Relationships between winter energetic condition and reproductive investment in a wild bird

Reproduction is an energetically demanding life history stage that comprises costly physiological and behavioural changes. Despite these costs, some individuals invest more in reproduction, and breed more successfully than others. To understand variation in reproductive investment, studies often evaluate factors during breeding, but conditions outside of the breeding season may also play a role. These linkages across life history stages remain poorly understood, particularly at the mechanistic level. Using wild populations of black-capped chickadees (Poecile atricapillus), we evaluated whether traits relating to energetic condition (i.e., circulating corticosterone concentrations, oxidative balance, haemolytic complement activity, and body condition) during the non-breeding season predicted reproductive investment in the subsequent breeding season. Winter haemolytic complement activity did not predict subsequent breeding investment. However, we found that females with high winter fat scores laid eggs sooner and fed their offspring more often, and females with high winter corticosterone concentrations produced lighter eggs. We also found that males with high winter oxidative stress had smaller clutches, that males with high winter fat fed their offspring more often and had nestlings with lower baseline corticosterone, and that males with high winter corticosterone concentrations had lighter eggs and smaller nestlings in the brood produced by their mate. These winter energetic traits may relate to reproductive investment via carry-over effects across seasons, and/or because they reflect an individual's phenotypic quality. Altogether, our findings suggest that in wild populations, conditions experienced outside of the breeding season may be important factors explaining variation in reproductive investment, even in a resident species. These linkages between life history stages could have important implications for the study of population dynamics, particularly in declining populations.