2009 NORTHEASTERN NATURALIST 16(4):535–552 Temporal Changes in Migratory Bird Use of Edges During Spring and Fall Seasons in Pennsylvania Gregory S. Keller1,2,*, Bradley D. Ross1,3, David S. Klute1,4, and Richard H. Yahner1,5 Abstract - Edge created through forest fragmentation can have significant impacts on the avian community, increasing predation and nest-parasitism rates and changing species richness and abundance patterns near edges. Although considerable research has demonstrated edge effects during the breeding season, few studies have considered how proximity to an edge affects migrant communities in stopover habitat. We studied bird communities in southern Pennsylvania (Adams, Chester, and Montgomery counties) to determine if richness and abundance of migrants were impacted by edges during spring and fall 1999–2001. The three categories of study sites were north-facing forest-herbaceous edges, south-facing forest-herbaceous edges, and interior forest (>300 m from an edge); sites were located in Gettysburg National Military Park-Eisenhower National Historic Site (GETT-EISE) and Valley Forge National Historical Park. During fall migration, Nearctic-Neotropic migrant species richness was significantly (P = 0.03) higher in interior forest compared to edges, whereas species richness of other guilds was not significantly different among edge types. During spring migration, richness of both permanent residents (P < 0.001) and temperate migrants (P < 0.001) were higher at both edges compared to interior sites. Only two species, Dendroica caerulescens (Black-throated Blue Warbler) (P = 0.03 during fall, P = 0.002 during spring) and Vireo olivaceus (Red-eyed Vireo) (P = 0.03 during fall, P = 0.05 during spring), showed differences among edge types during both spring and fall migration. Although the parks differed in amount of forest and in landscape composition, differences in richness and abundance patterns of birds between the parks also may be based on active management of Odocoileus virginianus Zimmerman (White-tailed Deer) at GETT-EISE that has resulted in vegetation differences. Overall, we found differential impacts of fragmentation (edge) on guilds and during different migratory periods, illustrating the importance of considering habitat use during each season individually for conservation of migratory songbirds. Introduction Habitat fragmentation has considerable impacts on forested landscapes in eastern North America by reducing overall amount of forested habitat, decreasing area of core habitat, and increasing the relative amount of edge habitat (Yahner 1995a). Changes in avian communities due to habitat fragmentation have been documented, based on fragment patch size and amount 1School of Forest Resources, The Pennsylvania State University, State College, PA 16802. 2Current address - 255 Grapevine Road, Department of Biology, Gordon College, Wenham, MA 01984. 3734 Partridge Lane, State College, PA 16803. 4Colorado Division of Wildlife, 6060 Broadway, Denver, CO 80216. *Corresponding author - greg.keller@gordon.edu. 536 Northeastern Naturalist Vol. 16, No. 4 of remaining forested habitat (e.g., Blake 1983, 1991; Galli et al. 1976; Robbins et al. 1989). Considerable research has focused on the impacts of human-induced edges on avian communities as a consequence of habitat fragmentation (Yahner 1988). For example, researchers have documented increased rates of nest parasitism (Brittingham and Temple 1983) and nest predation (Batáry and Báldi 2004, Gates and Gysel 1978) with proximity to edges during the breeding season, particularly in eastern deciduous forest. In addition, changes in bird abundance and density have been recorded with proximity to edges (Best et al. 1990, Kroodsma 1984, Parker et al. 2005). The primary focus of edge studies has been the breeding season (e.g., Brittingham and Temple 1983, Gates and Giffen 1991, Parker et al. 2005). Fewer studies on habitat use and distributions of birds have been conducted during migration, even though this period is vital for survival (Mehlman et al. 2005, Petit 2000, Sillett and Holmes 2002, Winker et al. 1992). In particular, the effects of increased edge habitats due to fragmentation that influence use of a given area by migrants have not received much attention. Studies