Flocking and Frugivory: The Effect of Social Groupings on Resource Use in the Common Bush-Tanager

Abstract

The Common Bush-Tanager (Chlorospingus ophthalmicus) displays flexibility in social behavior and prey choice. Thirty-three marked individuals were observed at Monteverde, Costa Rica over 15 consecutive months to determine the effect of social foraging (single individual, singleand mixed-species flock) on levels of frugivory and insectivory. I quantified the effects of social group composition and size, sex, and seasonality on the proportion of fruits and arthropods consumed by individuals. Common Bush-Tanagers spent one-third of their time in mixed-species flocks, consuming higher proportions of arthropods than individuals in single-species groups. Flock composition, rather than flock size, was the major factor influencing patterns of fruit and arthropod consumption. This study supports the hypothesis that birds may join mixed-species flocks in order to consume insect prey because when Common Bush-Tanagers participated in mixed-species flocks, they switched from frugivory to insectivory. Hence, the trophic position of species varies with the particular mix of single-species and mixed-species foraging groups available within a community.