INFLUENCE
OF RESOURCE
USE OF TREE-FALL
ABUNDANCE
ON
GAPS BY BIRDS IN AN
ISOLATED
WOODLOT
JOHN G. BLAKE• AND WILLIAM G. HOPPES2
Department
of Ecology,
Ethology,
andEvolution,
606EastHealeyStreet,University
of Illinois,
Champaign,
Illinois61820USA
ABSTR^CT.--The
occurrence
of birdsin forestunderstoryand tree-fallgapsduring spring
andfall migrationperiodswasdeterminedin an isolatedwoodlot.We usedmist-netcaptures
to testthe hypothesisthat birdsare attractedto gapsbecauseof higher resourcelevels.We
captured1,010birds (74 species)in springand 458 (44 species)in fall. Total capturesand
capturesper net were higher (P < 0.001) in gapsduring spring and fall. Mean number of
speciesper net washigher in gaps(P < 0.001)duringbothseasons,
but totalspeciesin gaps
(69 spring,43 fall) wasnot significantlyhigher than in forestunderstory(60 spring,28 fall).
Of 44 speciesrepresentedby adequatesamplesizes(n > 5) in spring,9 were significantly
(P < 0.05) more commonin gapsand 2 were more commonin forestunderstory.Nine of 17
specieswere capturedmore often (P < 0.05) in gapsduring fall. During spring, flycatchers,
groundinsectivores,foliageinsectivores,and granivore-omnivores
were capturedmore frequently(P < 0.05)in gaps.Flycatchers
showedno differencein fall, but other trophicgroups,
includingfrugivores,were capturedmorefrequently(P < 0.05) in gapsthan in forestunderstorysites.Barkforagersshowedno statisticalpreferencefor gapsor forestunderstoryin
springor fall. Total speciesper net and total capturesper net correlatedpositively(P < 0.05)
with densityof foliagein the lower canopyand negativelywith densityof upper canopy
foliage in both spring and fall. Total speciesand capturescorrelatedpositively (P < 0.05)
with insectabundancein spring and with fruit abundancein fall. Foliageinsectivorescorrelated positively with low canopy foliage and insectabundancein both spring and fall.
Capturesof frugivorescorrelatedwith fruit abundancein fall. Thesedatasupportthe hypothesisthat birds are attractedto tree-fall gapsbecauseof higher resourceabundanceand
provide further evidence of the importance of habitat heterogeneity to the structure and
compositionof bird communities.Received
7 November
1984,accepted
28 October1985.
GAPScaused by tree fails contribute to the
creation of a habitat mosaic in many forests
(Williamson
1975; Hartshorn 1978; Whitmore
1978; Runkle 1981, 1982; Brokaw 1985). Treefall gaps["a vertical 'hole' in the forest extend-
ing to within 2 m of the forest floor" (Brokaw
1982)] influence abundance and distribution of
bird speciesby maintaining habitat heterogeneity and by affecting abundance and distribution patternsof food resources(e.g. fruit and
insects).Resourcelevels may be higher in gaps
becauseof greater primary productivity associatedwith increasedlight levels (Fogden1972,
Halle et al. 1978). Previous studies in east-central Illinois (Willson et al. 1982, Martin and Karr
• Present address: Department of Zoology, Birge
Hall, University of Wisconsin, Madison, Wisconsin
53706 USA.
2 Present
address:
Center
for
Earth
and
Environ-
mental Science,Hudson Hall 102,StateUniversity of
New York, Plattsburgh,New York 12901USA.
328
1986) have demonstrated differences in assem-
blages of birds captured in forest gaps and
understory.Resultsof thesestudiessuggestthat
birds might be attracted to gaps becauseof
higher resourcelevels. Here, we test the prediction that abundanceof individuals in gaps
and forest understory correlates with abundance of resources in these locations. We used
mist nets to obtain concurrentsamplesof birds
in gaps and forest understorysitesand compared numberand speciescompositionof captures with
estimates
of insect and fruit
abun-
danceand with measuresof vegetationstructure
for the same sites.
Migration is energetically expensive, and
many birds must replenish fat reservesperiodically (Berthold 1975, Graber and Graber 1983,
Walsberg1983). Food suppliesoften are low or
unpredictable during migration (Walsberg
1983),particularlyin springwhen migrantsare
moving toward areaswhere local weather conditions may be severeand unpredictable. FindThe Auk 103: 328-340. April 1986
April 1986]
UseofTree-fall
Gaps
byBirds
ing adequatefood after a flight may be difficult
becausethe landing area often is unknown to
migrants. Consequently,it would be advantageousfor birds to recognizeand quickly select
foraging sites that are profitable (Martin and
Karr 1986). This may be particularly true for
migrantsin east-centralIllinois, where natural
habitat is limited and existsas isolatedpatches
329
known, all were well establishedand at least3-5 yr
old.
We nettedbirds 3 days/week,exceptwhen rain or
strongwinds precludeduse of nets, from about 15
min before
sunrise until
about noon.
We standard-
ized netting effort among daysto avoid problemsassociatedwith diurnal variability in capturerates(Karr
1981).We nettedbirdson 18 daysin spring(19 April
to 5 June) and on 24 days in fall (23 August to 31
in an agriculturalsetting(Blake 1986).Recent October). Nets were checked every hour, and capwork by Graberand Graber(1983) demonstrat- tured birds were identified, sexed (when possible),
ed that migrant warblersin spring experience weighed,bandedwith U.S. Fishand Wildlife Service
a net lossof energywhile foragingin woodlots aluminum leg bands,and releasedat point of capture.
in east-centralIllinois. By demonstratingdifWe comparedthe number of capturesin gap and
ferencesin resourcelevels between gapsand
forest understory nets using Chi-square tests (for
forest understory sites,and by demonstrating
samplesizes >_30) or FishersExactTest (for n < 30;
correlationsbetweencapturefrequencyand re- Sokal and Rohlf 1981).We correlatednumber of capsource level, we support the prediction that tures per net with measuresof habitat structure and
during migration birds selectas foraging sites resourceabundance (see below) using Spearman's
microhabitats
with
abundant
food resources.