during spring migration indicate that birds are influenced significantly by patch size and may avoid edges (Blake 1986, Keller and Yahner 2007, Martin 1980). In contrast, migrating birds during fall do not appear to be influenced by patch size and may frequent edges of forested habitats (Blake 1986; Rodewald and Brittingham 2002, 2004). This pattern may be a function of differences in vegetative characteristics, fruit abundance, and insect abundance and activity. These differences are likely based on microhabitat differences in relation to edge proximity and edge aspect (Martin 1980). In this paper, we investigated habitat use of edges compared to interior forest by songbirds during migration. The objectives of our study were to (1) determine the effects of forest-edge type (south-facing and north-facing deciduous forest/annual herbaceous edges and interior forest) and park characteristics on richness and abundance of migratory birds during spring and fall migration, and (2) compare differences in mixed-species flock structure based on edge type. We conducted this study at three national parks in Pennsylvania (Gettysburg National Military Park-Eisenhower National Historic Site combined and Valley Forge National Historical Park) to make comparisons of avian habitat use based on differences in land-use patterns and habitats of parks. Methods Field-site description and site selection We conducted this study at Gettysburg National Military Park-Eisenhower National Historic Site (GETT-EISE), Adams County, and Valley Forge National Historical Park (VAFO), Montgomery and Chester counties, in southern Pennsylvania. We treated GETT-EISE as one park because they are contiguous, have comparable management approaches, and have similar habitat compositions. The parks differ based on landscape composition: GETT-EISE is surrounded by a landscape dominated by agriculture, 2009 G.S. Keller, B.D. Ross, D.S. Klute, and R.H. Yahner 537 whereas VAFO is surrounded by residential and urban development (Yahner et al. 2001). Both parks are composed of rolling hills, with similar elevations and habitats; GETT-EISE is dominated by agricultural (55%) and forested (31%) habitats, whereas VAFO is dominated by open fields (57%) and forested habitat (38%) (Ross et al. 2003, Yahner et al. 2001). Forested habitats are composed of Quercus spp. (oaks), Carya spp. (hickories), and Liriodendron tulipifera L. (Tulip Poplar) at GETT-EISE and oaks, Tulip Poplar, Nyssa sylvatica Marsh (Black Gum), and Acer rubrum L. (Red Maple) at VAFO (Cypher 1986, Yahner et al. 1991). Understory at both parks is primarily composed of Cornus florida L. (Flowering Dogwood), Cercis canadensis L. (Redbud), Sassafras albidum Nuttall (Sassafras), and Lindera benzoin L. (Spicebush), as well as saplings of overstory trees. We established study sites comprised of 250-m transects at the junction of and parallel to deciduous forest-herbaceous edges and at the interior (>300 m from an edge) of deciduous-forest patches (>10 ha) at GETT-EISE and VAFO. We randomly selected four sites at north-facing edges, southfacing edges, and interior forest per park if available. Sites were separated by >250 m to maintain independence of bird observations. We established a total of 11 transects at GETT-EISE (4 interior, 4 south-facing, and 3 northfacing) both years, 12 sites at VAFO during year 1, and 11 sites at VAFO during year 2 (4 interior, 3 south-facing, and 4 north-facing). Bird surveys We surveyed birds five times along transects from sunrise to 4 hours after sunrise each season (25 August–10 October 1999–2000 for fall migration and 15 April–25 May 2000–2001 for spring migration) using fixed-width transects. Sites within a park were visited in random order to minimize confounding effects of time of day and weather, and all sites were surveyed within a park on a single day. At edge sites, individuals were counted if detected in forested habitat within 50 m of the transect or if flushed from herbaceous vegetation to forested habitat while we walked transects at each site. At interior sites, only birds detected within 50 m on north or east sides of transects or those flushed from the ground to north or east sides of transects were counted in order to match effort and approach used at edge sites. Birds