Why do male brown-headed cowbirds sing the most attractive songs when reared in isolation with only females, unable to hear male song?

Sexual competition often occurs within complex social environments where male displays can be received by potential mates, rivals, or both at once. In brown-headed cowbirds' breeding flocks, for example, multiple males sometimes vie directly for a single female's attention; at other times males have opportunities to sing to females without interference. It was tested whether cowbirds vary the intensity of their signalling across contexts like these. The songs were recorded from males courting females both with and without a male competitor in sight. The recordings were now played to solitary, naïve females in sound attenuation chambers, and also to a naïve aviary-housed flock. The songs males had produced when they could see their competitors were more attractive, eliciting more copulatory postures from naïve females and more approaches from birds in the flock.