VAN ROO, B.L.: The Effects of Exogenous Testosterone on Male Incubation in Blue-headed Vireos (Vireo solitarius)

Natural variation in circulating testosterone (T) is thought to play a proximate role in inter- and intra-specific variation in paternal care in birds. In previous studies, experimental manipulation of T in males has reduced rates of feeding young in several species. Fewer studies have experimentally tested the effects of testosterone on male incubation behavior, particularly in genetically monogamous passerine in which biparental care is thought to be essential to breeding success. Blue-headed vireo males were given subcutaneous implants filled with testosterone (n=13), flutamide (F, an androgen-receptor antagonist, n=13), or left empty (C, n=9). Testosterone treatment significantly reduced care and increased song relative to the other treatment groups, whereas treatment with flutamide resulted in trends opposite to T but not statistically different from the control group. Specifically, the proportion of total observed incubation time performed by males decreased from 0.54 to 0.48 to 0.34 for F, C, and T treatments, respectively. The average period of time that the nest was left unattended during incubation increased from 0.6 to 2.1 to 7.6 minutes for F, C, and T treatments, respectively. Consistent with previous studies, treatment of males with T significantly reduced rates of feeding offspring relative to F and C treatments. During both the incubation and the nestling stages, males treated with T spent more time singing and sang more phrases per minute than males in the F and C treatments. These results demonstrate that incubation is another form of parental care in males that can be altered by changes in circulating levels of testosterone.