Testosterone and prolactin in two songbirds that differ in paternal care: the Blue-headed Vireo and the Red-eyed Vireo.

We investigated the hypothesis that species differences in paternal care in birds may result from differences in concentrations of circulating testosterone (T) and prolactin (PRL). Concentrations of plasma T and PRL were compared in breeding Blue-headed Vireos (Vireo solitarius) and Red-eyed Vireos (Vireo olivaceus), passerine congeners with biparental and maternal incubation, respectively. In male Blue-headed Vireos, plasma T remained low from prenesting to fledgling stages; whereas in male Red-eyed Vireos, plasma T was highest during prenesting and progressively decreased during incubation, nestling, and fledgling stages. In male Blue-headed Vireos, plasma PRL was similar to that in female Blue-headed Vireos and was higher than in male Red-eyed Vireos at all breeding stages. Plasma PRL increased in male Red-eyed Vireos at the incubation stage and remained moderately elevated through the nestling and fledgling stages. In male Blue-headed Vireos, the combination of high PRL and low T during the prenesting stage may promote brood patch formation and nest building, and, at later stages, incubation and feeding of young. In male Red-eyed Vireos, high PRL and low T during incubation and nestling stages may facilitate the feeding of young, as seen in males of other species. Our observations support the hypothesis that differences in paternal care reflect differences in circulating T and PRL.