ORNISSCANDINAVICA 17: 165-174. Copenhagen1986

Patch utilization by migrating : resource oriented?

Thomas E. Martin and James R. Karr

Martin T. E. and Karr, J. R. 1986. Patch utilization by migrating birds: resource ori- ented? - Ornis Scand. 17: 165-174.

Use of gap (created by tree falls) and non-gap forest understory sites by migrating birds in central Illinois was studied during spring and autumn for three years (1978- 1980). Fruit and understory foliage were concentrated in gaps. Birds that relied on these resources (foliage-gleaning insectivores, frugivores in autumn) used gaps more than non-gaps. Birds that fed on food other than fruit and foliage insects ("frugi- vores" in spring, other insectivores) did not use gaps more than non-gaps. abundance varied markedly among gap and non-gap sites, potentially reflecting differences in site preferences. Site selection, as determined by bird abundances, was consistent (correlated) between years for birds that fed on items that were concen- trated in gaps but not for birds that did not rely on these patchy resources. Foliage density is a measure of foraging substrates for foliage-gleaning birds to search. Abun- dance of foliage-gleaning insectivores was highly correlated with foliage density in both spring and autumn. Frugivore abundance was highly correlated with fruiting fo- liage density during autumn when they are frugivorous, but not during spring when they are insectivorous. Insectivores not relying on foliage insects or fruit were un- correlated with either index of resource availability. These same relationships hold even when examining gap sites only. Thus, migrants can be consistent in their se- lection of foraging sites and this consistency appears to exist when resource densities are markedly different among sites (patchy) but not when resources are more disper- sed.

T. E. Martin, Dept of Zoology, Arizona State University, Tempe, Arizona 85287, U.S.A. J. R. Karr, Smithsonian Tropical Research Institute, Box 2072, Balboa, Re- public of Panama.

bers at the same time as their food and 1. Introduction peak (Graber Graber 1983). Migratory birds use considerable amounts of energy for When resources are patchy, foraging efficiency also is migration; they lose 1-4% of their gross body weight enhanced by selecting habitat patches with more abun- per hour of flying (Graber and Graber 1962, Hussell dant food resources (Charnov 1976, Krebs et al. 1978, 1969). As a result, migrants require periodical replen- Cowie and Krebs 1979, Martin 1985a). However, patch ishment of lost fat stores at stop-over sites to allow suc- selection by migrating birds is rarely studied. Instead, cessful completion of migration (Nisbet and Medway analyses of habitat selection during migration typically 1972, Berthold 1975). Individuals able to maximize have been restricted to general habitat patterns (e.g., their foraging efficiency (rate of food intake) at these Parnell 1969, Laursen 1976, Rappole and Warner 1976, stop-over sites increase their rate of fat deposition and Martin 1980). Only Willson et al. (1982) examined fine their chances of successful migration. One means of en- scale habitat selection patterns during migration. hancing foraging efficiency is to migrate when food is Willson et al. (1982) showed that migrants were more most abundant. Indeed, spring arrival of migrant war- abundant in understory of light gaps than in undisturbed blers in southern Illinois coincides with eruptions of forest understory. Light gaps are created by tree falls. their primary food (lepidopteran larvae); warbler num- More light reaches the forest floor in light gaps, causing