STUDY AREA AND METHODS
Study site.--We mist-netted birds in William Trelease Woods, an upland forest tract of about 24 ha
located 6.5 km northeast of Urbana (Champaign
County), Illinois. Principal trees in the woods are
sugarmaple (Acersaccharum),
hackberry(Celtisoccidentalis),
white ash(Fraxinus
americana),
slipperyelm
rank correlation (Sokal and Rohlf 1981).
We divided speciesinto six major trophic groups
in spring(flycatchers,ground insectivores,bark foragers,foliageinsectivores,omnivore-granivores,
and
nectarivores)and seven in fall (above six plus frugivores)basedon literature (Martin et al. 1951,Willson 1974, Willson et al. 1982) and personal observations (seeAppendix). Nectarivoreswere represented
by one species(Ruby-throatedHummingbird, Archilochuscolubris),and we did not include nectari-
(Ulmus rubra), basswood (Tilia americana), red oak
voresin the analysesof trophic groups.We catego(Quercusrubra), and buckeye (Aesculusglabra). The
rizedfrugivoresasprimary[thosethatdependheavily
understorysupportsa variety of species,including on fruit, i.e. more than 75%of foragingobservations
young individuals of the above species,pawpaw in fall (Hoppespers.obs.)]and secondaryfrugivores
(Asiminatriloba),and spicebush(Linderabenzoin).
[thosethatconsumefruit regularlybut lessoften than
Many speciespresent in TreleaseWoods produce primary frugivores,i.e. 25-50% of foragingobservafruits that are eatenby birds in late summerand fall.
tionsin fall (Hoppespers.obs.)]and comparednumTheseincludespicebush,hackberry,Virginia creeper bersof capturesof all frugivoresand numberof cap(Parthenocissus
quinquefolia),
wild grape(Vitusvulpina), turesof primaryand secondary
frugivoresseparately.
moonseed(Menispermum
canadensis),
catclaw (Smilax Becauseconsiderableattention has been placed on
hispida),poisonivy (Rhusradicans),and wahoo (Eu- the useof gapsby frugivoresin fall (e.g.Thompson
onymusatropurpureus).
Birds.--We used mist nets (12 m length, 2.6 rn
height, 36 mm mesh)to samplebirds using the lower
levels(bottom2-3 m) of gapsand forestunderstory
during spring and fall 1983. We believe that the advantagesgainedby simultaneous
samplingof all sites
outweigh any biasesassociatedwith mist-net sampies (Karr 1979, Martin and Karr 1986;but see Rem-
and Willson 1978, 1979), we also examined use of
gapsand forestunderstoryduring springby species
that are frugivorousin fall.
Vegetation.--We recorded vertical distribution of
vegetationat eachnet site following the methodsof
Karr (1971).Presenceor absenceof vegetationin each
of 14 height intervalswas noted at 40 pointsper net
(Fig. 1). Height intervals(meters)were 0-0.3, 0.3-0.9,
sen and Parker 1983). Also, comparisonswith pre- 0.9-1.5, 1.5-2.1, 2.1-2.7, 2.7-3.4, 3.4-4.0, 4.0-4.6, 4.6vious tree-fall gap studies are facilitated because 6.1, 6.1-9.1, 9.1-12.2, 12.2-15.2, 15.2-18.3, and >18.3.
similar techniqueswere used (Schemskeand Brokaw Percentagecoverfor a given height interval was cal1981, Willson et al. 1982, Martin and Karr 1986). Six
culatedas the percentageof all pointswith vegetanets were placed in gaps (1/gap) and 6 in forest tion presentat that height. We sampledvegetation
understory. Gap nets were positioned to minimize
profileseach week in spring and once every three
visibility while still remainingwithin the gaps.We weeksin fall, when foliagedistributionschangeless
did not placenetsin forestunderstorybetweengaps rapidly.
that were less than 40 rn apart or within 20 m of a
We recordedthe number of trees[individualswith
gap edge. Although exact age of the gaps is not a diameterat breastheight (DBH) of at least7.6 cm]
330
BLAKE
ANDHOPPES
.05hc•
circle
[Auk,Vol. 103
site (Fig. I). All individuals that producedfruit were
marked, and numbers of ripe and unripe fruit were
recordedat biweekly intervals in fall. No fruits were
presentduring spring.
Becausewe sampledfoliage, trees,and fruit up to
12.6 m from the center of each gap net (Fig. 1), our
samplessometimesincluded area beyond gap edges.
Thus,althoughgapswere definedasa "verticalhole"
in the vegetation,vegetationabove 2 m could be recordedat gap nets.
Insects.--Weused 15 x 15-cmplatesof plexiglass
coated with a thin layer of Tree Tanglefoot (Cody
1980,Hutto 1980) to sampleinsectabundance.Plates
were suspendedat about I and 2 m at four points
aroundeachnet (Fig. 1), for a total samplingarea of
1,800 cm2 (4 plates, 225 cm'/side) at each net site.
Traps were uncovered at dawn and checked shortly
after noon on 5 daysin springand 6 in fall. Captured
Ii ß II
insects were counted, identified to order, and mea-
Fig. I. Samplingdesignfor gapand forestunderstory nets.
sured to the nearestmillimeter. Traps were covered
with a thin sheet of plastic when not in use. We
recognizethat our data are not a direct measureof
the availability of insectseaten by all insectivorous
birds. However,
in a 0.05-ha circle (12.6 m radius) centered on the
middle of eachnet (Fig. I). Treeswere identifiedand
assignedto I of 7 size classes:
7.6-15, 15-23, 23-38,
38-53, 53-69, 69-84, and 84-102 cm DBH (James and
Shugart 1970).
