Automated radio tracking provides evidence for social pair bonds in an obligate brood parasite

Abstract

Social monogamy predominates in avian mating systems, but this strategy seems unlikely among brood parasites, where the emancipation from parental care should lead to an increase in numbers of mates for both sexes. Despite this, there is great diversity in mating systems in brood parasitic birds. The Screaming Cowbird Molothrus rufoaxillaris is a brood parasite that specializes almost exclusively on one host, the Greyish Baywing Agelaioides badius. Field observations and a radio telemetry study have indicated that male–female pairs spend a great deal of time together through the day, suggesting that they may be socially monogamous. We tested the hypothesis of social monogamy by radio-tracking male and female Screaming Cowbirds using an array of automated radiotracking receivers to collect data on spatial behaviour and social mating system. Monogamous pair bonds were recorded through social network analysis. These bonds endured throughout the entire breeding season, with males and females remaining associated throughout the day. In accordance with the hypothesis, paired Screaming Cowbirds spent significantly more time in proximity to their mates than to other conspecifics of the opposite sex, and had similar home-ranges. Social monogamy in the Screaming Cowbird differed markedly from the social polygyny/promiscuity exhibited by the sympatric congener Shiny Cowbird Molothrus bonariensis, a generalist brood parasite.