were surveyed on days with no precipitation and little wind (<15 kph). Species richness and abundance (number of individuals) of mixed-species flocks were noted during fall migration. A mixed-species flock was defined as a group of interacting (e.g., calling) birds of ≥2 species and ≥3 individuals. Vegetative characteristics Vegetative characteristics were measured during July 2000, using modified methods of James and Shugart (1970). We quantified vegetation within a 0.04-ha circular plot centered at 0, 125, and 250 m along each transect; for edge transects, sampling points were located 15 m into the forest from the edge. Within all plots, we measured number of logs and stumps, diameter at breast height (dbh) of snags, and dbh of overstory trees (>7.5 538 Northeastern Naturalist Vol. 16, No. 4 cm dbh). In addition, we counted the number of short shrubs (0.5–1.5 m tall, <2.5 cm dbh), tall shrubs (>1.5 m tall, <2.5 cm dbh), and understory trees (>1.5 m tall, 2.5–7.5 cm dbh) by species on the right side of north-to-south and east-to-west 1- x 22.8-m transects that intersected a sampling point. We also measured percent ground cover of leaf litter, herbaceous cover, logs, and bare ground and percent canopy cover at 2-m intervals with an ocular tube along each of the two transects. Data analysis Each species detected during bird surveys was classified according to life-history characteristics as non-migratory permanent resident, shortdistance temperate migrant, or long-distance Nearctic-Neotropic migrant (Andrle and Carroll 1988, Brauning 1992, Brewer et al. 1991, Ehrlich et al. 1988). A species was considered present at a site during a season if it was documented during at least one visit. We tested whether the avian community differed among the three edge categories and parks using ANOVA (Minitab 2004). Dependent variables were total species richness and richness within each guild. Independent variables for all analyses were edge category (south-facing edge, north-facing edge, or interior forest), park (GETT-EISE or VAFO), year (1999 or 2000 for fall and 2000 or 2001 for spring), and interactions among variables. We surveyed 23 sites during year 1 and 22 sites during year 2. Year was treated as an independent variable due to the possible annual variation during migration (Rodewald and Brittingham 2007). Data from fall and spring seasons were treated separately. Analysis of temperate and Nearctic-Neotropic migrant abundance was conducted individually for each species with ≥10 sightings during years combined at all sites combined (Sallabanks et al. 2000). We used average abundance (averaged across visits for a single site during fall and spring separately) as the dependent variable. As above, independent variables for all analyses were edge category, park, year, and interactions among variables. Because our visits to a site were separated by approximately a week, migratory individuals detected were probably different during each visit (Rodewald and Matthews 2005). However, to avoid double-counting residents that establish breeding territories during early spring or remain on territory during early fall, we used average abundance rather than total abundance. Because of this potential source of bias, we did not compare guilds directly. We identified differences in vegetative characteristics among edge categories and among parks that may have been relevant biologically to richness and abundance of migrant communities. The three vegetation sampling point values per site were averaged to create a single value for a characteristic at each site. Twelve characteristics were analyzed separately with ANOVA. Independent variables were edge type, park, and interaction between these two characteristics. Dependent variables were: number of logs and stumps; 2009 G.S. Keller, B.D. Ross, D.S. Klute, and R.H. Yahner 539 average dbh of snags and overstory trees; number of short shrubs, tall shrubs, and understory trees; percent cover of leaf litter, herbaceous vegetation, logs, and bare ground; and percent canopy cover For all analyses, results were considered significant at an alpha level of 0.05. For significant differences, we used Tukey’s test of multiple comparisons to identify differences among edge categories. F-statistics and P-values presented in