Fruit-producingshrubsand vines were sampled
along2-m-widetransects(80 m2total/site)at eachnet
we assume that the densities of in-
sectscaughtare correlatedwith densitiesof insectivorousbird food and that the resultsprovide a basis
for comparinginsectavailabilityamongsites(Hutto
1980).Efficiencyof sticky trapsvaries with such factors as wind speed (Johnson1950) and position of
board relative to vegetation (Login and Pickover
1977).We minimized effectsof thesefactorsby hang-
/•...,,,,,,
EARLY
SPRING
r•
W
Z
13
/•rest
I
LATE
FALL
SPRING
./'gap
31
O
20
•0
60
PERCENT
8'0
2'0
4'0
6'0
COVER
Fig. 2. Foliageprofilesfor gap and forestunderstorysites.
April 1986]
Useof Tree-fall
Gaps
byBirds
ing all boardsin the samepositionfor eachsampling
period and by not samplingon windy days.
Prior to testing for differencesbetween gap and
forest understorysites,we tested the data on vegetation and resourceabundancefor normality (Sha-
331
8O
piro-Wilks test).We used a Mann-Whitney test (standard normal deviate calculated,SND) when there was
6
a significant departure from normality and t-tests
when data did not depart from normal distribution.
We tested variances for equality (F-test) and used a
t' test (Sokal and Rohlf 1981) if variances were not
equal.
•,4c
c•
h
RESULTS
Vegetation.--Forest understory sites were
dominatedby sugarmaple, basswood,and elm;
gaps were dominated by oak, ash, and hackberry. Small trees (7.6-15 cm DBH) were dominant in gaps,and averagenumber per net site
was higher in gapsthan in forest locations.All
other size classes were more abundant
Gaps
in forest
20
'
September
2'3
October
Fig. 3. Abundanceof fruits (mean + ! SD) at gap
and understorysites.Differencesbetweengapsand
understorysiteswere significanton all dates(t > 6.6,
sites than in gaps, but total tree density was
not significantly higher (t = 1.41, P < 0.40) in P < 0.00! for all dates).
understory sites (œ= 56 trees/0.05 ha) than in
gaps(œ= 48/0.05 ha). Differencesin basalarea P < 0.05 for both periods).Vegetationdensity
per size classwere pronouncedbetween gaps in the middle canopywas not significantlydifand understory,and averagebasalarea over all ferentbetweengapsand forestunderstory.Gaps
size classeswas significantly higher (t = 4.0, had more ground vegetation than understory
P < 0.01) in understory sites (4.2 m2/0.05 ha) sitesin early fall (t ---2.40, P < 0.05), but differencesbetween gaps and forest understory
than in gaps(2.1 m2/0.05 ha).
Vegetationprofilesshoweda marked differ- sitesdecreasedas ground vegetation declined
ence between gapsand forest understory sites at all sites from early to late fall (Fig. 2). The
during all periods(Fig. 2). We compareddis- distribution of foliage in lower, middle, and
tribution of foliage over eight height intervals upper canopiesduring fall paralleled spring
in gaps and understory sites using ranked patterns.
Fruit and insect abundance.--Abundance
of
abundanceof foliage over the eight intervals.
Ranked abundances
were not correlated
befruit was greater in gaps than in forest undertween gapsand understorysitesduring spring story on all samplingdatesin fall (t > 6.6, P <
or fall
0.001 on each sampling date) (Fig. 3). AbunWe distinguishedfour foliage layers follow- dance of ripe fruits was highest early in Seping constructionof foliage proflies: ground (0tember and declined thereafter (Fig. 3; gaps:
0.9 m), low canopy(0.9-4.6 m), middle canopy r = -0.980, P < 0.001; understory:r = -0.894,
(4.6-9.1 m), and upper canopy (>9.1 m) (Fig. P < 0.01; fruit abundance over time). Fruit re2). All net sites had dense ground cover, and moval largely wascompletein understorysites
there was no significant difference in density after the first week in October,but somegaps
of ground foliage between gapsand nongaps. retainedfruit into December(Hoppesunpubl.
Gaps had more vegetation in the low canopy data). The major source of difference in fruit
during early (t = 4.16,P < 0.01),middle (Mann- abundancebetween gap and understorysites
Whitney test, SND = 2.39, P < 0.05), and late was the absenceof fruiting vines such as Vir(t = 3.41, P < 0.01) spring periods;understory ginia creeper,wild grape,and catclawin forest
siteshad more vegetationin the upper canopy understorysites.Understorysiteseither lacked
during all periods (early: t = 5.04, P < 0.001; fruit or had only a few fruiting spicebush
middle and late: Mann-Whitney, SND = 2.87, shrubs.
332
BLAKE
ANDHOPPES
70-
[Auk,Vol.103
Fall
Spring
50
30
10-
+:
g f
g f
g f
g f
g
13
20
31
May
g f
5
June
?,
g f
g f
g f
2
12
23
September
g f
g f
g f
2
15
30
October
Fig. 4. Number of insectscaptured(mean _+ 1 SD) on sticky traps (4 traps, totaling 0.18 m2/net site)
placedaround gap (g) and forestunderstory(f) nets.
Most insectscaptured in spring were small
(<2 mm) Diptera, although Coleoptera also
were well represented.By contrast, most insectscaptured in fall were large Diptera and
Coleoptera,although small Diptera also were
common. Average number of captures was
higher in gapsduring both springand fall (Fig.
4; Wilcoxon Signed Rankstest,P < 0.05 spring
and fall). Differencesbetween gapsand understory sites were more pronounced in spring,
when significant differences existed on 3 of 5
sampling dates, than in fall, when significant
differencesexistedon only 2 of 6 dates(Fig. 4).
Species
numbers
andcaptures
of birds.--We captured 1,010 birds (including 69 recaptures,
6.8%), representing 74 species,during 1,416
mist-net hours (1 MNH = 1 mist net open 1 h)
in spring, for an overall capture rate of 71 captures/100 MNH. In fall we caught 458 birds (14
recaptures,3.1%),representing44 species,during 1,504 MNH, for a capture rate of 31/100
MNH. Capture rates at individual nets ranged
from 7 to 161 birds/100 MNH in spring and
from 2 to 152 birds/100 MNH in fall. Only two
individuals,both in spring,were recapturedon
the sameday and at the samenet. Thus, inclusion of recapturesdoesnot bias resultsby introducing overrepresentation of individual
preferences.All specieswith number of captures in gap and forestunderstorynets are listed in the Appendix.