Received 7 January1985 Accepted 20 June 1985 ? ORNIS SCANDINAVICA

11 ORNIS SCANDINAVICA 17:2 (1986) 165 increasedgrowth and colonization by understoryveg- 1978-1980. Autumn was divided into early (before 15 etation (Hartshorn 1978, Thompson 1980, Runkle September) and late (after 15 September) subseasons. 1982). As a result, understoryfruits and foliage insects Nets (30 or 36 mm mesh, 4 shelves, 12 m long) were are more concentratedin light gap patchesthan in non- paired between gap and non-gap sites in the forest un- gaps. Thus, preferences for light gaps may reflect se- derstory. Gap nets were placed at the edge of gaps lection of patches with abundantfood by migrants. rather then in the centres to minimize net visibility as a Yet, the density of plants, and associated food re- bias on capture rates. Similar methods have been used sources, differsamong gap sites (Runkle 1982). If birds by Schemske and Brokaw (1981) and Willson et al. are truly selecting sites based on availability of re- (1982) to examine gap and non-gap use by birds. A core sources, then two resultsare predicted:(1) Birdsshould of 10 (5 pairs) nets was placed in the same locationsin be more abundantsat sites with greateramounts of the all seasons and years to examine consistency of site pref- food types they eat. (2) Birds shouldbe more abundant erences by birds. The gaps used for these locations were at the same sites each year, assuming differences in all old and well established with dense shrub under- qualityamong sites varies only a little among years. To story. In each season an additional gap:non-gapnet examine these predictions, we established permanent pairing was placed in a new gap (< 1 year old) that had gap and non-gapsites and monitoreduse of the under- not developed a shrub understory. story of these sites by migratingbirds during spring and A total of 5,212 mist net hours (MNH) was accrued autumn for three years. We focus on three questions. over the six seasons with 458, 476, and 765 MNH during First, do migratingbirds of differingforaging habits dif- springs, and 1205, 1344, and 964 MNH for autumns of fer in their abundanceat gap versusnon-gap understory 1978, 1979, and 1980, respectively. Nets were opened 30 sites? Second, are birdsconsistent in their site selection min before to 30 min after sunrise and left open for 4-6 (i.e., more abundantin the same sites) each year? Fi- hours in all seasons. nally, are differences in bird abundancesamong sites Vegetation cover was sampled during autumn 1979, correlatedwith differences in indexed resource abun- when plants were fruiting, using the point sample dance? method of Karr (1971) in which presence/absence of vegetation was noted at the following height intervals: G (ground), < 0.25, 0.25-0.5, 0.5-1, 1-2, 2-3, 3-5, 5- 2. Study area and methods 7.5, 7.5-10, 10-12.5, 12.5-15, 15-20, and > 20 m. Num- The study site was Trelease Woods, a 22-ha woodlot lo- ber of points in which vegetation was present relative to cated northeast of Urbana, Illinois, USA. Woodlots and total number of points sampled provided percent veg- other forest islandsare importantsources of forest habi- etation cover in each height interval. A total of 30 tat for migratingand breedingbirds in the mid-western points was sampled on three transects at each net. Two U.S. (Martin 1980, 1981, Blake 1983). Principaltree transects ran parallel to the net at a distance of 3-4 m species in TreleaseWoods includedoaks Quercusspp., from the net on each side with points taken every 1 m. sugar maple Acer saccharum, elm Ulmus, white ash The third transect ran perpendicular to and bisected the Fraxinus americana, basswood Tilia americana, and net with points taken every 2 m. Foliage of individual hackberry Celtis occidentalis. The understory included plants that actually bore bird-dispersed fruits (males paw paw Asimina triloba, spicebush Lindera benzoin, and some female individuals of some plants species did grape Vitis spp., moonseed Menispermum canadensis, not bear fruit) was tabulated separately to provide fo- pokeweed Phytolacca americana, Virginia creeper Par- liage cover of fruiting plants. Each site was ranked rela- thenocissus, and poison ivy Toxicodendron radicans. tive to total foliage cover and fruiting foliage cover in Most census proceduresused in studies of birdswere the understory (< 3 m). Analyses based on ranks are designedfor use duringbreeding season when most spe- preferred because true availabilities of resources to ani- cies are relativelysedentary and vocal. Studies during mals are difficult to measure accurately (Johnson 1980). other seasons and in regions such as tropical forest Abundance of birds at each net site were ranked where assumptionsof more classicalprocedures are not based on capture rates (birds/100 MNH) and compared met requiredifferent methods (Karr1979, 1981). Birds among seasons and years and with foliage and fruit during migration are highly transient and often are not cover rankings using a Spearman rank correlation. vocal, especially in autumn. Thus, we used mist-nets to Comparisons among seasons and years allow examin- measure avian use of patches because: (1) We were in- ation of whether birds are consistently more abundant terested in avian use of forest understory patches from 0 at the same net sites. Comparisons with fruit and foliage to 3 m above the ground and mist-nets sample the un- cover rankings allow examination of whether birds are derstory, (2) mist-netsdo not depend on vocalizations more abundant at sites with more fruit or foliage cover. or sedentary birds, and (3) mist-nets allow examination Correlations were based only on the 10 sites netted in all of avian use of small areas, which is a central focus of 6 seasons. this study. Comparisons of capture rates among net sites, micro- Patchuse by birdswas sampledduring spring (15 Ap- habitats, seasons and years were made using the Fisher ril - 29 May) and autumn (24 August - 18 October) of binomial probability test when sample size was small (n

166 ORNIS SCANDINAVICA 17:2 (1986) < 35) and by X2analysis for large samples. Food habit Tab. 1. Capturerates (birds/100mist-net hours) for birdscap- followed Willson et al. for tured in light gaps and undisturbed(non-gap) understoryin assignments (1982) except - 14 and late Warbler and American Redstart. spring,early (24 August September) (15 Septem- Yellow-rumped ber - 18 October) autumn1978-1980. Yellow-rumped Warblers (scientific names of all birds presented in Appendices A,B) were classified as fo- Autumn Spring liage-gleaning insectivores rather than frugivores be- Gap Non-Gap Gap Non-Gap cause Yellow-rumped Warblers were observed feeding on fruit less than 1% of the time during the periods of a) All birds (70 species) this study (n > 750 observations, unpublished data). Early 1978 37.0 14.7* 1978 53.6 21.9* Late 40.1 20.5* Yellow-rumps appeared to be more frugivorous after 1978 Early 1979 77.7 42.9* 1979 93.3 35.7* mid-October (TEM, pers. obs.). American Redstarts Late 1979 55.7 22.3* were also classified as foliage-gleaning insectivores be- ** ** cause (flycatching) represented less than 25% Early 1980 97.4 71.6* 1980 68.2 34.8* Late 1980 63.1 27.6* of their foraging maneuvers (n > 500 observations, un- ** published data, also Sherry 1979). b) Frugivores (12 species) Early 1978 6.8 4.1 1978 5.2 4.3 Late 1978 17.2 6.2* 3. Results ** 3.1. Vegetation Early 1979 9.9 6.2 1979 19.7 13.4 Late 1979 21.1 6.6* Light gaps had more (p < 0.001) foliage from ground to ** 3 m than sites 1), while sites had Early 1980 12.9 7.0 1980 10.2 8.1 non-gap (Fig. non-gap Late 1980 24.6 7.9* more foliage above 3 m. In addition, gaps had more (p * * < 0.01, t-test) fruit foliage (x = 32.1%, SE = 10.6) be- low 3 m than non-gap sites (x = 4.6%, SE = 6.7) due to c) Foliage-gleaning insectivores (30 species) a greater density of fruit plants in light gaps than non- Early 1978 15.8 2.6* 1978 33.9 12.4* Late 1978 6.8 3.3* gaps. Early 1979 28.5 10.2* 1979 49.2 6.7* Late 1979 7.8 3.2* 3.2. Use of gaps versusnon-gaps ** ** in Early 1980 35.3 20.7* 1980 32.9 8.1* Significantly more birds were captured gaps than in Late 1980 7.9 3.9 non-gaps for all seasons, subseasons, and years (Tab. ** **