the text illustrate differences for single variables within the full model (edge category, park, year, and interactions). Results Fall migration We detected 96 species during fall migration during both years and both parks combined. Total species richness (Fig. 1), richness of permanent residents, and richness of temperate migrants (Fig. 2) were not affected by edge type during fall. However, richness of Nearctic-Neotropic migrants was significantly higher at interior sites compared to edge sites (F = 4.07, P = 0.03), being more than two-fold that at north-facing edges. Figure 1. Total species richness (mean ± SE) during fall and spring migration in north-facing edges, south-facing edges, and interior forest at Gettysburg-National Military Park-Eisenhower National Historic Site and Valley Forge National Historical Site, PA combined during 1999–2001 (n = 23 sites during 1999–2000 and 22 sites during 2000-2001). Values for a season with the same superscripts are not signifi- cantly different based on Tukey’s test of pairwise comparisons. Differences that were not significant (P > 0.05) are designated N.S. 540 Northeastern Naturalist Vol. 16, No. 4 We analyzed abundance of 14 temperate migrants and found three species that were significantly affected by edge type during fall migration (Table 1). Common Yellowthroat (scientific names with authorities for all species mentioned given in Tables 1 and 2) and Eastern Phoebe were both more abundant at edges compared to interior forest, whereas Yellow-rumped Warbler was more abundant at south-facing edges. Blackand- white Warbler, Black-throated Blue Warbler, Red-eyed Vireo, and Wood Thrush were more abundant at interior compared to edge sites. In addition, Black-and-white Warbler and Black-throated Blue Warbler were significantly more common at VAFO, whereas Eastern Wood-Pewee was more abundant at GETT-EISE. Spring migration We detected 100 species during spring migration in both years and both parks combined. In contrast to fall, total species richness was higher at both edge types compared to interior forest (F = 10.89, P < 0.001; Fig. 1); richness of permanent residents and temperate migrants were both Figure 2. Species richness (mean ± SE) of permanent residents, temperate migrants, and Nearctic-Neotropic migrants in north-facing edges, south-facing edges, and interior forest during fall migration 1999 and 2000 at Gettysburg-National Military Park-Eisenhower National Historic Site and Valley Forge National Historical Site, PA combined (n = 23 sites during 1999 and 22 sites during 2000). Values for a given guild with the same superscripts are not significantly different based on Tukey’s test of pairwise comparisons. Differences that were not significant (P > 0.05) are designated N.S. 2009 G.S. Keller, B.D. Ross, D.S. Klute, and R.H. Yahner 541 Table 1. Average abundance of migratory species (mean ± SE) at north-facing and south-facing edges and interior sites at Gettysburg National Military Park-Eisenhower National Historic Site and Valley Forge National Historical Park, PA, during 1999 (n = 23 sites) and 2000 (n = 22 sites) fall migration (for species with at least 10 sightings at all sites combined). Edge P-value represents results from ANOVA among the three edge categories; “*” indicates a significant edge difference. Significant park differences (Park P-value) are noted with (V) for higher abundance at Valley Forge and (G) for higher abundance at Gettysburg-Eisenhower. Species Interior North South Edge P-value Park P-value Temperate migrants American Robin (Turdus migratorius L.) 1.81 ± 0.85 1.04 ± 0.47 0.73 ± 0.30 0.44 0.55 Blue-headed Vireo (Vireo solitarius Wilson) 0.11 ± 0.06 0.04 ± 0.04 0.00 ± 0.00 0.27 0.20 Common Grackle (Quiscalus quiscula L.) 1.25 ± 1.25 0.01 ± 0.01 0.00 ± 0.00 0.44 0.37 Common Yellowthroat (Geothlypis trichas L.) 0.00 ± 0.00 0.05 ± 0.04 0.04 ± 0.03 0.03* 0.19 Eastern Phoebe (Sayornis phoebe Latham) 0.00 ± 0.00 0.03 ± 0.02 0.16 ± 0.06 0.004* 0.58 Eastern Towhee (Pipilo erythrophthalmus L.) 0.08 ± 0.05 0.23 ± 0.10 0.20 ± 0.08 0.29 0.06 Golden-crowned Kinglet (Regulus satrapa Lichtenstein) 0.18 ± 0.05 0.16 ± 0.08 0.03 ± 0.05 0.19 0.55 Gray Catbird (Dumetella carolinensis L.) 0.08 ± 0.06 0.30 ± 0.15 0.43 ± 0.22 0.31 0.83 House Wren (Troglodytes aedon Vieillot) 