Pair Bonds and Factors Influencing the Diversity of Mating Systems in Brown-Headed Cowbirds

Abstract

Observations of radio-tagged Brown-headed Cowbirds (Molothrus ater) indicated that individual males and females engage in extended associations through the breeding season. Persistent courtship by males directed at particular females and extended time spent together by certain males and females suggests that cowbirds form pair bonds. When individuals of the pair are together, females are more likely to initiate departures than males, and males are more likely to follow their mates after they depart. These observations suggest that pair bonds are being maintained by males. There were four cases of monogamy, as well as two bigamous and at least one trigamous relationship in the local population. Radio-tracking data showed that the range of a male encompassed most or all of that of his mate(s). The mating system of this population is compared with those of other populations in an attempt to assess factors influencing the diversity of cowbird mating systems. We suggest that cowbird mating systems may be influenced by both the sex ratio of the population as well as the habitat in which populations are found.