LYNN, S.E.*; BENOWITZ-FREDERICKS, Z.M.; HAYWARD, L.S.; WINGFIELD, J.C.: Testosterone influences male behavior during incubation but not during the parental phase in a temperate passerine

Most monogamous male birds exhibit a peak in plasma testosterone (T) in the early spring coincident with territory establishment and pair bonding. Once nests are initiated, T declines precipitously and remains low as nestlings hatch and males become parental. A growing body of evidence suggests there is a trade-off between a male’s level of T and expression of paternal care: high T is associated with reduced paternal care in favor of enhanced sexual behavior. Male chestnut-collared longspurs (Calcarius ornatus) show a high degree of paternal care, including feeding and brooding young, acting as nest sentries, and aggressively defending the nest against predators. We experimentally altered the plasma level of T of male C. ornatus such that the early spring T peak was extended through the parental phase. T enhanced song behavior typical of the sexual phase and decreased nest sentry behavior during incubation, but did not interfere with brooding or feeding of nestlings. Thus, males responded behaviorally to T during incubation, but, unlike most other monogamous birds studied, did not respond to T during the parental phase. Through a series of male removal experiments, we determined that male chestnut-collared longspurs are critical for nestling survival. Based on these data and similar results in an Arctic passerine, we suggest that the pattern of males’ reduced responsiveness to T during the nestling phase may reflect the importance of male care in nest success.