0.01 ± 0.01 0.09 ± 0.06 0.09 ± 0.05 0.44 0.57 Northern Flicker (Colaptes auratus L.) 0.13 ± 0.04 0.37 ± 0.13 0.30 ± 0.11 0.22 0.19 Palm Warbler (Dendroica palmarum Gmelin) 0.01 ± 0.01 0.03 ± 0.03 0.13 ± 0.13 0.35 0.20 Ruby-crowned Kinglet (Regulus calendula L.) 0.10 ± 0.06 0.09 ± 0.05 0.13 ± 0.08 0.72 0.10 White-throated Sparrow (Zonotrichia albicollis Gmelin) 0.06 ± 0.06 0.84 ± 0.49 0.14 ± 0.09 0.15 0.09 Yellow-rumped Warbler (Dendroica coronata L.) 0.05 ± 0.04 0.00 ± 0.00 0.11 ± 0.05 0.03* 0.64 Nearctic-Neotropic migrants American Redstart (Setophaga ruticilla L.) 0.09 ± 0.04 0.03 ± 0.02 0.04 ± 0.03 0.43 0.08 Black-and-white Warbler (Mniotilta varia L.) 0.30 ± 0.11 0.06 ± 0.03 0.06 ± 0.05 0.02* 0.02 (V) Blackburnian Warbler (Dendroica fusca (Müller) 0.08 ± 0.04 0.00 ± 0.00 0.04 ± 0.02 0.16 0.26 Black-throated Blue Warbler (Dendroica caerulescens Gmelin) 0.53 ± 0.21 0.00 ± 0.00 0.17 ± 0.07 0.03* 0.04 (V) Black-throated Green Warbler (Dendroica virens Gmelin) 0.70 ± 0.33 0.07 ± 0.03 0.17 ± 0.07 0.13 0.93 Eastern Wood-Pewee (Contopus virens L.) 0.13 ± 0.08 0.27 ± 0.09 0.16 ± 0.06 0.10 0.001 (G) Magnolia Warbler (Dendroica magnolia Wilson) 0.31 ± 0.13 0.23 ± 0.10 0.09 ± 0.03 0.29 0.41 Red-eyed Vireo (Vireo olivaceus L.) 0.23 ± 0.07 0.07 ± 0.03 0.03 ± 0.02 0.03* 0.33 Wood Thrush (Hylocichla mustelina Gmelin) 0.73 ± 0.28 0.01 ± 0.01 0.04 ± 0.02 0.01* 0.16 542 Northeastern Naturalist Vol. 16, No. 4 Table 2. Average abundance of migratory species (mean ± SE) with at least 10 observations at north-facing and south-facing edges and interior sites at Gettysburg National Military Park-Eisenhower National Historic Site and Valley Forge National Historical Park, Pennsylvania, during 2000 (n = 23 sites) and 2001 (n = 22 sites) spring migration (species included with at least 10 sightings at all sites combined). Edge P-Value represents results from ANOVA among the three edge categories; “*” indicates a significant edge difference. Significant park differences (Park P-value) are noted with (V) for higher abundance at Valley Forge and (G) for higher abundance at Gettysburg-Eisenhower. Species Interior North South Edge P-value Park P-value Temperate migrants American Robin 0.29 ± 0.07 1.59 ± 0.37 0.97 ± 0.14 <0.001* <0.001 (V) Blue-gray Gnatcatcher (Polioptila caerulea L.) 0.13 ± 0.04 0.31 ± 0.13 0.34 ± 0.06 0.09 0.07 Brown-headed Cowbird (Molothrus ater Boddaert) 0.06 ± 0.03 0.26 ± 0.05 0.24 ± 0.07 0.01* 0.05 (G) Brown Thrasher (Toxostoma rufum L.) 0.01 ± 0.01 0.03 ± 0.02 0.17 ± 0.05 0.001* 0.49 Chipping Sparrow (Spizella passerine Bechstein) 0.00 ± 0.00 0.11 ± 0.04 0.17 ± 0.06 0.03* 0.14 Common Grackle 0.00 ± 0.00 0.11 ± 0.07 0.10 ± 0.04 0.22 0.88 Common Yellowthroat 0.00 ± 0.00 0.14 ± 0.07 0.07 ± 0.05 0.13 0.96 Dark-eyed Junco (Junco hyemalis L.) 0.00 ± 0.00 0.24 ± 0.21 0.04 ± 0.04 0.40 0.58 Eastern Phoebe 0.00 ± 0.00 0.05 ± 0.03 0.16 ± 0.08 0.12 0.17 Eastern Towhee 0.10 ± 0.05 0.27 ± 0.09 0.56 ± 0.09 0.002* 0.74 Gray Catbird 0.05 ± 0.02 0.31 ± 0.09 0.74 ± 0.14 <0.001* 0.23 House Wren 0.01 ± 0.01 0.49 ± 0.13 0.67 ± 0.17 0.001* 0.03 (G) Northern Flicker 0.06 ± 0.02 0.24 ± 0.08 0.26 ± 0.10 0.07 0.97 Ruby-crowned Kinglet 0.10 ± 0.04 0.16 ± 0.11 0.26 ± 0.08 0.48 0.02 (G) Red-winged Blackbird (Agelaius phoeniceus L.) 0.00 ± 0.00 0.10 ± 0.05 0.10 ± 0.07 0.17 0.01 (G) White-throated Sparrow 0.13 ± 0.07 0.49 ± 0.24 0.56 ± 0.24 0.20 0.15 Yellow-rumped Warbler 0.74 ± 0.25 0.29 ± 0.12 0.49 ± 0.16 0.23 0.26 2009 G.S. Keller, B.D. Ross, D.S. Klute, and R.H. Yahner 543 Table 2, continued. Species Interior North South Edge P-value Park P-value Nearctic-Neotropic migrants American Redstart 0.15 ± 0.07 0.09 ± 0.04 0.06 ± 0.05 0.45 0.42 Baltimore Oriole (Icterus galbula L.) 0.15 ± 0.04 0.31 ± 0.05 0.34 ± 0.06 0.02* 0.53 Black-and-white Warbler 0.04 ± 0.02 0.04 ± 0.02 0.09 ± 0.04 0.36 0.87 Blackburnian Warbler 0.09 ± 0.04 0.03 ± 0.02 0.01 ± 0.01 0.14 1.00 Blackpoll Warbler (Dendroica striata Forster) 0.04 ± 0.02 0.06 ± 0.03 0.04 ± 0.02 0.85 0.18 Black-throated Blue Warbler 0.29 ± 0.06 0.09 ± 0.04 0.03 ± 0.02 0.002* 0.78 Black-throated Green Warbler 0.20 ± 0.07 0.07 ± 0.06 0.10 ± 0.05 0.33 0.45 Eastern Wood-Pewee 0.21 ± 0.05 0.24 ± 0.06 0.14 ± 0.04 0.10 <0.001 (G) Great Crested Flycatcher (Myiarchus crinitus L.) 0.31 ± 0.07 0.16 ± 0.06 0.26 ± 0.07 0.23 <0.001 (G) Indigo Bunting (Passerina cyanea L.) 0.05 ± 0.05 0.07 ± 0.03 0.04 ± 0.02 0.63 0.03 (G) Ovenbird (Seiurus aurocapilla L.) 0.20 ± 0.05 0.23 ± 0.07 0.14 ± 0.07 0.78 0.08 Rose-breasted Grosbeak (Pheucticus ludovicianus L.) 0.10 ± 0.04 0.01 ± 0.01 0.06 ± 0.03 0.15 0.09 Red-eyed Vireo 0.80 ± 0.20 0.53 ± 0.13 0.29 ± 0.14 0.05* 0.004 (V) Scarlet Tanager (Piranga olivacea Gmelin) 0.40 ± 0.07 0.19 ± 0.05 0.14 ± 0.04 0.001* 0.84 Wood Thrush 0.64 ± 0.11 0.41 ± 0.09 0.60 ± 0.09 0.29 0.02 (G) 544 Northeastern Naturalist Vol. 16, No. 4 significantly higher at edge sites versus interior sites (Fig. 3). Total richness (F = 9.66, P = 0.004), richness of permanent residents (F = 16.45, P < 0.001), and richness of temperate migrants (F = 11.95, P = 0.001) were significantly higher at GETT-EISE compared to VAFO. In addition, we detected more temperate (F = 12.77, P = 0.001) and Nearctic-Neotropic (F = 6.67, P = 0.014) migrant species during spring 2001 compared to spring 2000. Abundances of 5 species (House Wren, Brown Thrasher, Gray Catbird, Chipping Sparrow, and Eastern Towhee) were significantly higher at southfacing edge sites compared to interior and north-facing edge sites during spring migration; abundance of American Robin was highest at north-facing edges, and Brown-headed Cowbird was more abundant at both edges compared to interior forest (Table 2). Four species were more abundant at GETT-EISE, whereas American Robin was more commonly encountered at VAFO. In contrast to temperate migrants, Nearctic-Neotropic migrants with significant differences in abundance were more common at interior sites compared to edges, with the exception of the Baltimore Oriole. Three species were significantly more abundant at interior sites. Four species were more abundant at GETT-EISE, and Red-eyed Vireo was more abundant at VAFO. Figure 3. Species richness (mean ± SE) of permanent residents, temperate migrants, and Nearctic-Neotropic migrants in north-facing edges, south-facing edges, and interior forest during spring migration 2000 and 2001 at Gettysburg-National Military Park-Eisenhower National Historic Site and Valley Forge National Historical Site, PA combined (n = 23 sites during 2000 and 22 sites during 2001). Values for a given guild with the same superscripts are not significantly different based on Tukey’s test of pairwise comparisons. Differences that were not significant (P > 0.05) are designated N.S. 2009 G.S. Keller, B.D. Ross, D.S. Klute, and R.H. Yahner 545 Flock structure We encountered 55 flocks during 1999 and 2000 fall-migration seasons, combined, including 32 at interior sites, 10 at north-facing edges, and 13 at south-facing edges. Flocks contained significantly more species (F = 4.89, P = 0.014) and more individuals (F = 4.08, P = 0.026) at interior sites than north-facing edges (Fig. 4). Although interior sites contained approximately 2 more species and 8 more individuals per flock compared to south-facing edges, this difference was not significant (P > 0.05). Vegetative characteristics Vegetation differed between edge types and parks (Fig. 5). Understory tree density was significantly (F = 3.77, P = 0.046) higher at interior sites compared to edges. Also, overstory tree density (F = 4.71, P = 0.045) and total basal area of overstory trees (F = 13.11, P = 0.002) were both signifi- cantly higher at VAFO versus GETT-EISE, whereas short-shrub density was higher at GETT-EISE (F = 46.74, P < 0.001). Discussion With continued forest fragmentation in eastern North America, studies on habitat use, preference, and avoidance of edges provide insight into the overall impacts on migratory songbirds. Although breeding birds have been a primary research focus on use of edge habitats by birds (Parker et al. 2005, Figure 4. Flock structure (mean ± SE) in north-facing edges, south-facing edges, and interior forest sites during fall migration 1999–2000 at Gettysburg National Military Park-Eisenhower National Historic Site and Valley Forge National Historical Park, PA combined (n = 55 flocks). Values with the same superscripts are not significantly different based on Tukey’s test of pairwise comparisons. 