In the company of cowbirds, Molothrus ater ater: robust patterns of sociability predict reproductive performance.

Many species exhibit behavioral tendencies that are stable over time and across contexts. Robust variation in sociability, or the propensity to approach others, is widespread across the vertebrates. Nonetheless, the influence of sociability on reproductive performance is largely unknown. In this study, we explore the relationship between sociability and reproductive behavior in flocks of Brown-headed Cowbirds (Molothrus ater ater). In spring 2011, we separated birds into three large aviaries based on the number of approaches they initiated and received during fall 2010. Females were separated into high, intermediate, and low sociable flocks, while male sociability was spread evenly across the three flocks. Here we report for the first time that different patterns of social approach tendencies in the fall predicted reproductive behavior in the spring. The high sociable flocks contained more laying females who produced more eggs in contrast to the other flocks. Male courtship behavior was comparable across the three flocks. These findings suggest that robust variation in sociability is an important factor in reproductive performance.