Vegetation Profiles d) Other insectivores (16 species) Early 1978 9.4 7.5 1978 8.5 2.9* Late 1978 13.9 7.7* Early 1979 36.8 25.7* 1979 10.9 4.6* Late 1979 23.6 10.1* ** ** Early 1980 44.8 44.8 1980 13.3 8.4* Late 1980 19.7 8.9* ** 'NO N-GAP e) Granivores (12 species) Early 1978 4.9 0.4* 1978 6.0 2.4* -.4 Late 1978 2.1 3.3 I Early 1979 2.6 0.8* 1979 13.4 10.9 I Late 1979 3.1 2.5 LJ Early 1980 2.2 0.0* 1980 11.8 10.2 I Late 1980 5.4 3.4 ** **

* gap/non-gapcomparison: p < 0.05; ** early/lateautumn comparison: p < 0.05.

la). However, the extent and direction of habitat se- lection varied among groups of species with similar food 0 10 20 30 40 50 60 70 80 90 habits. O/o VEGETATION COVER Fig. 1. Vegetationprofiles of gaps and non-gapareas. Profiles are based on the percentvegetation cover in each of a series of 3.2.1. Frugivores height intervalsbased on point samples. Both as a group and as individual species, frugivores

11* 167 Tab. 2. Ratio of the numberof species that were more abun- non-gaps, indicating that other insectivores do not pre- dant in gaps to the numberof speciesthat were more abundant fer gaps (Tab. 2). in non-gapsfor each of the three years and for the three years Ovenbirds were more abundant in combined. refers to the of the significantly gaps Probability(Prob) probability than in several seasons Of combinedratio being even (1:1). non-gaps (Appendices A, B). the remaining 15 other insectivore species, only one 1978 1979 1980 Combined Prob species (Winter ) in only one subseason (late au- tumn 1978) was significantly more abundant in gaps Spring the ovenbird the Frugivores 0:0 1:0 0:0 1:0 ns (Appendices A, B). Thus, only among Foliage-gleaning species in the other insectivore group consistently used insectivores 4:0 9:0 6:0 19:0 <0.001 gaps significantly more than non-gaps. If ovenbirds (the Other insectivores 1:0 1:0 0:0 2:0 ns most abundant "other Granivores 1:0 0:0 0:0 1:0 ns insectivore", Appendices A, B) are excluded then abundance of "other insectivores" Early autumn Frugivores 0:0 0:0 0:0 0:0 ns was not greater (p > 0.05) in gaps in any of the spring or Foliage-gleaning early autumn seasons nor in two of the three late au- insectivores 3:0 5:0 1:0 9:0 0.002 tumn seasons. Other insectivores 0:0 1:0 0:1 1:1 ns Granivores 1:0 0:0 0:0 1:0 ns Other insectivores declined significantly (p < 0.05) in Late autumn abundance from early to late autumn in two of the three Frugivores 3:0 4:0 2:0 9:0 0.002 years, whether or not ovenbirds were included (Tab. Foliage-gleaning other insectivores did not use more than insectivores 1:0 1:0 0:0 2:0 ns ld). Thus, gap Other insectivores 1:0 1:0 1:0 3:0 ns non-gap understory, in contrast to foliage-gleaning in- Granivores 1:0 0:0 0:0 1:0 ns sectivores, but other insectivores did decline in abun- dance during autumn similar to foliage-gleaning insec- tivores. showed patterns of capture that varied with season and habitat. More frugivores were captured in late than 3.2.4. Granivores early autumn (Tab. lb). Frugivores were captured more Granivores were captured more frequently in gaps than often in gaps than non-gaps during late autumn, but not non-gaps in early autumn in all three years, but not dur- during spring or early autumn (Tab. lb). These trends ing spring or late autumn (Tab. le). Comparison of the were reflected by the individual frugivore species. Only number of species that were more abundant in gaps to the Wood Thrush in spring 1979 showed significant the number that were more abundant in non-gaps indi- habitat discrimination outside the late autumn period, cated gaps were not preferred in any season (Tab. 2). while several species were more abundant in gaps in all Thus, a preference for gaps by granivores appears of the late autumn seasons (Appendices A, B). The weak. number of frugivore species that were more abundant in In summary, all insectivores (foliage-gleaning and gaps was significantly greater than the number of spe- other) were more abundant in early than late autumn, cies that were more abundant in non-gaps in late au- while frugivores were more abundant in late than early tumn, but not early autumn or spring (Tab. 2). Thus, autumn, and granivores exhibited no trend. Foliage- frugivores, as a group, used gaps more than non-gaps in gleaning insectivores used gaps more than non-gaps in late autumn, but not in spring or early autumn. all seasons. Frugivores used gaps more than non-gaps only during late autumn. Granivores used gaps more in 3.2.2. Foliage-gleaning insectivores early autumn, and other insectivores did not use gaps More foliage-gleaning insectivores were captured in more than non-gaps in any season. gaps than non-gaps and in early than late autumn (Tab. lc). Several foliage-gleaning species were significantly more abundant in gaps than non-gaps in every season of the number of (Appendices A, B). Comparison spe- Tab. 3. of site selectionbetween cies that were more abundant in to the number that Consistency succeedingyears gaps based on rankcorrelations of capturerates among 10 net sites were more abundant in non-gaps showed insectivores for all birds and individualfood habitsgroups. used gaps more than non-gaps (Tab. 2). Thus, foliage- Autumn gleaning insectivores in the understory clearly use gap Spring more than non-gap sites. 1978-19791979-1980 1978-1979 1979-1980

