
O/o VEGETATION
COVER
Fig. 1. Vegetationprofilesof gaps and non-gapareas. Profiles
are basedon the percentvegetationcoverin each of a seriesof
height intervalsbased on point samples.
11*
la). However, the extent and direction of habitat selection varied among groups of species with similar food
habits.
3.2.1. Frugivores
Both as a group and as individual species, frugivores
167
Tab. 2. Ratio of the numberof species that were more abundantin gapsto the numberof speciesthatwere moreabundant
in non-gapsfor each of the three yearsand for the three years
combined. Probability(Prob) refers to the probabilityof the
combinedratio being even (1:1).
1978 1979 1980 Combined Prob
Spring
Frugivores
Foliage-gleaning
insectivores
Other insectivores
Granivores
Early autumn
Frugivores
Foliage-gleaning
insectivores
Other insectivores
Granivores
0:0
1:0
0:0
1:0
ns
4:0
1:0
1:0
9:0
1:0
0:0
6:0
0:0
0:0
19:0
2:0
1:0
<0.001
ns
ns
0:0
0:0
0:0
0:0
ns
3:0
0:0
1:0
5:0
1:0
0:0
1:0
0:1
0:0
9:0
1:1
1:0
0.002
ns
ns
3:0
4:0
2:0
9:0
0.002
1:0
1:0
1:0
1:0
1:0
0:0
0:0
1:0
0:0
2:0
3:0
1:0
ns
ns
ns
Late autumn
Frugivores
Foliage-gleaning
insectivores
Other insectivores
Granivores
showed patterns of capture that varied with season and
habitat. More frugivores were captured in late than
early autumn (Tab. lb). Frugivores were captured more
often in gaps than non-gaps during late autumn, but not
during spring or early autumn (Tab. lb). These trends
were reflected by the individual frugivore species. Only
the Wood Thrush in spring 1979 showed significant
habitat discrimination outside the late autumn period,
while several species were more abundant in gaps in all
of the late autumn seasons (Appendices A, B). The
number of frugivore species that were more abundant in
gaps was significantly greater than the number of species that were more abundant in non-gaps in late autumn, but not early autumn or spring (Tab. 2). Thus,
frugivores, as a group, used gaps more than non-gaps in
late autumn, but not in spring or early autumn.
3.2.2. Foliage-gleaning insectivores
More foliage-gleaning insectivores were captured in
gaps than non-gaps and in early than late autumn (Tab.
lc). Several foliage-gleaning species were significantly
more abundant in gaps than non-gaps in every season
(Appendices A, B). Comparison of the number of species that were more abundant in gaps to the number that
were more abundant in non-gaps showed insectivores
used gaps more than non-gaps (Tab. 2). Thus, foliagegleaning insectivores in the understory clearly use gap
more than non-gap sites.
3.2.3. Other insectivores
Other insectivores generally were captured more frequently in gaps than non-gaps, although the difference
was not significant in all cases (Tab. ld). The numbers
of species that were more abundant in gaps were not
greater than the numbers that were more abundant in
168
non-gaps, indicating that other insectivores do not prefer gaps (Tab. 2).
Ovenbirds were significantly more abundant in gaps
than non-gaps in several seasons (Appendices A, B). Of
the remaining 15 other insectivore species, only one
species (Winter Wren) in only one subseason (late autumn 1978) was significantly more abundant in gaps
(Appendices A, B). Thus, only the ovenbird among the
species in the other insectivore group consistently used
gaps significantly more than non-gaps. If ovenbirds (the
most abundant "other insectivore", Appendices A, B)
are excluded then abundance of "other insectivores"
was not greater (p > 0.05) in gaps in any of the spring or
early autumn seasons nor in two of the three late autumn seasons.
Other insectivores declined significantly (p < 0.05) in
abundance from early to late autumn in two of the three
years, whether or not ovenbirds were included (Tab.
ld). Thus, other insectivores did not use gap more than
non-gap understory, in contrast to foliage-gleaning insectivores, but other insectivores did decline in abundance during autumn similar to foliage-gleaning insectivores.
