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Abstract

Relationships between social behavior and ecology are of great interest to ethologists and behavioral ecologists. For example, limited comparative data have already resulted in the development of broad explanations for possible interrelationships between patterns of social interaction and food supply in diverse species. Here, we analyzed agonistic behavior, dominance relationships, and social organization in evening grosbeaks (Coccothraustes vespertinus) living near Boulder, Colorado U.S.A. We found marked seasonal variations in mean rates of agonistic behavior, mean sex ratios, and mean flock sizes. Rates of agonistic encounters were positively correlated with group size and the number of adult males present. Generally, adult males dominated (>>) adult females, young males, and young females; young males >> adult and young females; and young females >> adult females. Adult males were most aggressive during the beginning of the breeding season.

The formation of large flocks of grosbeaks most likely facilitates locating the dispersed and locally irruptive food on which these birds depend, and may also confer protection against predators. We suggest that flocks of grosbeaks composed of mainly transient birds are maintained by (1) low rates of noncontact aggressive encounters and (2) the rare occurrence of potentially disruptive fights, both of which appear to result from birds' being able to assess the likely outcome of an encounter with another animal using phenotypic cues such as wing length, weight, and especially plumage. The ability to predict the outcome of an aggressive encounter may also be important in the integration of young birds into flocks.