3.2.3. Other insectivores All birds 0.84** 0.90** 0.91** 0.95** were more fre- Foliage-gleaning Other insectivores generally captured insectivores 0.87** 0.88** 0.90** 0.81** quently in gaps than non-gaps, although the difference Frugivores 0.03 0.60 0.98** 0.95** was not significant in all cases (Tab. ld). The numbers Other insectivores 0.55 0.77* 0.49 0.65 of that were more abundant in gaps were not species * greater than the numbers that were more abundant in p < 0.05; **p< 0.01.

168 ORNIS SCANDINAVICA 17:2 (1986) Tab. 4. Rank correlationsof capturerates with fruitingand to- correlated with both total and fruit foliage during au- tal understoryfoliage densitiesamong 10 net sites for all birds tumn in all three years. Moreover, frugivores were and individualfood habit See text for anges of v aria- groups. more highly correlated (p < 0.01) with fruit foliage than tion in capturerates and foliage and fruit densities. total foliage during autumn (Tab. 4) when they are fru- Foliage- givorous (Thompson and Willson 1978, 1979, Baird gleaning Other 1980, E. Stiles 1980). All insectivoresinsectivores Frugivores Foliage-gleaning insectivores were correlated with to- Spring 1978 tal and fruit foliage during both spring and autumn in all Fruit 0.782** 0.039 0.724* 0.582 three years. Foliage-gleaners were more highly corre- Foliage 0.909** 0.288 0.888** 0.745* lated (p < 0.02) with total foliage than fruit foliage dur- five of the six seasons (Tab. Spring 1979 ing 4). Fruit 0.915** 0.545 0.830** 0.803** Other insectivores, which relied on foods other than Foliage 0.945** 0.548 0.933** 0.639* foliage insects and fruits and which were inconsistent in their site selection, did not exhibit any predictable asso- Spring 1980 Fruit 0.948** 0.285 0.855** 0.582 ciation with total or fruit foliage. Correlations were sig- Foliage 0.900** 0.139 0.939** 0.230 nificant in 7 of the 12 cases (Tab. 3) but showed no clear association with either fruit or total foliage. If Oven- Autumn 1978 birds were not included then other insectivores were not Fruit 0.900** 0.976** 0.806** 0.685* correlated < 0.05) with either total or fruit foliage in Foliage 0.855** 0.891** 0.855** 0.491 (p any spring or autumn. Autumn1979 Association of bird variation with foliage variation Fruit 0.909** 0.948** 0.803** 0.782** may simply reflect greater use of gap understory by 0.855** 0.830** 0.888** 0.745* Foliage birds and that gaps have greater foliage and fruit densi- Autumn 1980 ties than non-gaps. To test whether variation in bird Fruit 0.833** 0.979** 0.838** 0.652 abundances among sites actually tracked variation in to- Foliage 0.867** 0.813** 0.796** 0.785* tal understory and fruit foliage among sites, variation of bird abundances was examined relative to total and fruit * ** p < 0.05, p < 0.01. foliage for only the gap sites. Foliage-gleaning insec- tivore abundance among gap sites was correlated with total in both seasons 3.3. Site selection understory foliage (Tab. 5). Frugi-

3.3.1. Consistency of site selection among years Tab. 5. Rankcorrelations of capturerates with fruitingand to- Capture rates varied markedly among net-sites, from tal understoryfoliage densitiesamong 5 gap sites for all birds and individualfood habits 1.9 to 194.1 birds/100 MNH. If site selection by birds is groups. consistent and site varies a little quality only among Foliage- years, then abundance of birds at the same sites should gleaning Other be correlated among years. However, if site selection is All Frugivores insectivores insectivores random, no correlation should occur. Spring1978 Correlations of capture rates among the net sites Fruit 0.50 -0.30 0.50 0.00 show that foliage-gleaning insectivores were highly con- Foliage 0.90* 0.00 0.90* 0.60 sistent in their site selection between years in both sea- sons were not consistent in site se- Spring1979 (Tab. 3). Frugivores Fruit 0.90* 0.25 0.90* 0.48 consistent lection during spring, but exhibited prefer- Foliage 1.00** 0.80 1.00** 0.08 ences during autumn. Other insectivores were inconsistent during all seasons and years, except spring Spring1980 1979-1980. Fruit 0.90* 0.10 0.90* 0.68 Foliage 0.98** 0.40 1.00** 0.08 3.3.2. Association of bird abundance with fruit and Autumn1978 total understory foliage cover Fruit 0.88* 0.90* 0.20 0.70 0.38 0.90* 0.70 0.30 Foliage cover of fruiting shrubs in the understory (? 3 Foliage m) varied from 0 to 42% and total understory foliage Autumn1979 varied from 19 to 68%. Variation in number of birds Fruit 0.98** 0.98** 0.70 0.60 was correlated (p < 0.01) with variation in both total Foliage 0.90* 0.90* 0.90* 0.30 and for the 10 sites netted in all seasons fruiting foliage Autumn1980 (Tab. 4), but ecological groups differed in their re- Fruit 1.00** 0.88* 0.90* 0.90* sponse. Frugivores were not correlated with either total Foliage 0.70 0.38 0.90* 0.60 or fruit foliage during spring when they were inconsist- ent in their selection of sites. Frugivores, however, were * p < 0.05; ** p < 0.01.