3.2.4. Granivores
Granivores were captured more frequently in gaps than
non-gaps in early autumn in all three years, but not during spring or late autumn (Tab. le). Comparison of the
number of species that were more abundant in gaps to
the number that were more abundant in non-gaps indicated gaps were not preferred in any season (Tab. 2).
Thus, a preference for gaps by granivores appears
weak.
In summary, all insectivores (foliage-gleaning and
other) were more abundant in early than late autumn,
while frugivores were more abundant in late than early
autumn, and granivores exhibited no trend. Foliagegleaning insectivores used gaps more than non-gaps in
all seasons. Frugivores used gaps more than non-gaps
only during late autumn. Granivores used gaps more in
early autumn, and other insectivores did not use gaps
more than non-gaps in any season.
Tab. 3. Consistencyof site selectionbetweensucceedingyears
based on rankcorrelationsof captureratesamong10 net sites
for all birdsand individualfood habitsgroups.
Spring
Autumn
1978-19791979-19801978-19791979-1980
All birds
Foliage-gleaning
insectivores
Frugivores
Otherinsectivores
0.84**
0.90**
0.91**
0.95**
0.87**
0.03
0.55
0.88**
0.60
0.77*
0.90**
0.98**
0.49
0.81**
0.95**
0.65
* <
p 0.05; **p< 0.01.
ORNIS SCANDINAVICA 17:2 (1986)
Tab.4. Rankcorrelationsof capturerateswith fruitingandtotal understoryfoliage densitiesamong 10 net sites for all birds
and individualfood habit groups.See text for anges of v ariation in capturerates and foliage and fruit densities.
All
FoliageOther
gleaning
Frugivores insectivoresinsectivores
Spring 1978
Fruit
Foliage
0.782**
0.909**
0.039
0.288
0.724*
0.888**
0.582
0.745*
Spring 1979
Fruit
Foliage
0.915**
0.945**
0.545
0.548
0.830**
0.933**
0.803**
0.639*
0.948**
0.900**
0.285
0.139
0.855**
0.939**
0.582
0.230
0.976**
0.891**
0.806**
0.855**
0.685*
0.491
0.909**
0.855**
0.948**
0.830**
0.803**
0.888**
0.782**
0.745*
Autumn 1980
Fruit
0.833**
0.867**
Foliage
0.979**
0.813**
0.838**
0.796**
0.652
0.785*
Spring 1980
Fruit
Foliage
Autumn 1978
0.900**
Fruit
0.855**
Foliage
Autumn1979
Fruit
Foliage
* p < 0.05, ** p < 0.01.
3.3. Site selection
correlated with both total and fruit foliage during autumn in all three years. Moreover, frugivores were
more highly correlated (p < 0.01) with fruit foliage than
total foliage during autumn (Tab. 4) when they are frugivorous (Thompson and Willson 1978, 1979, Baird
1980, E. Stiles 1980).
Foliage-gleaning insectivores were correlated with total and fruit foliage during both spring and autumn in all
three years. Foliage-gleaners were more highly correlated (p < 0.02) with total foliage than fruit foliage during five of the six seasons (Tab. 4).
Other insectivores, which relied on foods other than
foliage insects and fruits and which were inconsistent in
their site selection, did not exhibit any predictable association with total or fruit foliage. Correlations were significant in 7 of the 12 cases (Tab. 3) but showed no clear
association with either fruit or total foliage. If Ovenbirds were not included then other insectivores were not
correlated (p < 0.05) with either total or fruit foliage in
any spring or autumn.
