PATTERNS OF FRUGIVORY AND ENERGETIC CONDITION IN NEARCTIC LANDBIRDS DURING AUTUMN MIGRATION

Many species of Nearctic landbirds exhibit behavioral plasticity durin g migration, presumably to compensate for energetic demands of migrato ry flight. At Block Island, Rhode Island, a northern temperate site, I quantified the extent of one form of behavioral plasticity in Nearcti c landbirds: dietary expansion from breeding season insectivory to hig h levels of frugivory during autumn. I also measured changes in energe tic condition of migrants using recapture methods and diurnal regressi ons of mass change. Based on analyses of 1,568 fecal samples collected from 1993-1995, frugivory during migration was frequent within many s pecies, extensive within individual birds, and widespread among taxa. Migratory species ranged from strict insectivory in the Winter Wren (T roglodytes troglodytes) to 96% fruit by volume in the fecal samples of Rose-breasted Grosbeaks (Pheucticus ludovicianus). Nine of 17 recaptu red species demonstrated mass gains between first and last recapture. Only the Red-eyed Vireo (Vireo olivaceus) gained significant mass with in a single day, suggesting that the site was suitable for migratory m ass gain if a species remained longer than one day. Highly omnivorous species, with greater than one-third of the diet as fruit, gained sign ificantly more mass while on Block Island than did strict insectivores , i.e., species with less than one-third of the diet as fruit. Insecti vorous species on average declined in energetic condition during stopo ver. The extent of frugivory for a species was positively correlated w ith average change in energetic condition and fat score per day, sugge sting 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 previou sly thought, and that dietary plasticity may be an adaptation to energ etic demands of migratory flights.