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.