Association of bird variation with foliage variation
may simply reflect greater use of gap understory by
birds and that gaps have greater foliage and fruit densities than non-gaps. To test whether variation in bird
abundances among sites actually tracked variation in total understory and fruit foliage among sites, variation of
bird abundances was examined relative to total and fruit
foliage for only the gap sites. Foliage-gleaning insectivore abundance among gap sites was correlated with
total understory foliage in both seasons (Tab. 5). Frugi-
3.3.1. Consistency of site selection among years
Capture rates varied markedly among net-sites, from
1.9 to 194.1 birds/100 MNH. If site selection by birds is
consistent and site quality varies only a little among
years, then abundance of birds at the same sites should
be correlated among years. However, if site selection is
random, no correlation should occur.
Correlations of capture rates among the net sites
show that foliage-gleaning insectivores were highly consistent in their site selection between years in both seasons (Tab. 3). Frugivores were not consistent in site selection during spring, but exhibited consistent preferences during autumn. Other insectivores were
inconsistent during all seasons and years, except spring
1979-1980.
Tab.5. Rankcorrelationsof capturerateswithfruitingandtotal understoryfoliage densitiesamong5 gap sites for all birds
and individualfood habitsgroups.
3.3.2. Association of bird abundance with fruit and
total understory foliage cover
Foliage cover of fruiting shrubs in the understory (? 3
m) varied from 0 to 42% and total understory foliage
varied from 19 to 68%. Variation in number of birds
was correlated (p < 0.01) with variation in both total
and fruiting foliage for the 10 sites netted in all seasons
(Tab. 4), but ecological groups differed in their response. Frugivores were not correlated with either total
or fruit foliage during spring when they were inconsistent in their selection of sites. Frugivores, however, were
Autumn1978
ORNIS SCANDINAVICA 17:2 (1986)
All
Foliagegleaning
Other
Frugivores insectivores insectivores
Spring1978
Fruit
Foliage
0.50
0.90*
-0.30
0.00
0.50
0.90*
0.00
0.60
Spring1979
Fruit
Foliage
0.90*
1.00**
0.25
0.80
0.90*
1.00**
0.48
0.08
Spring1980
Fruit
Foliage
0.90*
0.98**
0.10
0.40
0.90*
1.00**
0.68
0.08
0.88*
0.38
0.90*
0.90*
0.20
0.70
0.70
0.30
Fruit
0.98**
0.98**
0.70
0.60
Foliage
0.90*
0.90*
0.90*
0.30
0.88*
0.38
0.90*
0.90*
0.90*
0.60
Fruit
Foliage
Autumn1979
Autumn1980
Fruit
1.00**
0.70
Foliage
* p < 0.05; ** p < 0.01.
169
vore abundances were not correlated with either foliage
measure during spring, but were correlated with fruit
foliage in all three autumn seasons. Other insectivores
were not correlated with either measure during either
season.
Thus, migrants that rely on foliage insects or fruit appeared to be more abundant in sites having more of the
respective food resource. Migrants that do not rely on
foliage insects or fruit were not more abundant in sites
with more foliage and/or fruit.
3.4. New gaps
Birds may prefer understory of light gaps over undisturbed forest understory simply because more light is
available to see and find fruit and insect resources. If
true, then migrants should use areas with high light intensity without regard to understory foliage and/or fruit
availability. This possibility was tested by establishing
nets in newly created light gaps (less than 1 yr old) in
each season, except autumn 1980. Thus, these new gaps
had high light intensity due to canopy gaps, but understory foliage and fruit density did not differ (p > 0.50)
from non-gaps because of the lack of time for establishment of light-released plants.
In all cases, new light gaps had lower capture rates
than older gaps (Tab. 6). New light gaps were included
in previous analyses with the result that overall capture
rates in all gaps (Tab. la) were less than capture rates in
old gaps (Tab. 6). Thus, earlier analyses of gap preferences were conservative. Capture rate in new gaps was
significantly higher than the mean for all non-gaps in
two of the five cases and insignificantly higher in the
other three cases (Tab. 6). Thus, birds may be attracted
to gaps because of light conditions present in such areas,
Tab. 6. Capturerates (birds/100MNH) in new gaps relativeto
the mean of old gaps and non-gaps.Significantdifferencesreflect differencein the new gap relativeto old gaps or new gap
relativeto non-gap.
