Effects of singing on copulation success and egg production in brown-headed cowbirds, Molothrus ater

We examined the relationship between singing and reproductive success in cowbirds. We amassed data from 17 captive flocks (164 males, 167 females) that we have studied over 4 years. For each flock, we conducted extensive observations on social interactions as the birds competed, courted, and reproduced. We collected and incubated all eggs laid during the breeding season and performed parentage analyses on the 7 flocks with the highest levels of egg production. Finally, we measured males' song quality in playback tests. Here, we assessed what aspects of singing were associated with 1) copulation success and 2) offspring production. Results differed for these 2 measures of reproductive success because of high variance in egg production within and across groups. The overall amount of songs males directed to females, a measure of courtship persistence, was the one variable strongly associated with males' copulation success. For offspring production, there was significant between-flock variability that was more pronounced than the within-group variability. The one variable that was found to be strongly associated with eggs laid within and across groups was the amount of countersinging males produced; a measure of male--male singing competition. Song attractiveness did not predict any unique variance in either measure of reproductive success. The relationship between female egg production and male competition suggests that females may be trading off current versus future reproduction based on the opportunities available in groups to evaluate males' competitive abilities. Copyright 2010, Oxford University Press.