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INFLUENCE OF RESOURCE ABUNDANCE ON USE OF -FALL GAPS BY BIRDS IN AN ISOLATED WOODLOT

JOHN G. BLAKE• AND WILLIAM G. HOPPES2 Departmentof ,Ethology, and Evolution, 606 East Healey Street, University of Illinois, Champaign,Illinois 61820 USA

ABSTR^CT.--Theoccurrence of birdsin forestunderstory and tree-fallgaps during spring and fall migrationperiods was determined in an isolatedwoodlot. We usedmist-net captures to testthe hypothesisthat birds are attractedto gapsbecause of higher resourcelevels. We captured1,010 birds (74 species)in spring and 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)during both seasons,but total speciesin gaps (69 spring,43 fall) was not significantlyhigher than in forestunderstory (60 spring, 28 fall). Of 44 speciesrepresented by adequatesample sizes (n > 5) in spring,9 were significantly (P < 0.05) more commonin gapsand 2 were more commonin understory.Nine of 17 specieswere capturedmore often (P < 0.05) in gapsduring fall. During spring, flycatchers, ground insectivores,foliage insectivores,and granivore-omnivoreswere capturedmore fre- quently(P < 0.05) in gaps.Flycatchers showed no differencein fall, but other trophicgroups, including frugivores,were capturedmore frequently (P < 0.05) in gapsthan in forestunder- storysites. Bark foragersshowed no statisticalpreference for gapsor forestunderstory in springor fall. Total speciesper net and total capturesper net correlatedpositively (P < 0.05) with densityof foliagein the lower canopyand negativelywith densityof upper foliage in both spring and fall. Total speciesand capturescorrelated positively (P < 0.05) with insectabundance in spring and with fruit abundancein fall. Foliageinsectivores cor- related positively with low canopy foliage and insectabundance in both spring and fall. Capturesof frugivorescorrelated with fruit abundancein fall. Thesedata supportthe hy- pothesisthat birds are attractedto tree-fall gapsbecause of higher resourceabundance and provide further evidence of the importance of heterogeneity to the structure and compositionof bird communities.Received 7 November1984, accepted 28 October1985.

GAPScaused by tree fails contribute to the 1986) have demonstrated differences in assem- creation of a habitat mosaic in many blages of birds captured in forest gaps and (Williamson 1975; Hartshorn 1978; Whitmore understory.Results of thesestudies suggest that 1978; Runkle 1981, 1982; Brokaw 1985). Tree- birds might be attracted to gaps becauseof fall gaps ["a vertical 'hole' in the forest extend- higher resourcelevels. Here, we test the pre- ing to within 2 m of the forest floor" (Brokaw diction that abundanceof individuals in gaps 1982)] influence abundance and distribution of and forest understory correlates with abun- bird speciesby maintaining habitat heteroge- dance of resources in these locations. We used neity and by affecting abundance and distri- mist nets to obtain concurrentsamples of birds bution patterns of food resources(e.g. fruit and in gaps and forest understory sites and com- insects).Resource levels may be higher in gaps pared number and speciescomposition of cap- becauseof greater primary productivity asso- tures with estimates of insect and fruit abun- ciatedwith increasedlight levels (Fogden 1972, danceand with measuresof vegetationstructure Halle et al. 1978). Previous studies in east-cen- for the same sites. tral Illinois (Willson et al. 1982, Martin and Karr Migration is energetically expensive, and many birds must replenish fat reservesperiod- ically (Berthold 1975, Graber and Graber 1983, • Present address: Department of Zoology, Birge Hall, University of Wisconsin, Madison, Wisconsin Walsberg1983). Food suppliesoften are low or 53706 USA. unpredictable during migration (Walsberg 2 Present address: Center for Earth and Environ- 1983),particularly in springwhen migrantsare mental Science,Hudson Hall 102, StateUniversity of moving toward areaswhere local weather con- New York, Plattsburgh,New York 12901 USA. ditions may be severeand unpredictable. Find-

