A phylogenetically controlled test of hypotheses for behavioral insensitivity to testosterone in birds.

In most male birds that exhibit paternal care, extending the spring testosterone (T) peak throughout the breeding season reduces nestling provisioning. However, in some species, this trade-off between high T and expression of paternal care is absent. For example, during some or all of the nestling period, T did not affect paternal behavior in Male Lapland longspurs (Calcarius lapponicus), chestnut-collared longspurs (Calcarius ornatus), and great tits (Parus major). Two ecological constraints have been hypothesized to drive insensitivity to T after eggs hatch: (1) a short breeding season that limits breeding opportunities, and (2) a need for paternal care to ensure reproductive success. However, because two of the three species that exhibit T insensitivity are closely related, potential phylogenetic confounds limit determination of which, if either, factor constrains some males to T insensitivity. We examined the effects of supplementary T on paternal behavior in the Snow Bunting (Plectrophenax nivalis), a member of the monophyletic Calcarius/Plectrophenax clade. Male Snow Buntings are constrained to a short breeding season, but paternal care is not essential for survival of nestlings. We administered exogenous T during the parental phase to mimic the early spring T peak. T treatment increased song rates and interfered with paternal behavior such that nestlings of T-implanted males grew more slowly than controls. Our data suggest that T insensitivity in this clade is related to relatively recent constraints of the breeding environment (i.e., not simply common ancestry) and that the necessity of paternal care in some species may be a strong selective factor driving behavioral insensitivity to T during the parental phase.