The Wilson Bulletin
Wilson Ornithological Society
Foraging in single- or mixed-species flocks is common among many bird species, and the advantages of being in these flocks have been the focus of several investigations (reviewed by Moynihan 1962, Bertram 1978). It has been hypothesized that birds occurring in mixed-species flocks gain advantages from (1) reduced predation risk (e.g., Moynihan 1962, Morse 1970), (2) increased foraging efficiency (e.g., Moynihan 1962, Morse 1970), or (3) social learning (e.g., Ward and Zahavi 1973). Here, we report the anomalous occurrence of flocks consisting of species not typically found in the types of aggregations we describe and suggest the possible function of these aggregations with respect to these hypotheses.