ORNIS SCANDINAVICA 17:2 (1986) 169 vore abundances were not correlated with either foliage but the higher capture rates in old gaps suggest that they measure during spring, but were correlated with fruit may remain in gaps only if recource abundance is high. foliage in all three autumn seasons. Other insectivores were not correlated with either measure during either 4. Discussion season. 4.1. Responseto resourceavailability Thus, migrants that rely on foliage insects or fruit ap- peared to be more abundant in sites having more of the Some birds using gaps primarily forage in forest can- respective food resource. Migrants that do not rely on opies, but they move down when breaks in the canopy foliage insects or fruit were not more abundant in sites occur (F. Stiles 1980). Consequently, gaps may not be as with more foliage and/or fruit. preferred as non-gap forest canopy by these species. However, our objectives were not to determine whether were over individual 3.4. New gaps gaps preferred non-gaps by spe- cies. Rather, when birds use forest understory, we were Birds may prefer understory of light gaps over undis- concerned with determining whether their use of under- turbed forest understory simply because more light is story patches was consistently related to resource avail- available to see and find fruit and insect resources. If ability. Indeed, three primary results indicate migrating true, then migrants should use areas with high light in- birds are resource oriented in their choice of patches tensity without regard to understory foliage and/or fruit (also see Martin 1985b). availability. This possibility was tested by establishing First, birds that depend on resources concentrated in nets in newly created light gaps (less than 1 yr old) in gaps use gaps more than non-gaps (Tabs 1, 2). Foliage- each season, except autumn 1980. Thus, these new gaps gleaning insectivores used gaps more than non-gaps had high light intensity due to canopy gaps, but under- during all seasons. Frugivores are frugivorous primarily story foliage and fruit density did not differ (p > 0.50) during late autumn (after 15 September) when fruits are from non-gaps because of the lack of time for estab- readily available (Thompson and Willson 1979, Baird lishment of light-released plants. 1980, Moore and Willson 1982) and they used gaps In all cases, new light gaps had lower capture rates more than non-gaps during this period. "Frugivores" than older gaps (Tab. 6). New light gaps were included are primarily insectivorous during spring and they ap- in previous analyses with the result that overall capture peared to include large proportions of insects in their rates in all gaps (Tab. la) were less than capture rates in diet during early autumn based on examination of feces old gaps (Tab. 6). Thus, earlier analyses of gap prefer- of captured individuals (TEM, unpubl. data). Most spe- ences were conservative. Capture rate in new gaps was cies designated as "frugivores" depend on insects other significantly higher than the mean for all non-gaps in than foliage insects when they are insectivorous. Conse- two of the five cases and insignificantly higher in the quently, "frugivores" did not use gaps more than non- other three cases (Tab. 6). Thus, birds may be attracted gaps during spring and early autumn when they were in- to gaps because of light conditions present in such areas, sectivorous. Similarly, other insectivores primarily in- cluded flycatchers and bark drillers and gleaners. Little reason exists to expect their resources to be concen- Tab. 6. rates in new Capture (birds/100MNH) gaps relativeto trated in and did not use more than non- the mean of old gaps and non-gaps.Significant differences re- gaps they gaps flect differencein the new gap relativeto old gaps or new gap gaps. Thus, migrants used gaps more than non-gaps relativeto non-gap. during periods when they relied on resources that were concentrated in gaps (foliage insects, fruits), but mi- Spring Autumn grants that did not rely on these patchy resources did Capture Capture not use gaps or non-gaps more. rate SE rate SE Second, consistency in relative abundances of birds at sites among years (Tab. documents that birds must be 1978 3) based on some characteristic asso- New Gap 33.0 19.8 choosing patches Old Gaps 64.5* 11.23 45.4* 7.55 ciated with the patches. Food abundance seems the Non-gaps 18.1* 5.78 17.4 2.18 likely cause of the consistent patch choice because mi- grants were more abundant at the same 1979 consistently sites in when relied on resources New Gap 41.2 47.4 succeeding years they Old Gaps 118.2* 27.20 81.6* 11.79 (fruit, foliage insects) that were concentrated in the Non-gaps 34.5 5.08 31.8* 6.02 patches sampled in this study. Migrants that relied on resources that were to be concentrated in the 1980 unlikely included within our were not New Gap 34.9 patches sampling regime Old Gaps 73.7* 12.99 consistent in their site choice among years. Non-gaps 33.8 7.34 Third, correlations with resource indices indicate that site selection is related to resource availability (Tab. 4). * p<0.05. Foliage density only indexes foliar insect availability,