328 The Auk 103: 328-340. April 1986 April 1986] Useof Tree-fallGaps by Birds 329 ing adequatefood after a flight may be difficult known, all were well establishedand at least 3-5 yr becausethe landing area often is unknown to old. migrants. Consequently,it would be advanta- We nettedbirds 3 days/week,except when rain or geousfor birds to recognizeand quickly select strong precludeduse of nets, from about 15 min before sunrise until about noon. We standard- foraging sites that are profitable (Martin and ized netting effort among daysto avoid problemsas- Karr 1986). This may be particularly true for sociatedwith diurnal variability in capturerates (Karr migrants in east-centralIllinois, where natural 1981).We nettedbirds on 18 daysin spring(19 April habitat is limited and existsas isolatedpatches 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 cap- work by Graberand Graber(1983) demonstrat- tured birds were identified, sexed (when possible), ed that migrant warblersin spring experience weighed,banded with U.S. Fish and Wildlife Service a net lossof energywhile foragingin woodlots aluminum leg bands, and releasedat point of cap- in east-centralIllinois. By demonstratingdif- ture. ferencesin resourcelevels between gapsand We comparedthe number of capturesin gap and forest understory sites, and by demonstrating forest understory nets using Chi-square tests (for samplesizes >_30) or FishersExact Test (for n < 30; correlationsbetween capturefrequency and re- Sokal and Rohlf 1981).We correlatednumber of cap- source level, we support the prediction that tures per net with measuresof habitat structure and during migration birds selectas foraging sites resource abundance (see below) using Spearman's microhabitats with abundant food resources. rank correlation (Sokal and Rohlf 1981). We divided speciesinto six major trophic groups STUDY AREA AND METHODS in spring (flycatchers,ground insectivores,bark for- agers,foliage insectivores, omnivore-granivores, and Study site.--We mist-netted birds in William Tre- nectarivores)and seven in fall (above six plus fru- lease Woods, an upland forest tract of about 24 ha givores)based on literature (Martin et al. 1951, Will- located 6.5 km northeast of Urbana (Champaign son 1974, Willson et al. 1982) and personal observa- County), Illinois. Principal in the woods are tions (seeAppendix). Nectarivoreswere represented sugarmaple (Acersaccharum), hackberry (Celtis occi- by one species(Ruby-throated Hummingbird, Ar- dentalis),white ash (Fraxinusamericana), slippery elm chilochuscolubris), and we did not include nectari- (Ulmus rubra), basswood (Tilia americana), red oak vores in the analysesof trophic groups.We catego- (Quercusrubra), and buckeye (Aesculusglabra). The rized frugivoresas primary [those that dependheavily understorysupports a 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(Lindera benzoin). [thosethat consumefruit regularlybut lessoften than Many speciespresent in Trelease Woods produce primary frugivores,i.e. 25-50% of foraging observa- fruits that are eaten by birds in late summerand fall. tionsin fall (Hoppespers. obs.)] and comparednum- Theseinclude spicebush, hackberry, Virginia creeper bers of capturesof all frugivoresand number of cap- (Parthenocissusquinquefolia), wild grape(Vitus vulpina), turesof primaryand secondaryfrugivores separately. moonseed(Menispermum canadensis), catclaw (Smilax Becauseconsiderable attention has been placed on hispida),poison ivy (Rhusradicans), and wahoo (Eu- the useof gapsby frugivoresin fall (e.g.Thompson onymusatropurpureus). and Willson 1978, 1979), we also examined use of Birds.--We used mist nets (12 m length, 2.6 rn gapsand forestunderstory during springby species height, 36 mm mesh)to samplebirds using the lower that are frugivorous in fall. levels(bottom 2-3 m) of gapsand forestunderstory Vegetation.--We recorded vertical distribution of during spring and fall 1983. We believe that the ad- vegetationat eachnet site following the methodsof vantagesgained by simultaneoussampling of all sites Karr (1971).Presence or absenceof vegetationin each outweigh any biases associatedwith mist-net sam- of 14 height intervalswas noted at 40 points per net pies (Karr 1979, Martin and Karr 1986; but see Rem- (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, 4.6- vious 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 Percentagecover for a given height interval was cal- 1981, Willson et al. 1982, Martin and Karr 1986). Six culatedas the percentageof all points with vegeta- nets 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 foliagedistributions change less did not placenets in forestunderstory between gaps rapidly. that were less than 40 rn apart or within 20 m of a We recordedthe number of trees[individuals with gap edge. Although exact age of the gaps is not a diameterat breastheight (DBH) of at least7.6 cm] 330 BLAKEAND HOPPES [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 samplessometimes included area beyond gap edges. Thus,although gaps were definedas a "verticalhole" in the vegetation,vegetation above 2 m could be re- cordedat gap nets. Insects.--Weused 15 x 15-cmplates of plexiglass coated with a thin layer of Tree Tanglefoot (Cody 1980,Hutto 1980) to sampleinsect abundance. Plates were suspended at about I and 2 m at four points .05hc•circle Ii ß II around eachnet (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 insects were counted, identified to order, and mea- sured to the nearestmillimeter. Traps were covered Fig. I. Samplingdesign for gapand forestunder- with a thin sheet of plastic when not in use. We story nets. recognize that our data are not a direct measureof the availability of insectseaten by all insectivorous birds. However, we assume that the densities of in- in a 0.05-ha circle (12.6 m radius) centered on the sectscaught are correlatedwith densitiesof insectiv- middle of eachnet (Fig. I). Treeswere identified and orousbird food and that the results provide a basis assignedto I of 7 size classes:7.6-15, 15-23, 23-38, for comparinginsect availability amongsites (Hutto 38-53, 53-69, 69-84, and 84-102 cm DBH (James and 1980).Efficiency of sticky traps varies with such fac- Shugart 1970). tors as speed (Johnson 1950) and position of Fruit-producingshrubs and vines were sampled board relative to vegetation (Login and Pickover along2-m-wide transects (80 m2 total/site) at eachnet 1977).We minimized effectsof thesefactors by hang-

r• /•...,,,,,,EARLYSPRING W

Z

13 I /•rest LATE SPRING FALL

3- ./'gap 1

O 20 •0 60 8'0 2'0 4'0 6'0 PERCENT COVER Fig. 2. Foliageprofiles for gap and forestunderstory sites. April 1986] Useof Tree-fallGaps by Birds 331

ing all boardsin the sameposition for eachsampling 8O period and by not samplingon windy days. Prior to testing for differencesbetween gap and forest understory sites, we tested the data on vege- tation and resourceabundance for normality (Sha- piro-Wilks test). We used a Mann-Whitney test (stan- 6 dard normal deviate calculated,SND) when there was a significant departure from normality and t-tests Gaps 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 •,4c c• equal.

