Dietary breadth of frugivorous birds in relation to their feeding strategies in the lowland forests of central Honshu, Japan

Abstract

Variation in the dietary breadth of frugivorous birds, i.e. variation in the degree of dietary specialization, is a key factor for understanding the construction of the architecture of diffused fruit–bird interactions. However, little information is available on the dietary breadth of frugivorous birds in a community and the avian attributes influencing it. In this study, we evaluated and compared the dietary breadth of 23 frugivorous birds (including both seed dispersers and seed predators) in relation to their feeding strategies (depending on which the birds were categorized into four types as gulpers, grinders, crushers and peckers) and other avian attributes. In particular, we hypothesized that feeding types with long fruit‐handling time (i.e. crushers and peckers) have a narrow dietary breadth. Our analysis was based on the data obtained through long‐term volunteer monitoring in the lowland forests of Kanagawa prefecture, Japan. We evaluated the dietary breadth of the birds while controlling for differences in sampling efforts and seasonal variation in the available fruit assemblages among the birds. Our analysis revealed that there was a large variation in dietary breadth among the birds, even after controlling for differences in sampling efforts and seasonal variation in the available fruit assemblages. Feeding strategies of the birds defined their dietary breadth to some degree, but we found little support for the hypothesis that fruit handling determines dietary breadth. We also observed a large variation in dietary breadth among the gulpers, which act mainly as seed dispersers for plants, and this variation could not be explained fully by other avian attributes. Our results showed that variation in the dietary breadth of frugivorous birds is partly determined by their feeding strategies and suggested that other unknown factors may play a role in determining dietary breadth variation and in structuring fruit–bird interaction networks.