170 ORNIS SCANDINAVICA 17:2 (1986) but it directlymeasures availability of foliage substrates they feed on a resourcethat is concentratedin gaps and for insects to use and for birds to search. At the same they show no preference for gaps during spring when time, it may index cover from predation (see below). their food is not concentratedin gaps. Second, prefer- Regardlessof the resourcethat this index measures,se- ences for gapscan not reflectmist-nests intercepting dif- lection of sites by foliage-gleaninginsectivores was cor- ferent foliage profiles in gaps as comparedto non-gaps related with it in all seasons. The fruit index also is an becausethe correlationswith resourceindices exist even indirect measure of food availability,but selection of when the gap sites are examinedseparately (Tab. 5). sites by frugivoreswas correlatedwith it in all three au- tumn seasons. Further,frugivores were not simply re- 4.3. Resource tracking spondingto understoryfoliage because they were more highly correlatedwith the fruit index than the total fo- If migrantsare selecting sites based on resource avail- liage index in all three years. In addition,they were not ability, then how do they assess resource abundance? correlatedwith the fruit index duringspring when they Resource samplingmay be necessaryin new or chang- did not consume fruits. Other insectivoresdid not feed ing environments(Heinrich 1976, Oster and Heinrich on fruitsor foliage insects and they were not correlated 1976, Krebs et al. 1978), and once patches have been with either index. Finally, the above relationshipsalso sampledbirds use patches relative to their profitability existed when gap sites were examinedseparately (Tab. (Smith and Sweatman 1974, Cowie and Krebs 1979, 5) indicatingmigrants are respondingto differencesin Krebs et al. 1978). However, northern migrantsthat resourcesamong sites and not just between gap versus stop over at a site during migrationcan not know the non-gap understory.Thus, abundancesof migrantsat distributionof resources. the understorysites were closely related to indexed re- Migrantsoften move in mixed-speciesflocks (Morse source availabilitywhen migrantsconsumed resources 1970) and flockingmay aid in resourcetracking. Flocks measuredby the indices, but not when they consumed may form a sort of informationcenter for patch prof- other resources. itability. The informationcenter hypothesiswas origi- nally suggested to operate in communallyroosting or colonially nesting birds (e.g. Ward and Zahavi 1973, 4.2. Is patch choice related to factors other than food Krebs 1974, Hunt and Hunt 1976, Erwin 1978, Wiksne resources? and Janaus1980, Anderssonand Gotmark1980). How- ever, the that individuals information 4.2.1. sites or cover hypothesis convey Perching safe as to the location of rich food sources has been ques- Birds may select sites with more foliage because they tioned (Evans 1982, Bayer 1982) and even refuted for providegreater numbers of perchingsites or more cover some bird species by experimentaltest (Andersson et from predation. This alternativeis unlikely for frugi- al. 1981). On the other hand, local enhancementoccurs, vores and other insectivores.Other insectivoresshowed where individualsare attractedto groupsof successfully no preferencefor sites with greaterdensities of foliage feeding individuals(Krebs et al. 1972, Krebs 1974, In- in any season. Frugivoresprefer sites with greaterdensi- glis and Isaacson 1978, Andersson et al. 1981, Evans ties of fruit foliage rather than total foliage duringau- 1982). Thus, the success of birds feeding in "richer" tumn and they show no preferencefor sites with more gaps may attractincreasing numbers of other birds. foliage duringspring. Foliage-gleaninginsectivores, on In addition, individuals may stay longer in richer the other hand, were correlatedwith total understory patches and, consequently, more individualsaccumu- foliage and, thus, may be respondingto cover in addi- late at good areas. This accumulationmay also increase tion to or instead of food abundance.However, the in- the probability of attracting other individuals. The creasedfood requirementsof migrantsduring migration slightly higher capture rates in new gaps than in non- may requiremore investmentin foragingat the expense gaps (Tab. 6) suggests that birds may use light as one of other activitiessuch as anti-predatorbehavior (Met- cue for sampling.However, the highercapture rates in calfe and Furness 1984). The fact that frugivoresand old gaps relative to new gaps indicates that birds only other insectivoresdid not base patch choice on cover, spend a short time in the patch if resources are poor. per se, also suggests that, within a habitat, cover may The above alternativesneed to be tested by manipul- not be as importantas food for patch choice duringmi- atingresource richness in gaps and non-gapsand quanti- gration. fying the recruitmentof birds to resourceavailability. Overall, this study shows that migratorybirds are 4.2.2. Mist-net bias consistent in their patch choice and that variationsin Preferencesfor sites may reflect a bias due to the sam- abundanceof migrantsamong sites generally reflects plingmethod (i.e., mist-netting).However, this alterna- variationsin indices of resource abundanceswhen the tive is unlikelyfor two reasons.First, species designated resourcesare patchy. as frugivoresare primarilyground foragers and, thus, are equally susceptibleto net capturesin all sites. Yet Acknowledgements- We thank E. A. Porter for field assis- they show a preference for gaps during autumn when tance and T. Alerstam, G. Hogstedt, R. A. Johnson, J. N.

ORNIS SCANDINAVICA 17:2 (1986) 171 Thompson and M. F. Willson for constructive comments on the Laursen, K. 1976. Feeding ecology of the (Regulus manuscript. This work was supported by grants from Wildlife regulus) during spring migration in Denmark. - Vogelwarte Management Institute to T. E. Martin, Max McGraw Wildlife 28: 180-190. Foundation to T. E. Martin and J. R. Karr, and U. S. Forest Martin, T. E. 1980. Diversity and abundance of spring mi- Service to J. R. Karr. gratory birds using habitat islands on the Great Plains. - Condor82: 430-439. 5. References - 1981. Limitation in small habitat islands: Chance or compe- tition? - Auk 98: 715-734. Andersson, M. and Gotmark, F. 1980. Social organization and - 1985a. Resource selection by tropical frugivorous birds: In- foraging ecology in the arctic skua Stercorarius parasiticus: tegrating multiple interactions. - Oecologia, Berl. 66: 563- a test of the food defendability hypothesis. - Oikos 35: 63- 573. 71. - 1985b. 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Coloniality in terns: the role of social feed- tween behavior, physiology and weather in avian transients ing. - Condor 80: 211-215. at a migration stopover site. - Oecologia, Berl. 26: 193- - 1982. Foraging-flock recruitment at a Black-billed Gull 212. colony: Implications for the information center hypothesis. Runkle, J. R. 1982. Patterns of disturbance in some old-growth - Auk 99: 24-30. mesic forests of eastern north America. - Ecology 63: 1533- Graber, R. R. and Graber, J. W. 1962. Weight characteristics 1546. of birds killed in nocturnal migration. - Wills. Bull. 74: 74- Schemske, D. W. and Brokaw, N. 1981. Treefalls and the dis- 78. tribution of understory birds in a tropical forest. - Ecology Hartshorn, G. S. 1978. Tree falls and tropical forest dynamics. 62: 938-945. - In: Tomlinson, P. B. and Zimmerman, M. H. (eds). Trop- Sherry, T. W. 1979. Competitive interactions and adaptive ical trees as living systems. Cambridge Univ. 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172 ORNIS SCANDINAVICA 17:2 (1986) Appendix A. Capture rates (birds/100 MNH) in gap and non-gap sites for all bird species captured during spring migration of 1978-1980. *p < 0.05.