RESULTS h 20

Vegetation.--Forest understory sites were dominatedby sugarmaple, basswood,and elm; gaps were dominated by oak, ash, and hack- ' 2'3 berry. Small trees (7.6-15 cm DBH) were dom- inant in gaps,and averagenumber per net site September October was higher in gapsthan in forest locations.All Fig. 3. Abundanceof fruits (mean + ! SD) at gap other size classes were more abundant in forest and understorysites. Differences between gaps and sites than in gaps, but total tree density was understorysites were significanton all dates(t > 6.6, 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).Vegetation density per size classwere pronouncedbetween gaps in the middle canopywas not significantlydif- and understory,and averagebasal area over all ferent betweengaps and 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) sites in early fall (t ---2.40, P < 0.05), but dif- than in gaps(2.1 m2/0.05 ha). ferencesbetween gaps and forest understory Vegetationprofiles showed a marked differ- sites decreasedas 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. abundanceof foliage over the eight intervals. Fruit and insect abundance.--Abundance of Ranked abundances were not correlated be- fruit was greater in gaps than in forest under- tween gapsand understorysites during spring story on all sampling datesin fall (t > 6.6, P < or fall 0.001 on each sampling date) (Fig. 3). Abun- We distinguishedfour foliage layers follow- dance of ripe fruits was highest early in Sep- ing constructionof foliage proflies: ground (0- tember 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 re- 2). All net sites had dense ground cover, and moval largely was completein understorysites there was no significant difference in density after the first week in October,but somegaps of ground foliage between gapsand nongaps. retained fruit into December(Hoppes unpubl. 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 vegetation in the upper canopy understorysites. Understory siteseither 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, . 332 BLAKEAND HOPPES [Auk,Vol. 103

70- Spring Fall

50

30 - 10- +: ?,

g f g f g f g f g f g f g f g f g f g f g f g 13 20 31 5 2 12 23 2 15 30 May June September October

Fig. 4. Number of insectscaptured (mean _+ 1 SD) on sticky traps (4 traps, totaling 0.18 m2/net site) placed around gap (g) and forest understory (f) nets.

Most insectscaptured in spring were small from 2 to 152 birds/100 MNH in fall. Only two (<2 mm) Diptera, although Coleoptera also individuals,both in spring,were recapturedon were well represented. By contrast, most in- the sameday and at the samenet. Thus, inclu- sectscaptured in fall were large Diptera and sion of recapturesdoes not bias resultsby in- Coleoptera, although small Diptera also were troducing overrepresentation of individual common. Average number of captures was preferences.All specieswith number of cap- higher in gapsduring both springand fall (Fig. tures in gap and forest understorynets are list- 4; Wilcoxon Signed Rankstest, P < 0.05 spring ed in the Appendix. and fall). Differencesbetween gapsand under- Gap-understorycomparisons.--Number of cap- story sites were more pronounced in spring, tureswas higher in gap than in understorynets when significant differences existed on 3 of 5 on 16 of 18 datesin spring and 20 of 24 dates sampling dates, than in fall, when significant in fall. Total number of capturesand average differencesexisted on only 2 of 6 dates(Fig. 4). numberof capturesper net were higher in gaps Speciesnumbers and capturesof birds.--We cap- than in understorynets during both springand tured 1,010 birds (including 69 recaptures, fall (Table 1). Similarly, average number of 6.8%), representing 74 species, during 1,416 speciescaptured per net was higher in gap nets mist-net hours (1 MNH = 1 mist net open 1 h) (Table 1). In both seasons, however, the total in spring, for an overall capture rate of 71 cap- number of speciescaptured in all gap netswas tures/100 MNH. In fall we caught 458 birds (14 not significantly greater than the number cap- recaptures,3.1%), representing 44 species,dur- tured in all understory nets (Table 2), indicat- ing 1,504 MNH, for a capture rate of 31/100 ing that a lower similarity in speciescomposi- MNH. Capture rates at individual nets ranged tion existedamong forest understorysites than from 7 to 161 birds/100 MNH in spring and among gaps.

TABLE1. Capturesin gap and forestunderstory nets during spring (S) and fall (F).

Gap Forest Test P Total captures (S) 628 382 X2 = 60.0 <0.001 (F) 356 102 X2= 143.0 <0.001 Captures/net (S) 105 64 t = 7.1 <0.001 (F) 58 17 t = 5.9 <0.001 Total species (S) 69 60 X2 = 0.004 >0.50 (F) 43 28 X2 = 3.2 <0.10 Species/net (S) 38 27 t = 9.1 <0.001 (F) 20 8 t = 5.9 <0.001 April1986] Useof Tree-fall Gaps by Birds 333

T^BI•E2. Specieswith markeddifferences (P < 0.01) in capturefrequency between gap and forestunderstory nets in TreleaseWoods, spring and fall 1983.

Spring Fall Captures Captures Speciesa Gap Forest P Gap Forest P Yellow-bellied Flycatcher 20 4 <0.001 Least Flycatcher 18 9 < 0.070 White-breasted Nuthatch 0 5 < 0.062 Golden-crownedKinglet 48 7 < 0.001 Ruby-crownedKinglet 10 2 <0.032 21 1 <0.001 Gray-cheekedThrush 14 6 <0.074 Swainson's Thrush 49 31 <0.024 13 4 <0.037 Wood Thrush 8 20 <0.023 Gray Catbird 30 9 <0.001 Red-eyed Vireo 10 0 <0.002 Magnolia Warbler 22 9 < 0.018 22 2 < 0.001 Yellow-rumped Warbler 8 17 < 0.064 Blackburnian Warbler 25 5 <0.001 Palm Warbler 0 7 <0.015 American Redstart 40 5 <0.001 7 0 <0.015 Ovenbird 76 22 < 0.001 Northern Waterthrush 23 2 <0.001 Canada Warbler 16 4 <0.005 7 1 <0.032 White-throated Sparrow 19 9 <0.051 American Goldfinch 6 0 <0.031 Scientificnames are given in the Appendix.

