Social dominance, mating and spacing systems, female fecundity, and vocal dialects in captive and free-ranging brown-headed cowbirds.

Abstract

Molothrus ater shows a diurnal cycle in sociality, being relatively asocial in the morning and social in the afternoon. Although cowbirds vary geographically in the extent of their daily movements, all populations share a similar opportunistic approach to their use of space for breeding and feeding. Female cowbirds have a similarly high rate of egg laying throughout the species' range, despite considerable variation in habitats and host species. Yearling males have lower breeding success and body weights than adults in some parts of the US West, whereas such differences may not occur in the East. Cowbirds have highly divergent vocal dialects in some parts of California, but little vocal variation elsewhere in the state. -from Authors