Spring
Capture
Autumn
rate
Capture
SE
rate
SE
1978
New Gap
Old Gaps
Non-gaps
33.0
64.5*
18.1*
11.23
5.78
19.8
45.4*
17.4
7.55
2.18
1979
New Gap
Old Gaps
Non-gaps
41.2
118.2*
34.5
27.20
5.08
47.4
81.6*
31.8*
11.79
6.02
1980
New Gap
Old Gaps
Non-gaps
34.9
73.7*
33.8
12.99
7.34
* p<0.05.
170
but the higher capture rates in old gaps suggest that they
may remain in gaps only if recource abundance is high.
4. Discussion
4.1. Responseto resourceavailability
Some birds using gaps primarily forage in forest canopies, but they move down when breaks in the canopy
occur (F. Stiles 1980). Consequently, gaps may not be as
preferred as non-gap forest canopy by these species.
However, our objectives were not to determine whether
gaps were preferred over non-gaps by individual species. Rather, when birds use forest understory, we were
concerned with determining whether their use of understory patches was consistently related to resource availability. Indeed, three primary results indicate migrating
birds are resource oriented in their choice of patches
(also see Martin 1985b).
First, birds that depend on resources concentrated in
gaps use gaps more than non-gaps (Tabs 1, 2). Foliagegleaning insectivores used gaps more than non-gaps
during all seasons. Frugivores are frugivorous primarily
during late autumn (after 15 September) when fruits are
readily available (Thompson and Willson 1979, Baird
1980, Moore and Willson 1982) and they used gaps
more than non-gaps during this period. "Frugivores"
are primarily insectivorous during spring and they appeared to include large proportions of insects in their
diet during early autumn based on examination of feces
of captured individuals (TEM, unpubl. data). Most species designated as "frugivores" depend on insects other
than foliage insects when they are insectivorous. Consequently, "frugivores" did not use gaps more than nongaps during spring and early autumn when they were insectivorous. Similarly, other insectivores primarily included flycatchers and bark drillers and gleaners. Little
reason exists to expect their resources to be concentrated in gaps and they did not use gaps more than nongaps. Thus, migrants used gaps more than non-gaps
during periods when they relied on resources that were
concentrated in gaps (foliage insects, fruits), but migrants that did not rely on these patchy resources did
not use gaps or non-gaps more.
Second, consistency in relative abundances of birds at
sites among years (Tab. 3) documents that birds must be
choosing patches based on some characteristic associated with the patches. Food abundance seems the
likely cause of the consistent patch choice because migrants were consistently more abundant at the same
sites in succeeding years when they relied on resources
(fruit, foliage insects) that were concentrated in the
patches sampled in this study. Migrants that relied on
resources that were unlikely to be concentrated in the
patches included within our sampling regime were not
consistent in their site choice among years.
Third, correlations with resource indices indicate that
site selection is related to resource availability (Tab. 4).
Foliage density only indexes foliar insect availability,
ORNIS SCANDINAVICA 17:2 (1986)
but it directlymeasuresavailabilityof foliage substrates
for insects to use and for birds to search. At the same
time, it may index cover from predation (see below).
Regardlessof the resourcethat this index measures,selection of sites by foliage-gleaninginsectivoreswas correlatedwith it in all seasons. The fruit index also is an
indirect measure of food availability,but selection of
sites by frugivoreswas correlatedwith it in all three autumn seasons. Further,frugivoreswere not simply respondingto understoryfoliage becausethey were more
highly correlatedwith the fruit index than the total foliage index in all three years. In addition,they were not
correlatedwith the fruit index duringspringwhen they
did not consumefruits. Other insectivoresdid not feed
on fruitsor foliage insects and they were not correlated
with either index. Finally, the above relationshipsalso
existed when gap sites were examinedseparately(Tab.