Gap-understory
comparisons.--Number
of captureswashigher in gapthan in understorynets
on 16 of 18 datesin spring and 20 of 24 dates
in fall. Total number of capturesand average
numberof capturesper net were higher in gaps
than in understorynetsduring both springand
fall (Table 1). Similarly, average number of
speciescapturedper net was higher in gap nets
(Table 1). In both seasons, however, the total
numberof speciescapturedin all gap netswas
not significantlygreater than the number captured in all understory nets (Table 2), indicating that a lower similarity in speciescomposition existedamong forestunderstorysitesthan
among
gaps.
TABLE
1. Capturesin gap and forestunderstorynetsduring spring (S) and fall (F).
Total captures
Captures/net
Total species
Species/net
Gap
Forest
Test
P
(S)
628
382
X2 = 60.0
<0.001
(F)
356
102
X2= 143.0
<0.001
(S)
105
64
t = 7.1
<0.001
(F)
58
17
(S)
69
60
X2 = 0.004
>0.50
(F)
43
28
X2 = 3.2
<0.10
(S)
(F)
38
20
27
8
t = 9.1
t = 5.9
t = 5.9
<0.001
<0.001
<0.001
April1986]
UseofTree-fall
Gaps
byBirds
333
T^BI•E
2. Specieswith markeddifferences(P < 0.01)in capturefrequencybetweengapand forestunderstory
nets in TreleaseWoods,spring and fall 1983.
Spring
Fall
Captures
Captures
Species
a
Gap
Forest
P
Yellow-bellied Flycatcher
Least Flycatcher
20
18
4
9
<0.001
< 0.070
White-breasted
0
5
< 0.062
Golden-crownedKinglet
Ruby-crownedKinglet
Gray-cheekedThrush
Nuthatch
10
14
2
6
<0.032
<0.074
Swainson's Thrush
Wood Thrush
49
8
31
20
Gray Catbird
Red-eyed Vireo
Magnolia Warbler
Yellow-rumped Warbler
30
9
<0.001
22
8
9
17
< 0.018
< 0.064
0
40
7
5
<0.015
<0.001
23
16
2
4
<0.001
<0.005
19
9
<0.051
6
0
<0.031
Blackburnian Warbler
Palm Warbler
American Redstart
Ovenbird
Northern Waterthrush
Canada Warbler
White-throated Sparrow
American
Goldfinch
<0.024
<0.023
Gap
Forest
P
48
21
7
1
< 0.001
<0.001
13
4
<0.037
10
22
0
2
<0.002
< 0.001
25
5
<0.001
7
76
0
22
<0.015
< 0.001
7
1
<0.032
Scientificnamesare given in the Appendix.
A minimum of 6 captures is required to attain significance at the 5% level (Fisher Exact
Test, 2-tailed). During spring, 44 specieshad 6
or more captures, and we expected 2.2 species
to show a significant difference (P < 0.05) in
captures between gaps and understory nets
basedon chancealone. In this study, 11 species
displayeda differencein capturefrequencyat
the 5% level and 16 at the 10% level (Table 2),
significantly more than expectedby chance
alone. Two species,Wood Thrush (Hylocichla
mustelina)and Palm Warbler (Dendroicapalmarum),were capturedsignificantlymore often in
forest understory than in gap sites (Table 2).
Two other species, White-breasted Nuthatch
(Sitta carolinensis)
and Yellow-rumped Warbler
(Dendroicacoronata),also were captured more
often in understorynets,but differenceswere
not significant. The thrush and nuthatch nest
in Trelease Woods, and territories
are located
16 species(Table 2) were caught more often in
gap nets.
Speciescapturedsignificantly more often in
gapswere not simply canopyspeciesfollowing
the edge of vegetation downward. In fact, most
speciesthat showeda preferencefor gaps (Table 2) were birds of the ground or lower vegetation levels. When all species(including those
representedby 1-5 captures)were considered,
51 specieswere captured more often in gaps
and 16 in forestunderstorynets, a ratio significantly different from even (X2 -- 18.3, P <
0.001). In addition, although 14 specieswere
captured only in gap nets, only 5 were restricted to forestunderstory(X2 = 4.26, P < 0.05).
Seventeen specieswere represented by at
least 6 capturesin fall, and based on chance
alone we expected1 speciesto show a significant (P < 0.05) difference in number of capturesbetween gapsand understorysites.However, 9 speciesdisplayedsignificantdifferences
in capturefrequencybetween gapsand understorysites(Table 2), and all were capturedmore
frequently in gaps. Only 2 species, White-
within undisturbed forest (JGBpets. obs.), accounting for the greater number of capturesin
forestunderstorysites.Also,the nuthatchpreferentiallyforageson largetrees(JGBpets.obs.),
which are uncommon in gaps. Both warblers breasted and Red-breasted nuthatches (Sitta
arrived in Trelease Woods before differences in
canadensis),
were captured more frequently in
vegetationbetweengapsand forestunderstory forestunderstorythan in gaps(seeAppendix).
sites were apparent. The remaining 12 of the Thirty-sevenspecieswere capturedmore often
334
BLAKE
ANDHOPPES
[Auk,Vol. 103
TABLE3. Capturesby trophic groups, in gap and forest understorynets during spring (S) and fall (F).
Numbers of capturesof speciesthat are frugivorousin fall are shown as a group and subdividedinto
primary and secondarysubgroups.'
