Patterns of frugivory and energetic condition in nearctic landbirds during autumn migration

Many species of Nearctic landbirds exhibit behavioral plasticity during migration, presumably to compensate for energetic demands of migratory flight. At Block Island, Rhode Island, a northern temperate site, I quantified the extent of one form of behavioral plasticity in Nearctic landbirds: dietary expansion from breeding season insectivory to high levels of frugivory during autumn. I also measured changes in energetic condition of migrants using recapture methods and diurnal regressions of mass change. Based on analyses of 1,568 fecal samples collected from 1993-1995, frugivory during migration was frequent within many species, extensive within individual birds, and widespread among taxa. Migratory species ranged from strict insectivory in the Winter Wren (Troglodytes troglodytes) to 96% fruit by volume in the fecal samples of Rose-breasted Grosbeaks (Pheucticus ludovicianus). Nine of 17 recaptured species demonstrated mass gains between first and last recapture. Only the Red-eyed Vireo (Vireo olivaceus) gained significant mass within a single day, suggesting that the site was suitable for migratory mass gain if a species remained longer than one day. Highly omnivorous species, with greater than one-third of the diet as fruit, gained significantly more mass while on Block Island than did strict insectivores, i.e., species with less than one-third of the diet as fruit. Insectivorous species on average declined in energetic condition during stopover. The extent of frugivory for a species was positively correlated with average change in energetic condition and fat score per day, suggesting that frugivory allowed species to gain mass more efficiently and extensively than exclusive insectivory. I conclude that frugivory in Nearctic landbirds is far more extensive during migration than previously thought, and that dietary plasticity may be an adaptation to energetic demands of migratory flights.