5) indicatingmigrantsare respondingto differencesin
resourcesamong sites and not just between gap versus
non-gap understory.Thus, abundancesof migrantsat
the understorysites were closely related to indexed resource availabilitywhen migrantsconsumed resources
measuredby the indices, but not when they consumed
other resources.
4.2. Is patch choice related to factors other than food
resources?
4.2.1. Perching sites or safe cover
Birds may select sites with more foliage because they
providegreaternumbersof perchingsites or more cover
from predation. This alternativeis unlikely for frugivores and other insectivores.Otherinsectivoresshowed
no preferencefor sites with greaterdensitiesof foliage
in any season. Frugivoresprefersites withgreaterdensities of fruit foliage ratherthan total foliage duringautumn and they show no preferencefor sites with more
foliage duringspring. Foliage-gleaninginsectivores,on
the other hand, were correlatedwith total understory
foliage and, thus, may be respondingto cover in addition to or insteadof food abundance.However, the increasedfood requirementsof migrantsduringmigration
mayrequiremore investmentin foragingat the expense
of other activitiessuch as anti-predatorbehavior(Metcalfe and Furness 1984). The fact that frugivoresand
other insectivoresdid not base patch choice on cover,
per se, also suggests that, within a habitat, cover may
not be as importantas food for patchchoice duringmigration.
4.2.2. Mist-net bias
they feed on a resourcethat is concentratedin gaps and
they show no preference for gaps duringspring when
their food is not concentratedin gaps. Second, preferences for gapscan not reflectmist-nestsinterceptingdifferent foliage profilesin gaps as comparedto non-gaps
becausethe correlationswith resourceindicesexist even
when the gap sites are examinedseparately(Tab. 5).
4.3. Resource tracking
If migrantsare selecting sites based on resource availability, then how do they assess resource abundance?
Resource samplingmay be necessaryin new or changing environments(Heinrich 1976, Oster and Heinrich
1976, Krebs et al. 1978), and once patches have been
sampledbirds use patches relativeto their profitability
(Smith and Sweatman 1974, Cowie and Krebs 1979,
Krebs et al. 1978). However, northern migrantsthat
stop over at a site duringmigrationcan not know the
distributionof resources.
Migrantsoften move in mixed-speciesflocks (Morse
1970)and flockingmay aid in resourcetracking.Flocks
may form a sort of informationcenter for patch profitability. The informationcenter hypothesiswas originally suggested to operate in communallyroosting or
colonially nesting birds (e.g. Ward and Zahavi 1973,
Krebs 1974, Hunt and Hunt 1976, Erwin 1978, Wiksne
and Janaus1980, Anderssonand Gotmark1980). However, the hypothesisthat individualsconveyinformation
as to the location of rich food sources has been questioned (Evans 1982, Bayer 1982) and even refuted for
some bird species by experimentaltest (Andersson et
al. 1981). On the otherhand,local enhancementoccurs,
whereindividualsare attractedto groupsof successfully
feeding individuals(Krebs et al. 1972, Krebs 1974, Inglis and Isaacson 1978, Andersson et al. 1981, Evans
1982). Thus, the success of birds feeding in "richer"
gaps may attractincreasingnumbersof other birds.
In addition, individualsmay stay longer in richer
patches and, consequently, more individualsaccumulate at good areas. This accumulationmay also increase
the probability of attracting other individuals. The
slightly higher capture rates in new gaps than in nongaps (Tab. 6) suggests that birds may use light as one
cue for sampling.However, the highercapturerates in
old gaps relative to new gaps indicatesthat birds only
spend a short time in the patch if resourcesare poor.
The above alternativesneed to be tested by manipulatingresourcerichnessin gapsand non-gapsandquantifying the recruitmentof birdsto resourceavailability.
