Hormonal Correlates of Breeding Status, Nest Construction, and Parental Care in Multiple-Brooded Northern Mockingbirds, Mimus polyglottos

Blood samples were taken from free-living male and female mockingbirds to determine correlations between plasma steroids and breeding behavior. Plasma levels of testosterone (T), estradiol (E2), progesterone (P), and corticosterone (B) were assessed during pre-breeding courtship, while males built nests, while females incubated, while parents fed young, and while males built new nests as pairs continued to care for dependent fledglings, Samples were also taken from unmated males holding spring territories. In males, T and P fluctuated with breeding phase, and in all groups, male T levels exceeded basal values measured in autumn. But, when mated males built nests in the absence of young, T levels were higher than in all other conditions, though only for early broods. In females too, T and P levels changed with breeding phase; T levels rose during courtship and while males built first nests and were intermediate during nest construction for later broods. Male and female P levels rose during courtship and when males built nests with young present. While feeding young in the nest, males showed significantly higher corticosterone levels than females, though in females, both P and B levels increased during incubation for replacement broods. These findings strengthen an earlier finding that T may activate male nest construction, and they suggest that breeding behavior may be related to T in females as well. Progesterone data raise the possibility that P too, may be linked to breeding behavior in male and female mockingbirds. Sex differences in corticosterone levels during the nestling period may reveal subtle costs associated with male mockingbirds' highly aggressive defense against predators.