Captures
Trophic group
Flycatchers
Bark foragers
Season
Species
b
Gap
Forest
X2
P
S
8.5
80
22
33.0
< 0.001
F
5
9
4
1.9
<0.50
S
8.5
24
26
0.1
> 0.50
F
4
10
7
0.5
>0.50
S
F
12
2
204
79
137
24
13.2
29.4
<0.001
<0.001
S
33
194
105
26.5
<0.001
F
16
165
35
89.4
Granivore-omnivores
S
F
11
5
119
23
90
9
4.0
6.1
< 0.05
< 0.025
Frugivores(total)
S
16
180
129
8.4
<0.005
F
10
66
23
Ground
insectivores
Foliage insectivores
Primary
Secondary
<0.001
20.8
< 0.001
<0.001
S
9
146
92
12.3
F
8
48
17
69.1
S
7
34
37
0.1
> 0.50
F
2
18
6
6.0
<0.025
<0.001
aFrugivoresincludespeciesthat regularlyconsumefruit during fall migration.Primaryfrugivoresinclude:
Northern Flicker, thrushes (6 species),Gray Catbird, and Brown Thrasher. Secondaryfrugivores include:
Red-belliedand Red-headedwoodpeckers,EuropeanStarling,Red-eyedVireo, Yellow-rumpedWarbler,Scarlet Tanager, and Northern Oriole.
bSeveralspecieswere assignedto two groupsequally (seeAppendix).
in gaps and only 2 in understory sites (X2 = primary and total frugivoreswere capturedsig31.4, P < 0.001),when all specieswere consid- nificantly more often in gaps (Table 3).
ered. Sixteen specieswere captured only in
Vegetation,resourceabundance,
and numberof
gaps,while 1 was capturedonly in forest sites captures
ofbirds.--Foliagedensityin the ground
(X2 = 13.2, P < 0.001).
and middle canopy layers varied little among
The distribution of capturesamong five ma- sites, and no consistent difference existed bejor trophic groups during spring differed be- tween gap and forest sites. Consequently,we
tween gap and forest sites (X2 = 20.8, 4 df, P <
examined correlations between capture rates
0.001).Bark foragersdisplayedno differencein and foliage density in lower- and upper-canofrequency of capturesbetween gap and forest py layers only, using each net as a single samnets (Table 3). Other groups, particularly fly- ple point (n = 12).
catchers,foliage insectivores,and ground inDuring spring, species richness and total
sectivores,were caughtsignificantlymore often number of capturescorrelated positively with
in gap nets (Table 3). Speciesthat are frugivo- density of vegetation in the lower layer and
rous in fall were captured more often in gaps negatively with density of upper-canopy foduring spring (Table 3). When separatedinto liage (Table 4). Similar patterns were evident
primary and secondary frugivores, primary for number of captures of flycatchers, foliage
frugivores preferred gaps,but secondaryfru- insectivores,and primary frugivores. Ground
givoresdisplayedno differencein capturesbe- insectivores showed no correlation with lowlevel foliage but correlatednegatively with uptween gapsand forest understory (Table 3).
The number of individuals captured in all per-canopyfoliage (Table 4).
We comparedtotal number of capturesdurmajor trophic groups was higher in gaps than
in forest sites during fall (Table 3). Unlike ing early, middle, and late spring migration
spring, however, the distribution of captures with foliage densitiesin lower- and upper-canamongmajortrophic groupsdid not differ be- opies during these periods. Number of captween gapsand forest(X2= 7.98, 5 dL P > 0.05). tures per net correlatedpositively with foliage
Ground insectivores,foliage insectivores,and in low strataduring early (Spearmanrs= 0.684,
April1986]
UseofTree-fall
Gaps
byBirds
335
TABLE
4. Correlations
betweennumberof capturesand foliagedensityin lower and uppercanopies(LC,
UC), insectabundance(IN), and fruit abundance(FR) at gap and forestunderstorynets.Only significant
correlations
(Spearman's
r) areshown:P < 0.05,r > 0.587;P < 0.01,r > 0.727;andP < 0.001,r > 0.846.
SeeTable 3 for descriptionof frugivore categories.
Spring
LC
Totalspecies
Totalcaptures
Flycatchers
0.63
0.61
0.80
Ground insectivores
Foliageinsectivores
-0.81
-0.84
-0.88
IN
LC
0.62
0.60
0.76
-0.61
0.67
-0.81
Granivore-omnivores
Frugivores(total)
Frugivores(primary)
Fall
UC
0.74
UC
IN
FR
0.64
0.63
0.65
0.72
-0.73
-0.64
-0.63
0.71
-0.73
0.72
0.68
-0.73
0.59
0.59
0.62
0.70
-0.89
0.67
0.68
0.58
-0.66
-0.69
The numberof capturesper net during early,
late periods(r• = 0.684, P < 0.05) and correlated middle, and late fall migration correlatedposnegatively with foliage density in the upper itively with foliage density in the lower level
canopy (early, rs = -0.474, P < 0.20; middle, (early, rs= 0.64, P < 0.05;middle, rs= 0.56, P <
0.10; late, rs = 0.49, P < 0.20). The number of
rs= -0.734, P < 0.01; late, rs = -0.787, P <
P < 0.05), middle (rs = 0.542, P < 0.10), and
0.01).
When all net siteswere included, only number of foliage insectivorescorrelated signifi-
captures correlated negatively with foliage
density in the upper level (early, rs = -0.45,
P < 0.20; middle, rs = -0.70,
P < 0.05; late,
cantlywith insectabundance(rs= 0.614,P <
rs= -0.70, P < 0.05). The number of captures
0.05). However, number of insects captured at
of only 2 trophic groups,ground insectivores
and foliage insectivores,correlated positively
one gap sitewas significantlyless(t = 2.2, P <
0.05) than the mean for other gap sites. The
reasonsfor the unusuallylow numberof insect
capturesat one gap are not clear, but the positions of the insect traps or other factorsunrelated to actual insect abundancein the gap
(e.g. microclimatic conditions) may have unduly influencedthe effectivenessof the traps.
As a consequence,we reexaminedcorrelations
between bird community variables and insect
abundanceat the remaining 11 sites (see Discussion).Both speciesrichnessand number of
captures correlated significantly with insect
abundance (Table 4). Among trophic groups,
flycatchers,in particular,showeda strongcor-
with number of insects captured at each net
site. Severalgroups,however, correlatedpositively with fruit abundanceper net (Table 4).