Overall, this study shows that migratorybirds are
consistent in their patch choice and that variationsin
abundanceof migrantsamong sites generally reflects
variationsin indices of resourceabundanceswhen the
resourcesare patchy.
Preferencesfor sites may reflect a bias due to the samplingmethod(i.e., mist-netting).However,this alternative is unlikelyfor two reasons.First,speciesdesignated
as frugivoresare primarilyground foragersand, thus,
are equally susceptibleto net capturesin all sites. Yet Acknowledgements- We thank E. A. Porter for field assisthey show a preferencefor gaps during autumnwhen tance and T. Alerstam, G. Hogstedt, R. A. Johnson, J. N.
ORNIS SCANDINAVICA 17:2 (1986)
171
Thompson and M. F. Willson for constructive comments on the
manuscript. This work was supported by grants from Wildlife
Management Institute to T. E. Martin, Max McGraw Wildlife
Foundation to T. E. Martin and J. R. Karr, and U. S. Forest
Service to J. R. Karr.
Laursen, K. 1976. Feeding ecology of the goldcrest (Regulus
regulus) during spring migration in Denmark. - Vogelwarte
28: 180-190.
Martin, T. E. 1980. Diversity and abundance of spring migratory birds using habitat islands on the Great Plains. -
Condor82: 430-439.
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ORNIS SCANDINAVICA 17:2 (1986)
Appendix A. Capture rates (birds/100 MNH) in gap and non-gap sites for all bird species captured during spring migration of
1978-1980. *p < 0.05.
Frugivores
Common Flicker
Colaptes auratus
Veery
Catharus fuscenscens
Gray-cheeked Thrush
C. minimus
Swainson's Thrush
C. ustulatus
Hermit Thrush
C. guttatus
Wood Thrush
Hylocichla mustelina
American Robin
Turdus migratorius
Gray Catbird
Dumetella carolinensis
Brown Thrasher
Toxostoma rufum
Red-eyed Vireo
Vireo olivaceus
Summer Tanager
Piranga rubra
Scarlet Tanager
P. olivacea
Foliage-gleaning insectivores
Ruby-crowned Kinglet
Regulus calendula
Solitary Vireo
Vireo solitarius
Blue-winged Warbler
Vermivora pinus
Golden-winged Warbler
V. chrysoptera
Tennessee Warbler
V. peregrina
Orange-crowned Warbler
V. celata
Nashville Warbler
V. ruficapilla
Northern Parula
Parula americana
Chestnut-sided Warbler
Dendroica pennsylvanica
Magnolia Warbler
D. magnolia
Cape May Warbler
D. tigrina
Yellow-rumped Warbler
D. coronata
Black-throated Green Warbler
D. virens
Blackburnian Warbler
D. fusca
Palm Warbler
D. palmarum
Bay-breasted Warbler
D. castanea
Cerulean Warbler
D. cerulea
American Redstart
Setophaga ruticilla
Worm-eating Warbler
Helmitheros vermivorus
Northern Waterthrush