546 Northeastern Naturalist Vol. 16, No. 4 Sallabanks et al. 2000), studies during migration are critical as researchers try to link habitat patterns in multiple seasons (Marra et al. 1998). In this study, we found that patterns of habitat use of edges by guilds and by individual species differ considerably between migratory seasons. Use of edges compared to interior forest may be based on several factors, such as dual habitat use, differences in foraging, and differences in vegetation structure. Small and Hunter (1989) documented higher richness close to powerline openings, suggesting in their study that species use both forest and open habitats. However, mowed lawns and heavily-grazed pastures Figure 5. Significant differences (P < 0.05) in vegetative characteristics (mean ± SE) based on (a) edge type at both parks combined and (b) the park at Gettysburg-National Military Park-Eisenhower National Historic Site and Valley Forge National Historical Site, PA (n = 23 sites). Values for a characteristic with the same superscripts in (a) are not significantly different based on Tukey’s test of pairwise comparisons. 2009 G.S. Keller, B.D. Ross, D.S. Klute, and R.H. Yahner 547 formed the herbaceous openings at edges in our study; greater use of edges by migrants was probably not due to use of both habitats at edges, perhaps with the exceptions of American Robin, Brown-headed Cowbird, and Chipping Sparrow. Common Yellowthroat, Eastern Phoebe, Eastern Towhee, Gray Catbird, House Wren, and Yellow-rumped Warbler were not detected in herbaceous vegetation during our surveys, but used edge habitats at a high rate during migration. Differential use of edges also may be due to food availability and vegetation structure. For example, Yahner (1995b) found that during both winter and breeding seasons, ground-shrub-foraging species (e.g., Dark-eyed Junco, Gray Catbird) were more commonly encountered near edges, probably because of the higher density of shrubs and abundant weed seeds at edges. We found similar results for individual species during spring and fall migration (e.g., Common Yellowthroat, Gray Catbird, House Wren); however, we did not find a higher density of shrubs at edges, and density of understory trees was higher at interior sites. Rodewald and Brittingham (2007) documented higher attack and gleaning rates at edges compared to interior forest during fall migration for Yellow-rumped Warbler, suggesting higher habitat quality based on prey availability. We found a higher abundance of Yellow-rumped Warbler at edge sites as well during fall migration, but most other insectivores were more abundant at interior sites. Importantly, Yong et al. (1998) noted that abundance of Wilsonia pusilla Wilson (Wilson’s Warbler) does not necessarily relate to habitat quality during migration; therefore, further research is necessary to determine if edge habitats are higher quality for all species that were more abundant in this study, or if species-specific patterns limit the ability of researchers to generalize these results. Although edge type strongly influenced songbirds during migration in our study, the effects differed considerably between spring and fall. Use of edges by temperate migrants and interior sites by Nearctic-Neotropic migrants is not surprising, given differences in area sensitivity and use of shrubby habitats among migrants (Whitcomb et al. 1981). However, guilds did not necessarily maintain consistent responses to edge type between seasons. We documented interior use by Nearctic-Neotropical migrants during fall but not spring; on the contrary, permanent residents and temperate migrants used edges significantly more during spring but not during fall. In contrast to these findings, Rodewald and Brittingham (2004) observed that migratory birds during fall may generalize in habitat use (i.e., both edges and forest interior), and Keller (2001) and Keller and Yahner (2007) found a strong area effect during spring but not fall for Nearctic-Neotropic migrants, suggesting that migratory species may reduce area requirements during fall. Similarly, Swanson et al. (2003) found that captures of migrants were spread evenly throughout woodlots during spring, but were concentrated in edge microhabitats during fall migration. In part, given significant yearly differences in patterns of habitat use during migration (Rodewald and Brittingham 2007), longer-term studies may be necessary to fully