SPRING TREE SPECIES USE BY MIGRATING YELLOW-RUMPED WARBLERS IN RELATION TO PHENOLOGY AND FOOD AVAILABILITY

I examined the temporal pattern of migration and tree species preferences of Yellow-rumped Warblers (Dendroica coronata) in relation to tree and food phenology across three spring seasons (2001-2003) at a migration stopover site in cast-central Illinois, USA. Foraging Yellow-rumped Warblers used tree species in relation to the date that trees initiated bud break and the date caterpillars were most abundant in trees. The first arrival date of Yellow-rumped Warblers at the stopover site varied with date of bud break; duration of migration through the stopover site ranged from 31 to 47 days. The earliest Yellow-rumped Warbler migrants observed arrived at the stopover site before appearance of many arthropods, and foraged on the temporarily abundant adult stage of the hackberry psyllid (Pachypsylla spp. [Fletcher] [Homoptera]). Later migrants switched to foraging for caterpillars. The proportions of foraging observations among tree species were similar across all 3 years and, each year, tree species used by Yellow-rumped Warblers diversified as spring progressed. Yellow-rumped Warblers are short-distance migrants (most winter in the southern United States); my results indicate these birds may have an advantage over long-distance migrants in view of global climate change as they can arrive early at stopover habitats and exploit resources when they are most available.