Seiurus noveboracensis
Kentucky Warbler
Oporornis formosus
Spring 1978
Spring 1979
Spring 1980
Spring 1978
Spring 1979
Spring 1980
NonGap Gap
NonGap Gap
NonGap Gap
NonGap Gap
NonGap Gap
NonGap Gap
0.4
0.4
0.3
0.5
1.6
0.5
0.8
1.3
0.3
0.5
0.4
0.5
0.8
0.8
0.5
1.0
0.8
1.0
7.6
7.1
2.4
1.8
0.8
1.4
1.7
1.0
2.5
0.4
1.6
0.5
1.3
1.6
2.5
1.7
2.6
0.8
2.1
0.8
0.8
0.3
0.5
0.3
0.8
1.2
-
0.4
0.8
- 0.5
0.4
0.8
0.3
3.4
0.3*
0.4
-*
2.8
1.6
2.8
8.9
1.0
6.7
8.0
0.8*
0.8*
0.5*
2.4*
6.3
-*
0.4
2.8
0.5*
2.5
0.8
0.4
0.5
0.8
1.0
2.5
1.6
2.4
7.1
2.5*
1.3
5.6
2.9
2.1
0.3
-
0.3
-
1.0
0.5
0.3
-
2.6
0.5*
0.3
-
0.5
*
3.4
0.3
-
7.3
1.8*
0.8
0.3
0.3
-
0.5 0.5
0.5
-
0.3
-
4.4
1.6*
1.7
0.4
12 ORNISSCANDINAVICA17:2(1986)
-
2.9
1.3
0.4*
0.8
0.5
0.8
0.3
Connecticut Warbler
0. agilis
Mourning Warbler
O. philadelphia
Common Yellowthroat
0. trichas
Hooded Warbler
Wilsonia citrina
Wilson's Warbler
W. pusilla
Canada Warbler
W. canadensis
Northern Oriole
Icterus galbula
0.3
0.4
0.4
-
0.5
0.3
0.8
1.4
2.5
- 0.4
1.0
-*
0.8
0.8
0.3
Other insectivores
Red-headed Woodpecker
Melanerpes erythrocephalus
Red-bellied Woodpecker
M. carolinus
Yellow-bellied Sapsucker
1.2
Sphyrapicus varius
Downy Woodpecker
0.4
Picoides pubescens
Hairy Woodpecker
P. villosus
Eastern Wood Pewee
Contopus virens
Yellow-bellied Flycatcher
0.4
Empidonax flaviventris
Acadian Flycatcher
E. virescens
Great Crested Flycatcher
Myiarchus crinitus
Red-breasted Nuthatch
Sitta canadensis
Brown Creeper
Certhia americana
House Wren
Troglodytes aedon
Winter Wren
T. troglodytes
Black-and-White Warbler
Mniotilta varia
Ovenbird
Seiurus aurocapillus
Granivores
Blue Jay
Cyanocitta cristata
European Starling
Sturnus vulgaris
Northern Cardinal
Cardinalis cardinalis
Rose-breasted Grosbeak
Pheucticus ludovicianus
Indigo Bunting
Passerina cyanea
Lincoln's Sparrow
Melospiza lincolnii
Swamp Sparrow
M. georgiana
White-throated Sparrow
Zonotrichia albicollis
Dark-eyed Junco
Junco hyemalis
Common Grackle
Quiscalus quiscula
Brown-headed Cowbird
Molothrus ater
American goldfinch
Carduelis tristis
-
2.0
0.5
0.4
0.5
0.3
0.5
0.4
-
0.8
0.3
0.5
0.3
0.3
0.3
0.5
-
0.4
0.5
0.3
0.5
0.8
0.3
2.5
0.8
0.5
0.8
0.8
0.5
0.8
0.4
0.8
0.8
1.0*
6.3
1.7*
8.4
5.2
-*
0.8
0.8
1.3
1.0
2.4
1.3
0.8
0.5
1.0
0.4
-
1.3
1.3
0.8
2.4
2.4
5.9
6.3
3.4
3.9
2.5
0.8
0.5
0.5
0.3
0.8
-
0.4
0.8
1.0
2.5
0.3
0.4
2.5
0.8
0.3
0.3
0.8
173
Appendix B. Capture rates (birds/100 MNH) in gap and non-gap sites for all bird species captured during early (24 August - 14
September) and late (15 September - 18 October) autumn migrations of 1978-1980. * p < 0.05.