A minimum of 6 captures is required to at- 16 species(Table 2) were caught more often in tain significance at the 5% level (Fisher Exact gap nets. Test, 2-tailed). During spring, 44 specieshad 6 Speciescaptured significantly more often in or more captures, and we expected 2.2 species gapswere not simply canopyspecies following to show a significant difference (P < 0.05) in the edge of vegetation downward. In fact, most captures between gaps and understory nets speciesthat showed a preferencefor gaps (Ta- basedon chancealone. In this study, 11 species ble 2) were birds of the ground or lower vege- displayeda differencein capturefrequency at tation levels. When all species(including those the 5% level and 16 at the 10% level (Table 2), representedby 1-5 captures)were considered, significantly more than expected by chance 51 specieswere captured more often in gaps alone. Two species,Wood Thrush (Hylocichla and 16 in forest understory nets, a ratio signif- mustelina)and Palm Warbler (Dendroicapalma- icantly different from even (X2 -- 18.3, P < rum),were capturedsignificantly more often in 0.001). In addition, although 14 species were forest understory than in gap sites (Table 2). captured only in gap nets, only 5 were restrict- Two other species, White-breasted Nuthatch ed to forestunderstory (X 2 = 4.26, P < 0.05). (Sitta carolinensis)and Yellow-rumped Warbler Seventeen species were represented by at (Dendroicacoronata), also were captured more least 6 capturesin fall, and based on chance often in understorynets, but differenceswere alone we expected1 speciesto show a signifi- not significant. The thrush and nuthatch nest cant (P < 0.05) difference in number of cap- in Trelease Woods, and territories are located tures between gapsand understorysites. How- within undisturbed forest (JGBpets. obs.), ac- ever, 9 speciesdisplayed significant differences counting for the greater number of capturesin in capturefrequency between gapsand under- forestunderstory sites. Also, the nuthatchpref- storysites (Table 2), and all were capturedmore erentiallyforages on large trees(JGB pets. obs.), frequently in gaps. Only 2 species, White- 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 vegetationbetween gaps and forestunderstory forestunderstory than in gaps(see Appendix). sites were apparent. The remaining 12 of the Thirty-seven specieswere capturedmore often 334 BLAKEAND HOPPES [Auk,Vol. 103

TABLE3. Capturesby trophic groups, in gap and forest understory nets 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 Season Speciesb Gap Forest X2 P Flycatchers S 8.5 80 22 33.0 < 0.001 F 5 9 4 1.9 <0.50 Bark foragers S 8.5 24 26 0.1 > 0.50 F 4 10 7 0.5 >0.50 Ground insectivores S 12 204 137 13.2 <0.001 F 2 79 24 29.4 <0.001 Foliage insectivores S 33 194 105 26.5 <0.001 F 16 165 35 89.4 <0.001 Granivore-omnivores S 11 119 90 4.0 < 0.05 F 5 23 9 6.1 < 0.025 Frugivores(total) S 16 180 129 8.4 <0.005 F 10 66 23 20.8 < 0.001 Primary S 9 146 92 12.3 <0.001 F 8 48 17 69.1 <0.001 Secondary S 7 34 37 0.1 > 0.50 F 2 18 6 6.0 <0.025 aFrugivores include species that regularly consumefruit during fall migration.Primary frugivoresinclude: Northern Flicker, thrushes (6 species),Gray Catbird, and Brown Thrasher. Secondaryfrugivores include: Red-belliedand Red-headedwoodpeckers, European Starling, Red-eyed Vireo, Yellow-rumpedWarbler, Scar- let Tanager, and Northern Oriole. bSeveral species were assignedto two groupsequally (seeAppendix). in gaps and only 2 in understory sites (X2 = primary and total frugivoreswere capturedsig- 31.4, P < 0.001), when all specieswere consid- nificantly more often in gaps (Table 3). ered. Sixteen specieswere captured only in Vegetation,resource abundance, and numberof gaps,while 1 was captured only in forest sites capturesof birds.--Foliagedensity in 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 be- jor 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 foragersdisplayed no differencein and foliage density in lower- and upper-cano- frequency of capturesbetween gap and forest py layers only, using each net as a single sam- nets (Table 3). Other groups, particularly fly- ple point (n = 12). catchers,foliage insectivores,and ground in- During spring, species richness and total sectivores,were caughtsignificantly more 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 fo- during 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 givoresdisplayed no differencein capturesbe- insectivores showed no correlation with low- tween gapsand forest understory (Table 3). level foliage but correlatednegatively with up- The number of individuals captured in all per-canopyfoliage (Table 4). major trophic groups was higher in gaps than We comparedtotal number of capturesdur- 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-can- among major trophic groupsdid not differ be- opies during these periods. Number of cap- tween 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] Useof Tree-fall Gaps by Birds 335

TABLE4. Correlationsbetween number of capturesand foliagedensity in lower and uppercanopies (LC, UC), insectabundance (IN), and fruit abundance(FR) at gap and forestunderstory nets. Only significant correlations(Spearman's r) areshown: P < 0.05,r > 0.587;P < 0.01,r > 0.727;and P < 0.001,r > 0.846. SeeTable 3 for descriptionof frugivore categories.

