Fruit quality and consumption by songbirds during autumn migration

Seasonal fruits are an important food resource for small songbirds during autumn migration in southern New England. Therefore, conservation and management of important stopover sites used by migrating birds requires knowledge about nutritional requirements of songbirds and nutritional composition of commonly consumed fruits. We measured nutrient composition and energy density of nine common fruits on Block Island, Rhode Island, and conducted a field experiment to estimate consumption rates of three of these fruits by birds during autumn migration. Most common fruits on Block Island contained primarily carbohydrates (41.3-91.2% dry weight), and little protein (2.6-8.6%) and fat (0.9-3.7%), although three contained more fat: Myrica pennsylvanica (50.3%), Viburnum dentatum (41.3%), and Parthenocissus quinquefolia (23.6%). Bird consumption of high-fat, high-energy V. dentatum fruit and high-carbohydrate, low-energy Phytolacca americana fruit was greater than consumption of Aronia melanocarpa, a high-carbohydrate, low-energy fruit. We estimated that migratory birds on Block Island must eat up to four times their body mass in fruit wet weight each day to satisfy their energy requirements when eating low-energy fruits such as P. americana, and they cannot satisfy their protein requirements when eating only certain high-energy fruits such as V. dentatum. Our results suggest that many migratory birds must eat both fruits and insects to meet their dietary needs. Thus, shrubland habitat at important migratory stopover sites such as Block Island should be managed so that it contains a variety of preferred fruit-bearing shrubs and an adequate abundance of insects.