Late
Early
Autumn 1980
Autumn1979
Autumn 1978
Late
Early
Late
Early
Gap Non-Gap Gap Non-Gap Gap Non-Gap Gap Non-Gap Gap Non-Gap Gap Non-Gap
Frugivores
Common Flicker
Veery
Gray-cheeked Thrush
Swainson's Thrush
Hermit Thrush
Wood Thrush
American Robin
Gray Catbird
Brown Thrasher
Red-eyed Vireo
Scarlet Tanager
Foliage-gleaning insectivores
Golden-crowned Kinglet
Regulus satrapa
Ruby-crowned Kinglet
Golden-winged Warbler
Tennessee Warbler
Nashville Warbler
Chestnut-sided Warbler
Magnolia Warbler
Black-throated Blue Warbler
Yellow-rumped Warbler
Black-throated Green Warbler
Blackburnian Warbler
Bay-breasted Warbler
American Redstart
Northern Waterthrush
Kentucky Warbler
Common Yellowthroat
Wilson's Warbler
Canada Warbler
Other insectivores
Black-billed Cuckoo
Coccyzus erythrocephalus
Red-bellied Woodpecker
Downy Woodpecker
Eastern Wood Pewee
Yellow-bellied Flycatcher
Red-breasted Nuthatch
Brown Creeper
Winter Wren
Black-and-White Warbler
Ovenbird
Granivores
Blue Jay
Northern Cardinal
Rose-breasted Grosbeak
White-throated Sparrow
Dark-eyed Junco
Common Grackle
174
0.4
1.5
1.5
3.8
1.9
0.4
0.4
0.4
0. 4
0.4
0.9
0.3
1.2
6.5
4.2
2.4
0.3
1.2
0.3
0.3
1.5*
3.3
0.3*
0.3
*
0.3
0.3
-
0.6
0.4
2.6
0.4
-
-*
-
3.0
0.3
0.4
2.3
0.8
0.4
0.6
-
0.4
0.3
1.8
1.1
0.6
0.6
0.8
0.8
2.3
4.2
0.6
0.6
0.8
1.1
2.8
2.1
0.3
0.3
0.4
0.4
2.3
2.0
0.3
1.4
4.8
0.8
1.4
4.0
1.1
0.4*
0.4
0.8
0.4
1.7
0.3
2.5
0.6
0.9
2.8
6.6
4.4
2.2
1.9
0.9
0.3
0.3
0.6
0.3
1.1
4.5
0.4
0.6
1.7
1.4
3.7
0.6
0.6
0.6*
2.2*
1.6*
0.3*
0.3
-
2.6
1.7
6.0
0.9
0.9
3.4
2.6
-
0.3
0.9
0.9
1.0
-
0.3
1.1
1.4*
1.1
0.3*
0.6
0.3
0.3
2.5
0.3
0.3
0.6
1.3
0.3
0.6
0.6
0.3*
1.7*
0.3
0.3
0.3
0.6*
3.9
3.9*
1.0
0.3
0.8
1.1*
5.9
6.9
8.9
2.0
0.3
0.9
0.3
1.7
1.7
0.9
3.4
11.2
0.9
3.4
1.7*
0.9
1.0
2.0
-
0.9
7.8
0.9
2.6
4.3
0.9
1.0
8.6
5.2
3.0
1.0
3.0
0.3
0.3
1.3
0.9
0.3
0.8
0.3
2.3
0.6
0.3
0.3
1.5
0.4
0.6
0.6
0.8
7.2
2.1
1.1
6.0
0.3
0.3*
3.3
7.1
1.8
5.3
7.6
24.9
2.2
7.9
16.7*
0.3
0.3
0.3
0.9
0.3
-
0.3
0.6
1.3
-0.9
0.3
0.6
0.4
1.9
0.8
0.4
-
-
0.3
-
3.0*
0.6
0.3
_
0.6
0.3
-
-
2.6
~~
~~-0.9
1.3
0.6
0.6
1.3
3.7
14.9
0.6
0.3
3.4
0.9
1.0
1.0
2.8
5.0*
7.8
29.3
2.0
21.6*
22.4
2.0
2.0
13.8
0.3
0.6
0.3
1.9
0.3
0.3
0.3
1.6
1.7
1.7
1.0
1.0
4.9*
1.0
2.0
3.0
4.9
3.0
3.9
ORNISSCANDINAVICA17:2(1986)