Spring Fall LC UC IN LC UC IN FR Totalspecies 0.63 -0.81 0.62 0.65 -0.73 0.64 Total captures 0.61 -0.84 0.60 0.72 -0.64 0.63 Flycatchers 0.80 -0.88 0.76 -0.63 Ground insectivores -0.61 0.71 -0.73 0.72 0.59 Foliageinsectivores 0.67 -0.81 0.74 0.68 -0.73 0.59 Granivore-omnivores 0.62 Frugivores(total) -0.66 0.68 Frugivores(primary) 0.70 -0.89 0.67 -0.69 0.58

P < 0.05), middle (rs = 0.542, P < 0.10), and The numberof capturesper net during early, late periods(r• = 0.684, P < 0.05) and correlated middle, and late fall migration correlatedpos- negatively 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 < rs = -0.734, P < 0.01; late, rs = -0.787, P < 0.10; late, rs = 0.49, P < 0.20). The number of 0.01). captures correlated negatively with foliage When all net siteswere included, only num- density in the upper level (early, rs = -0.45, ber of foliage insectivores correlated signifi- P < 0.20; middle, rs = -0.70, P < 0.05; late, cantly with 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 one gap site was significantlyless (t = 2.2, P < and foliage insectivores,correlated positively 0.05) than the mean for other gap sites. The with number of insects captured at each net reasonsfor the unusuallylow numberof insect site. Severalgroups, however, correlatedposi- capturesat one gap are not clear, but the po- tively with fruit abundanceper net (Table 4). sitions of the insect traps or other factorsun- When frugivores were separatedinto primary related to actual insect abundancein the gap and secondarygroups, only primary frugivores (e.g. microclimatic conditions) may have un- showed a significant associationwith fruit duly influencedthe effectivenessof the traps. abundance. As a consequence,we reexaminedcorrelations between bird community variables and insect DISCUSSION abundanceat the remaining 11 sites (see Dis- cussion).Both speciesrichness and number of Tree-fall gapsrepresent a distinct microhab- captures correlated significantly with insect itat that differs from the understory of sur- abundance (Table 4). Among trophic groups, rounding forest in vegetationstructure (e.g. fo- flycatchers,in particular,showed a strongcor- liage density, tree size distributions), relation with insect abundance (Table 4). When speciescomposition, microclimatic conditions, all sites were included, correlations were sim- and resourceabundance (Geiger 1950,Denslow ilar in direction but were not as strongas when 1980, Schemske and Brokaw 1981, Brokaw 1982, the one site was eliminated. Chazdon and Fetcher 1984, Martin and Karr Fall patterns were similar to spring with re- 1986). Recent studies (e.g. Karr and Freemark spectto correlationsbetween number of species 1983) have demonstratedthat birds may select and capturesand measuresof habitat structure habitatsbased on slight differencesin vegeta- (Table 4). Two trophic groups,ground insecti- tion or microclimate and thus there is reason votes and foliage insectivores,correlated with to believe that birds are capableof recognizing foliage density in the lower layer, while 5 and selectingtree-fall gapsas a distinct micro- groups correlatednegatively with upper-can- habitat in which to forage (Martin and Karr opy foliage (Table 4). Unlike spring, number of 1986). capturesof all frugivores combined correlated Microclimatic conditions within a gap are a negativelywith upper-canopyfoliage. function of gap size (in relation to canopy 336 BLAKEAND HOPPES [Auk, Vol. 103 height), shape, orientation, and vegetation colonists to mature forest species (Cates and structure,particularly with respectto how these Orians 1975, Hartshorn 1978). Thus, gaps not factors determine the daily duration of direct only supportgreater concentrationsof foliage insolation (Geiger 1950, Lee 1978, Chazdon and than similar areaswithin forest understory but Fetcher 1984). Amount of light, highest daily also support relatively higher proportions of temperatures, and amount of precipitation foliage that are palatable to a broad variety of reaching the ground are higher in gaps than in herbivorous invertebrates. Although foliage adjacentforest understoryand also differ with- density does not directly measureabundance in different sectionsof a gap (Geiger 1950,Den- of foliage insects,it can be taken as an indirect slow 1980, Chazdon and 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 substrates for insects and searching encesin microclimate between gaps and forest substratesfor foliage-gleaning birds. understory are particularly pronounced close Most insectivorous groups occurred more to the ground, within the range of vegetation frequently in gapsin spring and fall, support- sampled by mist nets, and are influenced ing the hypothesisthat birds selectgaps as prof- strongly 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 densitiespro- Schulz 1960 in Denslow 1980, Denslow 1980). vide indirect evidence that a greater resource Differences in amount of radiation penetrat- abundanceattracts many speciesto gaps (Mar- ing 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 lessabundant in 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 species concentrated in gaps in Panama- understory.As the canopy closeswith contin- nian forest, but those speciesconsidered gap ued leaf production,however, the differencein specialistswere insectivorousto a great extent. amount of radiation penetrating below the can- A greater abundanceof foliage and insects opy in gaps and undisturbed forest increases reduces the amount of time required for (Anderson 1964). searchingand travel and results in a faster rate Birds that use lower levels of a forest may be of food ingestion. This may be particularly im- attracted to gaps because of a greater abun- portant during migration, when energy re- danceof resourcesor becauseresources may 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 may be greaterpres- 1982). 