Payoff asymmetries in contests among male brown-headed cowbirds

Summary

Morphometric analyses and behavioral observations of a marked population of brood parasitic brown-headed cowbirds (Molothrus ater) revealed several male attributes correlated with dominance. Males that won larger percentages of the contests in which they were involved tended to be older and larger than other males. Dominant males arrived earlier on the breeding grounds and became engaged in more contests than subordinate males. If a female was present during a contest among males, the male which she was accompanying (her “consort”, and mate in most cases) was more likely to win the contest than were any of his opponents. Multiple regression analyses were performed to determine any significant partial effects of these attributes on male dominance. Age and involvement in more contests exerted significant partial effects in some analyses, but only the presence of a female consort did so consistently. I suggest that the female is a resource of greater value to the male with whom she is consorting than to other males. Her value is greater in terms of present and future benefits because he has already expended some or all of the effort necessary to establish a pair bond. He will have to expend less future effort to remain her mate than would another male to become her mate. Her presence during contests induces him to persist (or escalate) and to dominate his male opponents.