Spring 1978 Spring 1979 Spring 1980 Spring 1978 Spring 1979 Spring 1980 Non- Non- Non- Non- Non- Non- Gap Gap Gap Gap Gap Gap Gap Gap Gap Gap Gap Gap

Frugivores Connecticut Warbler Common Flicker 0. agilis 0.3 Colaptes auratus 0.4 0.4 0.3 0.5 Mourning Warbler Veery O. philadelphia 0.4 - 0.4 - 0.5 Catharus fuscenscens 1.6 0.5 0.8 1.3 0.3 0.5 Common Yellowthroat Gray-cheeked Thrush 0. trichas C. minimus 0.4 0.5 0.8 0.8 0.5 1.0 Hooded Warbler Swainson's Thrush Wilsonia citrina 0.8 0.3 C. ustulatus 0.8 1.0 7.6 7.1 2.4 1.8 Wilson's Warbler Hermit Thrush W. pusilla - 0.4 1.0 C. guttatus 0.8 1.4 1.7 0.4 1.6 0.5 Canada Warbler Wood Thrush W. canadensis 1.4 2.5 -* 0.8 0.8 Hylocichla mustelina 1.0 2.5 1.3 1.6 Northern Oriole American Robin Icterus galbula 0.3 Turdus migratorius 2.5 1.7 2.6 0.8 Gray Catbird Other insectivores Dumetella carolinensis 2.1 0.8 0.8 0.3 Red-headed Brown Thrasher Melanerpes erythrocephalus 0.5 Toxostoma rufum 0.5 0.3 Red-bellied Woodpecker Red-eyed M. carolinus 0.4 0.5 Vireo olivaceus 1.2 0.8 Yellow-bellied Sapsucker Summer Tanager Sphyrapicus varius 1.2 0.3 Piranga rubra 0.4 - 0.3 Downy Woodpecker Scarlet Tanager Picoides pubescens 0.4 0.5 0.4 - 0.8 0.3 P. olivacea 0.8 0.4 - 0.5 Hairy Woodpecker P. villosus 0.5 0.3 Foliage-gleaning insectivores Eastern Wood Pewee Ruby-crowned Contopus virens 0.5 Regulus calendula 0.8 3.4 0.3* Yellow-bellied Flycatcher Solitary Vireo Empidonax flaviventris 0.4 - 0.4 0.5 0.3 0.3 Vireo solitarius 0.4 Acadian Flycatcher Blue-winged Warbler E. virescens 0.3 Vermivora pinus 0.3 - Great Crested Flycatcher Golden-winged Warbler Myiarchus crinitus 0.5 0.8 V. chrysoptera 0.3 - Red-breasted Tennessee Warbler Sitta canadensis V. peregrina 2.8 -* 6.7 0.8* 1.0 0.5 Brown Creeper Orange-crowned Warbler Certhia americana 0.3 V. celata 0.3 - House Wren Nashville Warbler Troglodytes aedon 2.5 0.8 0.5 0.8 V. ruficapilla 1.6 1.0 8.0 0.8* 2.6 0.5* Winter Wren Northern Parula T. troglodytes 0.8 0.5 Parula americana 0.3 - Black-and-White Warbler Chestnut-sided Warbler Mniotilta varia 1.0 0.8 0.4 0.8 0.8 Dendroica pennsylvanica 2.8 0.5* 0.5 - Ovenbird Magnolia Warbler Seiurus aurocapillus 1.0* 6.3 1.7* 8.4 5.2 D. magnolia 8.9 2.4* 6.3 -* 3.4 * Cape May Warbler Granivores D. tigrina 0.4 0.3 - Blue Jay Yellow-rumped Warbler Cyanocitta cristata 2.0 -* 0.8 0.8 1.3 1.0 D. coronata 2.8 0.5* 2.5 0.8 7.3 1.8* European Starling Black-throated Green War- Sturnus vulgaris 2.4 1.3 bler Northern Cardinal D. virens 0.5 0.4 0.8 0.3 Cardinalis cardinalis 0.8 0.5 Blackburnian Warbler Rose-breasted Grosbeak D. fusca 0.8 1.0 2.5 0.3 - Pheucticus ludovicianus 0.4 - 1.3 1.3 0.8 Palm Warbler Indigo Bunting D. palmarum 1.6 2.4 7.1 2.5* 0.5 0.5 Passerina cyanea 2.4 2.4 5.9 6.3 3.4 3.9 Bay-breasted Warbler Lincoln's Sparrow D. castanea 1.3 0.5 - Melospiza lincolnii 2.5 0.8 0.5 0.5 Cerulean Warbler Swamp Sparrow D. cerulea 0.3 - M. georgiana 0.3 American Redstart White-throated Sparrow Setophaga ruticilla 5.6 2.9 2.1 4.4 1.6* Zonotrichia albicollis 0.8 - 0.4 0.8 1.0 2.5 Worm-eating Warbler Dark-eyed Junco Helmitheros vermivorus 1.7 Junco hyemalis 0.3 Northern Waterthrush Common Grackle Seiurus noveboracensis 0.4 - 2.9 0.4* 0.8 0.5 Quiscalus quiscula 0.4 - Kentucky Warbler Brown-headed Cowbird Oporornis formosus 1.3 0.8 0.3 Molothrus ater 2.5 0.8 0.3 0.3 American goldfinch Carduelis tristis 0.8

