9411650 Morse The decline of many populations of Neotropical migratory songbirds has triggered much research over the last decade into the biology of these species on their temperate breeding grounds and their tropical wintering areas. The interim migration period has received much less attention, despite the extreme energetic demands and the very divergent behavior of migrants during this transient period. During migration, many songbirds diversify their breeding-season behaviors to select different habitats, use more foraging behaviors, and feed on more varied diet types than at other times of the year. The ecological factors behind the evolution of such behavioral plasticity and the ecological and conservation implications these divergent behaviors remain speculative. The proposed research will investigate these questions in one aspect of behavioral diversification in autumn migrating songbirds - the shift in diet from eating insects to one of eating fruit. Preliminary research on Block Island, Rhode Island suggests that unpredictability in resource abundance at migratory stopover sites and increased energy intake on expanded diets during the demanding migration period have led to behavioral plasticity in songbirds. This research will be furthered with continued monitoring of insect and fruit abundance at stopover sites to monitor different degrees of predictability of the two resources during Autumn, 1994 and 1995. Furthermore, migrants will be fed experimental diets with varying degrees of fruit to determine the extent they can fatten on expanded diets, thus testing the hypothesis that diversification of ingested food types has resulted from selection to maximize energy intake during migration. Finally, experiments will allow evaluation (through doubly-labeled water techniques) of the amount of energy saved by migrant birds in feeding on diverse diets. These techniques allow estimation of the energy expenditure of birds which can be follow ed under situations where birds have monotypic and diverse diets available to them. These studies will provide insight into the evolutionary pressures for behavioral plasticity as well as important information about the biology of migrating songbirds. This in turn will allow better conservation decisions to be made regarding these declining populations. ***