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. of evidencesuggest that suchresources are more Leaf litter often is abundant in gaps, and abundantor concentratedin gapsthan in forest consequentlysoil and litter invertebrates are understory. likely to be abundantas well (Bultmanand Uetz Lepidopteran larvae form a major compo- 1984 and references therein). Higher soil and nent of the diet of many foliage-gleaning near-groundtemperatures in gaps, relative to species,and warbler migration in spring coin- forest understory, also may increaseinsect ac- cides with the peak emergence of larvae (Gra- tivity levels over that presentin forest under- ber and Graber 1983). Tree-fall gaps support story. If soil invertebratesare more abundant relatively dense foliage in low levels, much of in gapsthan in forestunderstory locations, par- it produced by pioneer or early successional ticularly early in spring, this is a likely expla- species.Plant speciesare not equally palatable nation for the greater abundanceof ground in- to generalistherbivores, and a continuum of sectivoresin gaps over forest understory. palatability exists from weedy speciesto gap Finally, our study provides direct evidence April1986] Useof Tree-fall Gaps by Birds 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 majorcomponent of the diet of many was smaller than the others, and differences in flycatchersin Illinois (Graber et al. 1974), and microclimate directly related to gap size may flycatchersshowed a strong correlation with have influenced insect activity levels. Despite insectabundance during spring. Higher light the apparentlylow insectabundance, bird cap- levels and a greaternumber of low (under 4.6 tures were still higher than in forest under- m) perches, combined with a greater abun- story sitesand more comparableto other gap danceof insects,may make gapsan especially sites.It is possiblethat birds were attracted to profitablelocation for flycatchinginsectivores. the gap becauseof higher light levels and fo- During fall, differences in insect abundance liage density, relative to surrounding forest were lesspronounced between gapsand forest, understory, on the expectation that insect and thus it would be lessadvantageous for fly- abundance would be greater. However, corre- catchersto concentratetheir activities in gaps lation between bird and flying insect abun- than in understory sites. Fall migration char- dance was strongestfor flycatcherswhen the acteristicallyis much less pronounced than one unusual net was eliminated, suggestingas- spring migration (Graber et al. 1974), and the sessment of actual abundance. Further, in a lack of correlationbetween flycatchersand in- comparisonof bird use of new and old gaps, sectabundance may reflect the low number of Martin and Karr (1986) found evidence to sug- flycatchers caught in fall (n = 13). Little is gest that birds were not simply respondingto known of the habitatpreferences of migrants, higher light levels in gaps. and migrants may selectdifferent (e.g. Birdsmay be attractedto gapsbecause of in- forest edge) in fall than in spring (e.g. Austin creasedcover in lower levels, perhaps as pro- 1970). 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 fly- al. 1951; Thompson and Willson 1978, 1979), catchersand foliage insectivoreswere correlat- and the availability and diversity of fruits is ed with low foliage density in spring, how- greater 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 gapsappears to attractmany foliageinsectivores with foliage densityand 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.Those speciesmost 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 speciesmay consumefruits when they are en- Much of the impetus for this study came from re- countered but may not actively searchfor areas searchconducted by T. E. Martin, and his thoughts and ideas contributed much to this paper. We thank with high fruit concentrations.Total speciesand D. Enstrom,T. Hoppes,L. Larson,and B. A. Loiselle total capturesper net correlated with insect for assistancein the field. This paper has benefited abundancein spring and with fruit abundance from commentsof C. Faanes,D. Levey,B. A. Loiselle, in fall, reflecting the switch in diet of a sub- S. B. Vander Wall, and M. F. Willson. Preparation of stantial proportion of birds captured during this manuscriptwas supportedby a Guyer Fellow- spring and fall migration. ship provided to the senior author by the Depart- Birds may select foraging locations on the ment of Zoology,University of Wisconsin. basisof actual resourcelevels (e.g. insectabun- dance) or on the basis of an indirect index of LITERATURE CITED resourceabundance (e.g. light levels, foliage density) (Martin and Karr 1986). Fewer insects AMERICAN ORNITHOLOGISTS' UNION. 1983. Check-list were captured on sticky traps at one gap than of North American birds, 6th ed. Washington, at remaining gaps during spring. The actual D.C., Amer. Ornithol. Union. 338 BLAKEAND HOPPES [Auk,Vol. 103