12 ORNISSCANDINAVICA 17:2 (1986) 173 Appendix B. Capture rates (birds/100 MNH) in gap and non-gap sites for all bird species captured during early (24 August - 14 September) and late (15 September - 18 October) autumn migrations of 1978-1980. * p < 0.05. Autumn 1978 Autumn 1979 Autumn 1980

Early Late Early Late Early Late Gap Non-Gap Gap Non-Gap Gap Non-Gap Gap Non-Gap Gap Non-Gap Gap Non-Gap

Frugivores Common Flicker 0.4 0.9 0.6 - 0.6 Veery 1.5 1.5 0.3 0.3 1.7 1.7 0.9 0.6 2.6 0.9 Gray-cheeked Thrush 1.2 0.3 1.4 0.3 2.8 0.6* 1.7 0.9 5.9 Swainson's Thrush 3.8 1.9 6.5 1.5* 3.7 2.5 6.6 2.2* 6.0 3.4 6.9 3.9 Hermit Thrush 4.2 3.3 4.4 1.6* 8.9 3.9* Wood Thrush 0.4 0.4 2.4 0.3* 2.0 1.1 2.2 0.3* 2.6 - 2.0 American Robin 0.3 0.3 1.9 0.3 Gray Catbird 1.2 * 0.9 - 1.0 Brown Thrasher 0.4 Red-eyed Vireo 0. 4 0.4 - 0.3 0.6 0.6 0.3 0.3 0.9 0.9 Scarlet Tanager 0.3 - 0.3

Foliage-gleaning insectivores Golden-crowned Kinglet 0.4 0.6 0.6 0.3 1.0 Regulus satrapa Ruby-crowned Kinglet 0.3 - 0.3 Golden-winged Warbler 1.1 1.4 0.8 1.7 0.9 Tennessee Warbler 4.5 0.4* 4.8 1.1* 0.3 - 1.7 Nashville Warbler 0.4 0.8 Chestnut-sided Warbler 0.4 1.4 1.1 0.3 - 3.4 3.4 1.0 Magnolia Warbler 2.6 0.8 3.0 -* 4.0 1.4* 2.5 0.3* 11.2 1.7* 2.0 1.0 Black-throated Blue Warbler 0.4 0.4 0.3 - 1.1 1.1 0.3 0.6 0.9 0.9 - 3.0 Yellow-rumped Warbler 0.3 0.3 Black-throated Green Warbler 0.4 0.8 0.6 0.6 Blackburnian Warbler 0.8 0.6 - 0.3 Bay-breasted Warbler 2.3 0.6 2.1 2.3 0.3* 1.3 0.9 0.9 2.6 1.0 American Redstart 0.8 0.4 - 0.3 4.2 1.7* 0.3 0.3 7.8 4.3 Northern Waterthrush 0.4 0.3 - 0.6 0.3 0.9 0.9 Kentucky Warbler 0.6 Common Yellowthroat 0.4 0.3 0.8 0.3 0.3 Wilson's Warbler 0.4 1.1 0.3 Canada Warbler 2.3 1.8 0.6 2.8 0.6* 1.3 0.3 8.6 5.2 3.0

Other insectivores Black-billed 0.9 Coccyzus erythrocephalus Red-bellied Woodpecker 0.3 Downy Woodpecker 0.8 0.3 0.3 2.6 Eastern Wood Pewee 0.6 0.3 _ ~~ ~~-0.9 Yellow-bellied Flycatcher 1.5 0.4 0.6 0.3 2.3 0.6 1.3 0.6 3.4 0.9 1.0 1.0 Red-breasted Nuthatch 0.6 0.3 0.6 - 1.0 Brown Creeper 0.3 - 0.3 - 0.6 0.3 Winter Wren 2.1 0.3* 2.2 1.3 2.0 2.0 Black-and-White Warbler 0.8 1.1 3.3 1.8 7.6 7.9 3.7 2.8 7.8 21.6* 2.0 1.0 Ovenbird 7.2 6.0 7.1 5.3 24.9 16.7* 14.9 5.0* 29.3 22.4 13.8 4.9*

Granivores Blue Jay 0.3 - 0.3 -0.9 Northern Cardinal 0.4 0.4 0.3 - 0.6 0.3 0.3 - 1.7 1.0 Rose-breasted Grosbeak 1.9 - 0.3 0.3 1.3 0.6 - 0.3 1.7 White-throated Sparrow 0.9 - 0.6 0.3 2.0 3.0 Dark-eyed Junco 0.3 0.3 3.0 Common Grackle 0.8 - 3.0* 0.6 - 1.9 1.6 4.9 3.9

174 ORNISSCANDINAVICA 17:2 (1986)