When frugivores were separatedinto primary
and secondarygroups,only primary frugivores
showed a significant associationwith fruit
abundance.
DISCUSSION
Tree-fall gapsrepresenta distinct microhabitat that differs from the understory of surrounding forestin vegetationstructure(e.g. foliage density, tree size distributions), plant
relation with insect abundance (Table 4). When
speciescomposition,microclimaticconditions,
all sites were included, correlations were sim-
and resourceabundance(Geiger 1950,Denslow
ilar in directionbut were not asstrongaswhen
1980, Schemske and Brokaw 1981, Brokaw 1982,
the one site was eliminated.
Chazdon and Fetcher 1984, Martin and Karr
Fall patternswere similar to spring with respectto correlationsbetweennumber of species
and capturesand measuresof habitat structure
(Table 4). Two trophic groups,ground insectivotes and foliage insectivores,correlatedwith
foliage density in the lower layer, while 5
groups correlatednegatively with upper-canopy foliage (Table 4). Unlike spring, number of
capturesof all frugivores combined correlated
negativelywith upper-canopyfoliage.
1986). Recent studies(e.g. Karr and Freemark
1983) have demonstratedthat birds may select
habitatsbasedon slight differencesin vegetation or microclimate
and thus there
is reason
to believe that birds are capableof recognizing
and selectingtree-fall gapsas a distinct microhabitat in which to forage (Martin and Karr
1986).
Microclimatic conditionswithin a gap are a
function of gap size (in relation to canopy
336
BLAKE
ANDHOPPES
height), shape, orientation, and vegetation
structure,particularly with respectto how these
factorsdetermine the daily duration of direct
insolation (Geiger 1950, Lee 1978, Chazdon and
Fetcher 1984). Amount of light, highest daily
temperatures, and amount of precipitation
reaching the ground are higher in gapsthan in
adjacentforestunderstoryand also differ with-
[Auk, Vol. 103
colonists to mature forest species (Cates and
Orians 1975, Hartshorn 1978). Thus, gaps not
only supportgreater concentrationsof foliage
than similar areaswithin forest understory but
also support relatively higher proportions of
foliage that are palatable to a broad variety of
herbivorous invertebrates. Although foliage
density does not directly measureabundance
in differentsectionsof a gap (Geiger1950,Den- of foliage insects,it can be taken as an indirect
slow 1980,Chazdonand Fetcher 1984).By con- measure of such abundance because foliage
trast, relative humidity is lower in gaps than density does directly measure availability of
in forest understory (Denslow 1980). Differ- feeding substratesfor insects and searching
encesin microclimate between gaps and forest substratesfor foliage-gleaning birds.
Most insectivorous groups occurred more
understory are particularly pronounced close
to the ground, within the range of vegetation frequently in gapsin spring and fall, supportsampled by mist nets, and are influenced ing the hypothesisthat birdsselectgapsasprofstrongly by vegetation structure.Soil temper- itable foraginglocations(Martin and Karr 1986).
atures and temperaturesclose to the ground Correlationsbetween insectivorousgroups (e.g.
generally increasewith gap size (Geiger 1950, foliage insectivores)and foliage densitiesproSchulz 1960 in Denslow 1980, Denslow 1980).
vide indirect evidence that a greater resource
Differencesin amount of radiation penetrat- abundanceattractsmany speciesto gaps (Maring to the forest floor in tree-fall gaps and un- tin and Karr 1986,this study).Furthermore,bark
disturbedforest vary with seasonand gap age, insectivoresexhibited little or no preference
and may change over time within a season for gaps in spring or fall and foraging sub(Geiger 1950, Chazdon and Fetcher 1984). In stratesfor this group are lessabundantin gaps,
temperate deciduous forests, the amount of making gaps less attractive as foraging sites.
light reaching the forest floor early in spring Schemskeand Brokaw (1981) found relatively
is only slightly greater in gaps than in forest few speciesconcentrated in gaps in Panamaunderstory.As the canopy closeswith contin- nian forest, but those speciesconsideredgap
ued leaf production,however, the differencein specialistswere insectivorousto a great extent.
A greater abundanceof foliage and insects
amount of radiation penetratingbelow the canopy in gaps and undisturbed forest increases reduces the amount of time required for
(Anderson 1964).
searchingand travel and resultsin a faster rate
Birds that use lower levels of a forest may be of food ingestion.This may be particularly imattracted to gaps becauseof a greater abun- portant during migration, when energy redanceof resourcesor becauseresourcesmay be quirementsare high (Graber and Graber 1983).
more accessibleor visible in gaps (i.e. because During spring, migrants are moving toward
of higher light levels) than in forestunderstory areaswhere food suppliesand weather may be
(Schemske and Brokaw 1981, Willson et al. unpredictable.Thus,there maybe greaterpres1982). During spring, most birds feed on in- sure during spring than during fall to select
sects and other invertebrates, and several lines
the most profitable sites in which to forage.
Leaf litter often is abundant in gaps, and
of evidencesuggestthat suchresourcesare more
abundantor concentratedin gapsthan in forest consequentlysoil and litter invertebrates are
likely to be abundantaswell (Bultmanand Uetz
understory.
Lepidopteran larvae form a major compo- 1984 and references therein). Higher soil and
nent of the diet of many foliage-gleaning near-groundtemperaturesin gaps,relative to
species,and warbler migration in spring coin- forest understory,also may increaseinsect accides with the peak emergence of larvae (Gra- tivity levels over that presentin forestunderber and Graber 1983). Tree-fall gaps support story. If soil invertebratesare more abundant
relatively densefoliage in low levels, much of in gapsthan in forestunderstorylocations,parit produced by pioneer or early successional ticularly early in spring, this is a likely explaspecies.Plant speciesare not equally palatable nation for the greaterabundanceof ground into generalistherbivores,and a continuum of sectivoresin gapsover forest understory.