ANDERSON, M. C. 1964. Studies of the woodland Neotropics (A. Keast and E. S. Morton, Eds.). light climate.II. Seasonalvariation in the light Washington,D.C., SmithsonianInst. Press. climate. J. Ecol. 52: 643-663. JAMES,F. C., & H. H. SHUGART,JR. 1970. A quanti- AUSTIN,G. T. 1970. Migration of warblersin south- tative method of habitat description.Audubon ern Nevada. Southwestern Natur. 15: 231-237. Field Notes 24: 727-736. BERTHOLD,P. 1975. Migration: control and meta- JoHNsoN,C.G. 1950. The comparisonof suctiontrap, bolic physiology.Pp. 77-128 in Avian biology, stickytrap, and townet for the quantitativesam- vol. 5 (D. S. Farner and J. R. King, Eds.).New pling of small airborneinsects. Ann. Appl. Biol. York, Academic Press. 37: 268-285. BLAKE,J. G. 1986. Species-arearelationship of mi- KARR,J. R. 1971. Structure of avian communities in selected Panama and Illinois habitats. Ecol. grantsin isolatedwoodlots. Wilson Bull. in press. BROKAW,N. 1982. Treefalls:frequency, timing, and Monogr. 41: 207-229. consequences.Pp. 101-108 in The ecologyof a 1979. On the use of mist nets in the study tropical forest: seasonalrhythms and long-term of bird communities.Inland Bird Banding51: 1- 10. changes(E.G. Leigh, Jr., A. S. Rand, and D. M. Windsor,Eds.). Washington, D.C., Smithsonian 1981. Surveying birds with mist nets. Stud. Avian Biol. 6: 62-67. Inst. Press. 1985. Gap-phase regeneration in tropical --, & K. E. FREEMARK. 1983. Habitat selection forest.Ecology 66: 682-687. and environmentalgradients: dynamics in the BULTMAN,T. L., &: G. W. UETZ. 1984. Effects of struc- "stable" tropics.Ecology 64: 1481-1494. LEE,R. 1978. Forestmicroclimatology. New York, ture and nutrient quality of litter on abundances Columbia Univ. Press. of litter-dwelling arthropods.Amer. Midi. Nat- ur. 111: 165-172. LOGIN,G. R., & C. A. PICKOVER.1977. Sticky traps CATES,R. G., • G. H, ORIANS. 1975. Successional and spider prey. Carolina Tips 40: 25-26. MARTIN, A. C., H. S. ZIM, & A. L. NELSON. 1951. statusand palatabilityof plantsto generalisther- Americanwildlife :a guide to wildlife food bivores. Ecology 56: 410-418. habits. New York, McGraw-Hill Book Co., Inc. CHAZDON,R. L, &:N. FETCHER.1984. Photosynthetic MARTIN, T. E., & J. R. KARR. 1986. Patch utilization light environmentsin a lowland tropical rain by migrating birds: resource oriented? Ornis forest in Costa Rica. J. Ecol. 72: 553-564. Scandinavicain press. CODY,M. L. 1980. Speciespacking in insectivorous REMSEN,J. V., JR.,& T. A. PARKERIII. 1983. Contri- bird communities:density, diversity, and pro- bution of river-createdhabitats to bird species ductivity. Proc. 17th Intern. Ornithol. Congr.: richnessin Amazonia.Biotropica 15: 223-231. 1071-1077. RUNKLE,J. R. 1981. Gap regenerationin some old- DENSLOW,J. $. 1980. Gap partitioning among trop- growth forestsof the easternUnited States.Ecol- ical trees. Biotropica 12 (Suppl.): 47- ogy 62: 1041-1051. 55. 1982. Patterns of disturbance in some old- FOGDEN,M.P. L. 1972. The seasonalityand popu- growth roesicforests of eastern North America. lation dynamicsof equatorial forest birds in Sa- Ecology63: 1533-1546. rawak. Ibis 114: 307-343. $CHEMSKE,D. W., & N. BROKAW. 1981. Treefalls and GEIGER,R. 1950. The climate near the ground. Cam- the distribution of understorybirds in a tropical bridge, Massachusetts,Harvard Univ. Press. forest. Ecology 62: 938-945. GRABER,J. W., & R. R. GRABER.1983. Feedingrates SCHULZ,J.P. 1960. Ecologicalstudies on rain forest of warblers in spring. Condor 85: 139-150. in northern Suriname. Verh. K. ned. Akad. Wet. GRABER,R. R., J. W. GRABER,&: E. L. KIRK. 1974. Il- (A. natkd.) 53: 1-267. linois birds: Tyrannidae. Illinois Nat. Hist. Surv. SOKAL,R. R., & F. J. ROHLF. 1981. Biometry: the Biol. Notes No. 86. principlesand practicesof statisticsin biological HALLE, F., R. A. A. OLDEMAN, & P. B. TOMLINSON. research, 2nd ed. San Francisco, W. H. Freeman 1978. Tropical treesand forests:an architectural & Co. analysis.Berlin, Springer-Verlag. THOMPSON,J. N., & M. F. WILLSON. 1978. Distur- HARTSHORN,G.S. 1978. Treefallsand tropical forest banceand the dispersalof fleshy fruits. Science dynamics.Pp. 617-638 in Tropical treesas living 200: 1161-1163. systems(P. B. Tomlinsonand M. H. Zimmerman, --, & . 1979. Evolutionof temperatefruit/ Eds.). Cambridge, England, Cambridge Univ. bird interactions:phenological strategies. Evo- Press. lution 33: 973-982. HUTTO, R. L. 1980. Winter habitat distribution of WALSBERG,G. E. 1983. Ecologicalenergetics: what migratory land birds in western Mexico, with are the questions?Pp. 135-158 in Perspectivesin speciesreference to small foliage-gleaning in- ornithology (A. H. Brush and G. A. Clark, Jr., sectivores.Pp. 181-203 in Migrant birds in the Eds.).New York, CambridgeUniv. Press. April1986] Useof Tree-fall Gaps by Birds 339