understand the patterns 548 Northeastern Naturalist Vol. 16, No. 4 of regional habitat use, and short-term studies may not be generalized to other regions or time periods. In addition, although birds are under similar migratory constraints during spring and fall, patterns from one migratory season should not be generalized to other seasons. Our findings that significant differences occurred between parks in abundances of individual species and in species richness (fall migration only), may be attributed to two main factors. First, land use surrounding the parks is considerably different and may impact regional patterns of migration; VAFO is 20 km northwest of Philadelphia and surrounded by residential and commercial developments, whereas GETT-EISE is surrounded by agricultural land (Yahner et al. 2001). Other researchers (Flather and Sauer 1996, Villard et al. 1999) have shown that the presence and abundance of bird species, particularly Nearctic-Neotropic migrants, is influenced by both local and landscape-level forest cover. Keller and Yahner (2007) found that during migration species are differentially influenced by the amount of forested habitat in the surrounding landscape in Pennsylvania; in this study, landscape-level differences between VAFO and GETT-EISE may significantly affect abundances of migratory birds, even though the amount of forest within parks is relatively similar. Second, GETT-EISE began a population reduction effort for Odocoileus virginianus Zimmerman (White-tailed Deer) in 1995, which decreased the deer population and increased tree regeneration (Niewinski et al. 2006). Because of this management effort, White-tailed Deer density differed considerably between GETT-EISE (19 deer/km2) and VAFO (70 deer/km2) at the time of our study (Lovallo and Tzilkowski 2003, Niewinski et al. 2006). The difference in shrub density between parks in our study is probably a result of this management. Holmes et al. (2008) documented significantly higher richness and density of woody plant species in forest with lower deer density in Michigan. Both Allomberta et al. (2005) and DeGraaf et al. (1991) found that high densities of deer caused a significant reduction in abundance of songbirds during the breeding season, specifically those species dependent on understory vegetation. In this study, migrants that nest or forage in the understory layer (House Wren, Ruby-crowned Kinglet, Indigo Bunting, Wood Thrush) were more abundant at GETT-EISE. In contrast, migrants typical of either open mature forest (Black-and-white Warbler, Black-throated Blue Warbler, Red-eyed Vireo) or open lawns (American Robin) were more abundant at VAFO. Further research is necessary to establish if the impacts of White-tailed Deer may extend beyond the breeding season, even affecting abundance of migratory transient species, such as Ruby-crowned Kinglet and Black-throated Blue Warbler, as may have occurred in our study. Flock structure and location varied among edge types in our study during fall. Although we did not quantify food availability, Rodewald and Brittingham (2002) found that flocks moved more slowly at edge sites, suggesting greater food resources at edges. In addition, flocks were larger and richness was higher at edge sites due to greater vegetative heterogeneity in 2009 G.S. Keller, B.D. Ross, D.S. Klute, and R.H. Yahner 549 their study. In contrast, our results imply that interior sites during fall were more important to Nearctic-Neotropic migrants. Flocks were located higher above ground, typically in the canopy, in interior forest compared to edges (G.S. Keller, pers. observ.), perhaps suggesting that sunlight penetration and microclimate might be a more important variable than edge type to flocks of migrating birds, at least in fall. North-facing edges had fewer species and individuals per flock and limited sunlight penetration, whereas south-facing edges and canopies of interior sites, which tended to receive full morning sunlight in fall, had larger flocks. Particularly given the low temperatures during the latter portion of fall migration in Pennsylvania (3–5 °C), any slight differences in sunlight penetration and temperature may increase activity of insect prey (Whitaker et al. 2000). 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