Finally, our study provides direct evidence
palatability exists from weedy speciesto gap
April1986]
UseofTree-fall
Gaps
byBirds
337
that flying insects,primarily Diptera and Co- causefor the low number is unknown and may
leoptera,are more abundantin gaps.Coleop- representsimple chanceevents.The gap itself
teraform a majorcomponentof the diet of many was smaller than the others, and differences in
flycatchersin Illinois (Graber et al. 1974), and
flycatchersshowed a strong correlation with
insectabundanceduring spring. Higher light
levelsand a greaternumber of low (under 4.6
m) perches, combined with a greater abundanceof insects,may make gapsan especially
profitablelocationfor flycatchinginsectivores.
During fall, differences in insect abundance
were lesspronouncedbetween gapsand forest,
and thus it would be lessadvantageousfor flycatchersto concentratetheir activitiesin gaps
than in understory sites. Fall migration characteristicallyis much less pronounced than
spring migration (Graber et al. 1974), and the
lack of correlationbetween flycatchersand insectabundancemay reflect the low number of
flycatchers caught in fall (n = 13). Little is
microclimatedirectly related to gap size may
have influenced insect activity levels. Despite
the apparentlylow insectabundance,bird captures were still higher than in forest understory sitesand more comparableto other gap
sites.It is possiblethat birds were attracted to
the gap becauseof higher light levels and foliage density, relative to surrounding forest
understory, on the expectation that insect
abundancewould be greater. However, correlation between bird and flying insect abundance was strongestfor flycatcherswhen the
one unusualnet was eliminated, suggestingas-
counteredbut may not actively searchfor areas
with high fruit concentrations.
Total speciesand
total capturesper net correlated with insect
abundancein springand with fruit abundance
in fall, reflecting the switch in diet of a substantial proportion of birds captured during
spring and fall migration.
Birds may select foraging locations on the
basisof actualresourcelevels (e.g. insectabun-
searchconductedby T. E. Martin, and his thoughts
sessment of actual abundance. Further,
in a
comparisonof bird use of new and old gaps,
Martin and Karr (1986) found evidence to sug-
gestthat birds were not simply respondingto
known of the habitatpreferencesof migrants, higher light levels in gaps.
and migrantsmay selectdifferent habitats(e.g.
Birdsmay be attractedto gapsbecauseof inforest edge) in fall than in spring (e.g. Austin creasedcover in lower levels, perhaps as pro1970).
tection from predators. If this were the case,
Many speciesin Illinois rely on fruits for a we might expect all trophic groups to show a
major part of their diet during fall (Martin et similar responseto density of foliage. Only flyal. 1951; Thompson and Willson 1978, 1979), catchersand foliage insectivoreswere correlatand the availability and diversity of fruits is ed with low foliage density in spring, howgreater in gaps than in forest understory
ever, and in fall only ground and foliage
(Thompsonand Willson 1978,1979).The abun- insectivoreswere. The positive correlation of
danceof fruits in gapsappearsto attractmany foliageinsectivoreswith foliagedensityand the
frugivores (Thompsonand Willson 1978, Mar- general lack of correlation exhibited by other
tin and Karr 1986), and our sampling of actual groups suggestthat availability of cover was
fruit abundanceand bird abundancesupports not a primary factor influencing distribution
this suggestion.Thosespeciesmost dependent patterns of individuals [see Martin and Karr
on fruit during fall ("primaryfrugivores")were (1986) for further discussionon this point].
more strongly correlatedwith fruit abundance
than were specieswith a more mixed fruit and
ACKNOWLEDGMENTS
insectdiet ("secondaryfrugivores"). The latter
Much of the impetus for this study came from respeciesmay consumefruits when they are en-
dance) or on the basis of an indirect index of
resourceabundance (e.g. light levels, foliage
density) (Martin and Karr 1986). Fewer insects
were capturedon sticky traps at one gap than
at remaining gaps during spring. The actual
and ideas contributed much to this paper. We thank
D. Enstrom,T. Hoppes,L. Larson,and B. A. Loiselle
for assistance
in the field. This paper has benefited
from commentsof C. Faanes,D. Levey,B. A. Loiselle,
S. B. Vander Wall, and M. F. Willson. Preparation of
this manuscriptwas supportedby a Guyer Fellowship provided to the senior author by the Department of Zoology,University of Wisconsin.
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APPENDIX.Number of capturesin gap and forestunderstorynetsin TreleaseWoods,Illinois, during spring
and fall migration,1983.Speciesthat belongto differenttrophicgroupsin spring(S) and fall (F) have
groupsindicatedfor both seasons.
Nomenclaturefollowsthe A.O.U. check-list(1983).
Captures
Gap
Species
Ruby-throatedHummingbird (Archilochus
colubris)
Red-headedWoodpecker(Melanerpeserythrocephalus)
Red-belliedWoodpecker(Melanerpes
carolinus)
Yellow-bellied Sapsucker(Sphyrapicus
varius)
Downy Woodpecker(Picoides
pubescens)
Hairy Woodpecker(Picoides
villosus)
Northern Flicker (Colaptesauratus)
Olive-sided Flycatcher(Contopus
borealis)
EasternWood-Pewee(Contopus
virens)
Yellow-belliedFlycatcher(Empidonax
fiaviventris)
Acadian Flycatcher(Empidonax
virescens)
Empidonax
sp.'