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APPENDIX.Number of capturesin gap and forestunderstory nets in TreleaseWoods, Illinois, during spring and fall migration,1983. Species that belong to different trophic groupsin spring (S) and fall (F) have groupsindicated for both seasons.Nomenclature follows the A.O.U. check-list(1983).

Captures Gap Forest Species S F S F Trophic group Ruby-throatedHummingbird (Archilochuscolubris) 7 4 2 Nectarivore Red-headedWoodpecker (Melanerpes erythrocephalus) 2 Bark forager Red-belliedWoodpecker (Melanerpes carolinus) 1 I Bark forager Yellow-bellied Sapsucker(Sphyrapicus varius) 3 4 Bark forager Downy Woodpecker(Picoides pubescens) 3 4 Bark forager Hairy Woodpecker(Picoides villosus) 2 1 Bark forager Northern Flicker (Colaptesauratus) 4 5 2 Ground and bark forager (S), frugivore (F) Olive-sided Flycatcher (Contopusborealis) 4 Flycatcher EasternWood-Pewee (Contopus virens) 6 Flycatcher Yellow-belliedFlycatcher (Empidonax fiaviventris) 20 Flycatcher Acadian Flycatcher(Empidonax virescens) 2 Flycatcher Empidonaxsp. ' 2 3 Flycatcher LeastFlycatcher (Empidonax minimus) 18 I 9 Flycatcher EasternPhoebe (Sayornis phoebe) I I 1 Flycatcher Great CrestedFlycatcher (Myiarchus crinitus) 7 3 Flycatcher Blue Jay (Cyanocittacristata) 8 2 5 Omnivore-granivore Red-breasted Nuthatch (Sitta canadensis) I Bark forager White-breasted Nuthatch (Sitta carolinensis) 5 Bark forager Brown Creeper (Certhiaamericana) 8 3 4 Bark forager Carolina Wren (Thryothorusludovicianus) 1 Foliage insectivore Winter Wren (Troglodytestroglodytes) i 3 Foliage insectivore Golden-crownedKinglet (Regulussatrapa) I 48 2 Foliage insectivore Ruby-crownedKinglet (Reguluscalendula) 10 21 2 Foliage insectivore Veery (Catharusfuscescens) 12 2 6 Ground insectivore (S), fru- givore (F) Gray-cheekedThrush (Catharusrainlinus) 14 4 6 Ground insectivore (S), fru- givore (F) Swainsoh's Thrush (Catharusustulatus) 49 13 31 Ground insectivore (S), fru- givore (F) Hermit Thrush (Catharusguttatus) I 12 Ground insectivore (S), fru- givore (F) Wood Thrush (Hylocichlamustelina) 8 3 20 Ground insectivore (S), fru- givore (F) AmericanRobin (Turdusmigratorius) 25 6 17 Ground insectivore (S), fru- givore (F) Gray Catbird (Dumetellacarolinensis) 30 3 9 Ground-foliageinsectivore (S), frugivore (F) Brown Thrasher(Toxostoma rufum) 3 Ground insectivore EuropeanStarling (Sturnusvulgaris) 4 5 Ground insectivore White-eyedVireo (Vireogriseus) i 2 Foliage insectivore SolitaryVireo (Vireosolitarius) 2 Foliage insectivore Warbling Vireo (Vireogilvus) 1 Foliage insectivore Red-eyedVireo (Vireoolivaceus) I0 I0 8 Foliage insectivore(S), fru- givore (F) Golden-winged Warbler (Vermivorachrysoptera) 2 Foliage insectivore TennesseeWarbler (Vermivoraperegrina) 9 I 3 Foliageinsectivore 340 BLAKEAND I-IOPPES [Auk, Vol. 103

APPENDIX. Continued.

Captures Gap Forest Species S F S F Trophic group Nashville Warbler (Vermivoraruficapilla) 11 4 8 Foliage insectivore Yellow Warbler (Dendroicapetechia) 1 Foliage insectivore Chestnut-sidedWarbler (Dendroicapensylvanica) 3 1 3 Foliage insectivore Magnolia Warbler (Dendroicamagnolia) 22 22 9 Foliage insectivore Black-throatedBlue Warbler (Dendroicacaerulescens) 2 Foliage insectivore Yellow-rumped Warbler (Dendroicacoronata) 8 8 17 6 Foliage insectivore (S), fru- givore (F) Black-throated Green Warbler (Dendroicavirens) 6 1 7 1 Foliage insectivore BlackburnJanWarbler (Dendroicafusca) 4 25 2 5 Foliage insectivore Palm Warbler (Dendroicapalmarum) 7 Foliage insectivore Bay-breastedWarbler (Dendroicacastanea) 9 19 4 16 Foliage insectivore Blackpoll Warbler (Dendroicastriata) 1 1 4 1 Foliage insectivore Black-and-white Warbler (Mniotilta varia) 5 6 4 4 Bark forager American Redstart(Setophaga ruticilla) 40 7 5 Flycatcherand foliage in- sectivore Worm-eating Warbler (Helmitherosvermivorus) 1 1 Foliage insectivore Ovenbird (Seiurusaurocapillus) 47 76 42 22 Ground insectivore Northern Waterthrush (Seiurusnoveboracensis) 23 3 2 2 Ground insectivore Louisiana Waterthrush (Seiurusmotacilla) 1 1 Ground insectivore Kentucky Warbler (Oporornisformosus) 6 4 Foliage insectivore Connecticut Warbler (Oporornisagilis) 3 Foliage insectivore Mourning Warbler (Oporornisphiladelphia) 11 5 Foliage insectivore Common Yellowthroat (Geothlypistrichas) 5 1 2 Foliage insectivore Hooded Warbler (Wilsoniacitrina) 1 Foliage insectivore Wilson's Warbler (Wilsoniapusilla) 1 1 Foliage insectivore Canada Warbler (Wilsoniacanadensis) 16 7 4 1 Foliage insectivore Yellow-breasted Chat (Icteria virens) 1 1 Foliage insectivore Scarlet Tanager (Pirangaolivacea) 7 3 Foliage insectivore Northern Cardinal (Cardinaliscardinalis) 7 2 Omnivore-gramvore Rose-breastedGrosbeak (Pheucticusludovicianus) 1 5 5 Omnivore-gramvore Indigo Bunting (Passerinacyanea) 66 56 Omnivore-gramvore Rufous-sidedTowhee (Pipiloerythrophthalmus) 3 Omnivore-gramvore Fox Sparrow (Passerellailiaca) 1 1 Omnivore-gramvore Lincoln's Sparrow (Melospizalincolnii) 2 Omnivore-gramvore Swamp Sparrow (Melospizageorgiana) 1 1 Omnivore-gramvore White-throated Sparrow (Zonotrichiaalbicollis) 19 1 9 1 Omnivore-gramvore Common Grackle (Quiscalusquiscula) 1 14 5 7 Omnivore-gramvore Brown-headed Cowbird (Molothrus ater) 7 5 Omnivore-gramvore Orchard Oriole (Icterusspurius) 1 Foliage insectivore Northern Oriole (Icterusgalbula) 4 Foliage insectivore American goldfinch (Carduelistristis) 6 Omnivore-granivore Probably Willow Flycatcher (Empidonaxtraillii).