The information communicated by displays and their effects on recipients may depend on individual behavioral components that have been ritualized into the displays. This study focused on a visual component, bill wiping, that often occurs at the end of the display that accompanies the perched song (hereinafter referred to as song) of male Brown-headed Cowbirds (Molothrus ater). Although bill wiping appears to be a frequent and ritualized component of the cowbird song display, it has received almost no attention despite extensive research on this species' singing behavior. Breeding season observations of group-housed cowbirds of the eastern and midwestern subspecies (M. a. ater) in a semi-naturalistic aviary and pairs of western birds (M. a. obscurus) put together temporarily in an acoustically isolated chamber demonstrated that bill wiping is regularly performed at the end of song displays and is socially modulated. It is much more common in male-than female-directed songs, which led us to hypothesize that it signals aggression. We supported this hypothesis by finding that male-directed songs are also more likely to be accompanied by head-up displays, a known signal of aggression and dominance. Nevertheless, the occurrence of bill wiping (and head-up displays) during female-directed songs, albeit at a low frequency, warrants future study.

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