Stage-structured multihormone phenotypes and a shared endocrine milieu underlie facultative male care in a biparental songbird.

Parental care varies across reproductive stages, between sexes, and even within individuals. This flexibility raises a key question: does caregiving correspond to distinct circulating hormone profiles, or does it occur within a shared hormonal background that supports rapid, context-dependent shifts in behavior? We investigated these alternatives in red-winged blackbirds (Agelaius phoeniceus), a species in which females always feed nestlings while males range from no provisioning to high rates of chick care. Using continuous nest video recordings (24-48 h) and a panel of circulating hormones (prolactin and four steroids), we examined how hormone profiles relate to breeding stage, sex, and paternal effort. First, courtship-stage males and nestling-stage males were clearly separated by their hormone profiles, confirming that breeding stage is a major axis of endocrine variation. Second, during the nestling-stage, provisioning females and males differed in hormone profiles and in how hormones related to parental effort, indicating sex-specific hormonal modulation within a shared parental context. Third, provisioning and non-provisioning males overlapped broadly in hormone profiles, and no single hormone reliably predicted whether a male provisioned. Instead, associations with parental effort emerged primarily through interactions among hormonal axes. Together, these results indicate that breeding stage and sex strongly organize circulating hormone profiles and that facultative male care operates within a broadly shared endocrine background rather than through discrete hormonal divergence between caregiving categories.

Duke Scholars

Author Sara E Lipshutz Biology