LeastFlycatcher(Empidonax
minimus)
EasternPhoebe(Sayornis
phoebe)
Great CrestedFlycatcher(Myiarchuscrinitus)
Blue Jay (Cyanocitta
cristata)
Red-breasted Nuthatch (Sitta canadensis)
White-breasted Nuthatch (Sitta carolinensis)
Brown Creeper (Certhiaamericana)
Forest
S
F
S
7
4
2
2
1
I
3
4
3
2
4
4
1
2
5
4
F
Trophic group
Nectarivore
Bark forager
Bark forager
Bark forager
Bark forager
Bark forager
Ground and bark forager
(S), frugivore (F)
Flycatcher
Flycatcher
Flycatcher
Flycatcher
6
20
2
2
18
I
7
3
I
I
9
1
3
8
2
5
Flycatcher
I
8
3
5
4
Flycatcher
Flycatcher
Flycatcher
Omnivore-granivore
Bark forager
Bark forager
Bark forager
Foliage insectivore
Foliage insectivore
Foliage insectivore
Foliage insectivore
Ground insectivore (S), frugivore (F)
Carolina Wren (Thryothorus
ludovicianus)
Winter Wren (Troglodytes
troglodytes)
Golden-crownedKinglet (Regulussatrapa)
Ruby-crownedKinglet (Regulus
calendula)
Veery (Catharus
fuscescens)
i
I
1
3
48
2
10
12
21
2
2
6
Gray-cheekedThrush (Catharusrainlinus)
14
4
6
Ground insectivore (S), fru-
Swainsoh's Thrush (Catharusustulatus)
49
13
31
Hermit Thrush (Catharus
guttatus)
I
12
Wood Thrush (Hylocichlamustelina)
8
3
20
AmericanRobin (Turdusmigratorius)
25
6
17
Ground insectivore (S), frugivore (F)
Ground insectivore (S), frugivore (F)
Ground insectivore (S), frugivore (F)
Ground insectivore (S), fru-
Gray Catbird (Dumetellacarolinensis)
30
3
9
givore (F)
Brown Thrasher(Toxostoma
rufum)
EuropeanStarling(Sturnusvulgaris)
White-eyedVireo (Vireogriseus)
SolitaryVireo (Vireosolitarius)
Warbling Vireo (Vireogilvus)
Red-eyedVireo (Vireoolivaceus)
Golden-winged Warbler (Vermivorachrysoptera)
TennesseeWarbler (Vermivoraperegrina)
3
4
i
2
1
I0
5
2
Ground
Ground
insectivore
insectivore
I0
8
Foliage insectivore
Foliage insectivore
Foliageinsectivore
Foliageinsectivore(S), frugivore (F)
Foliageinsectivore
I
3
Foliageinsectivore
2
9
givore (F)
Ground-foliageinsectivore
(S), frugivore (F)
340
APPENDIX.
BLAKE
ANDI-IOPPES
[Auk,Vol. 103
Continued.
Captures
Gap
Species
Nashville Warbler (Vermivoraruficapilla)
Yellow Warbler (Dendroicapetechia)
Chestnut-sidedWarbler (Dendroica
pensylvanica)
Magnolia Warbler (Dendroicamagnolia)
Black-throatedBlue Warbler (Dendroicacaerulescens)
Yellow-rumped Warbler (Dendroicacoronata)
Black-throated Green Warbler (Dendroicavirens)
BlackburnJanWarbler (Dendroicafusca)
Palm Warbler (Dendroicapalmarum)
Bay-breastedWarbler (Dendroicacastanea)
Blackpoll Warbler (Dendroicastriata)
Black-and-white Warbler (Mniotilta varia)
American Redstart(Setophaga
ruticilla)
Forest
S
F
S
F
11
4
8
3
1
3
22
2
8
22
9
8
17
6
6
4
1
25
7
2
7
1
5
9
1
19
1
4
4
16
1
5
40
6
7
4
5
4
1
Trophic group
Foliage insectivore
Foliage insectivore
Foliage insectivore
Foliage insectivore
Foliage insectivore
Foliage insectivore (S), frugivore (F)
Foliage insectivore
Foliage insectivore
Foliage insectivore
Foliage insectivore
Foliage insectivore
Bark forager
Flycatcherand foliage insectivore
Worm-eating Warbler (Helmitherosvermivorus)
Ovenbird (Seiurusaurocapillus)
Northern Waterthrush (Seiurusnoveboracensis)
Louisiana Waterthrush (Seiurusmotacilla)
Kentucky Warbler (Oporornis
formosus)
Connecticut Warbler (Oporornisagilis)
Mourning Warbler (Oporornis
philadelphia)
Common Yellowthroat (Geothlypistrichas)
Hooded Warbler (Wilsoniacitrina)
Wilson's Warbler (Wilsoniapusilla)
Canada Warbler (Wilsoniacanadensis)
Yellow-breasted Chat (Icteria virens)
Scarlet Tanager (Pirangaolivacea)
Northern Cardinal (Cardinaliscardinalis)
Rose-breastedGrosbeak (Pheucticusludovicianus)
Indigo Bunting (Passerina
cyanea)
Rufous-sidedTowhee (Pipiloerythrophthalmus)
Fox Sparrow (Passerella
iliaca)
Lincoln's Sparrow (Melospizalincolnii)
Swamp Sparrow (Melospizageorgiana)
White-throated Sparrow (Zonotrichia
albicollis)
Common Grackle (Quiscalusquiscula)
Brown-headed Cowbird (Molothrusater)
Orchard Oriole (Icterusspurius)
Northern Oriole (Icterusgalbula)
American goldfinch (Carduelistristis)
Probably Willow Flycatcher (Empidonaxtraillii).
1
47
23
1
76
3
42
2
Foliage insectivore
22
2
Ground
Ground
insectivore
insectivore
insectivore
1
1
Ground
6
4
Foliage insectivore
Foliage insectivore
Foliage insectivore
Foliage insectivore
Foliage insectivore
Foliage insectivore
Foliage insectivore
Foliage insectivore
Foliage insectivore
Omnivore-gramvore
Omnivore-gramvore
Omnivore-gramvore
Omnivore-gramvore
Omnivore-gramvore
Omnivore-gramvore
Omnivore-gramvore
Omnivore-gramvore
Omnivore-gramvore
Omnivore-gramvore
Foliage insectivore
Foliage insectivore
Omnivore-granivore
3
11
5
1
1
1
16
7
5
2
1
4
1
1
7
3
7
1
5
66
3
1
2
5
56
1
1
2
1
1
19
1
9
1
1
14
5
5
7
7
1
4
6