Endocrine Response of Captive Male Brown-Headed Cowbirds to Intrasexual Social Cues

The hormonal response to a spectrum of intrasexual social stimuli was studied in captive Brown-headed Cowbirds ( Molothrus ater ). Males were housed in individual cages in one of four conditions: (1) in a semicircle with other males (= visual + vocal stimuli), (2) in acoustic chambers with visual access to a devocalized companion (= visual stimuli only), (3) in chambers in which a tutor tape was played daily (= vocal stimuli only), and (4) in acoustic chambers with no other input (neither visual nor social stimuli). Blood samples were taken at 2-week intervals throughout the spring and early summer and assayed for testosterone. The results indicate that the greatest increase in testosterone was in the birds receiving visual plus vocal stimuli and in those receiving only visual stimuli; males receiving only vocal stimuli had a muted endocrine response, and those receiving no social stimulation exhibited the briefest response. The relatively small response to vocal stimulation is consistent with the absence of a behavioral response to simulated territorial intrusions (i.e., song playbacks) that has been shown in previous studies in male cowbirds.