HABITAT USE AND BEHAVIOR OF MIXED SPECIES LANDBIRD FLOCKS DURING FALL MIGRATION

Abstract

Little research has examined the ecology of mixed species flocks of migrant and resident landbirds during migratory periods. We studied habitat use and behavior of mixed species insectivorous landbird flocks during fall migration in central Pennsylvania. From late August to early October, 1998 and 1999, 220 flocks were observed for 30-min periods in six forest habitat types: mature forest interior, mature forest edge, mature forest agricultural edge, mature suburban forest, pole stage forest, and shrub/sapling stage forest. Sixty species were recorded in flocks that contained 2–24 species each (mean = 9.25 ± 0.29 SE). Flocks contained 2–181 individuals (mean = 22.12 ± 1.18 SE). Flocks in the six habitats had 49–61% Nearctic-Neotropical migrant individuals, 5–15% temperate migrants, and 23–37% residents. Abundance and species richness of migratory guilds (Nearctic-Neotropical migrants, temperate migrants, and resident species) within flocks were highest in structurally heterogeneous habitats (especially forest edge habitat) and were lowest in homogeneous pole stage forest. Of nine migrant species whose abundance varied significantly among habitats, six had highest abundance in flocks in forest edge habitat: Blue-headed Vireo (Vireo solitarius), Red-eyed Vireo (V. olivaceus), Blue-gray Gnatcatcher (Polioptila caerulea), Chestnut-sided Warbler (Dendroica pensylvanica), Black-throated Green Warbler (D. virens), and Magnolia Warbler (D. magnolia). Resident parids (Black-capped Chickadee, Poecile atricapillus, and/or Tufted Titmouse, Baeolophus bicolor) occurred within 82% of flocks and were observed leading 68% of these flocks. Movement rate (m/min) of flocks varied among habitats with flocks in edge-dominated habitats (forest edge, forest agricultural edge, and suburban forest) tending to have slower movement rates than in pole stage forest and forest interior, suggesting that food availability may have been greater in edge-dominated habitats. Consistently high species richness and abundance of migrant guilds and individual species strongly suggests that structurally diverse forest edge habitats were selected and provided relatively high quality stopover habitat for landbirds during fall migration.