Body condition dynamics and the cost‐of‐delay hypothesis in a temperate‐breeding duck

Pre‐breeding body condition is an important determinant of reproductive success in birds, largely through its influence on timing of breeding. Declines in clutch size and recruitment probability within breeding seasons indicate a tradeoff may exist between the number of young (clutch size) and quality of young (recruitment probability). We explored local drivers of pre‐breeding body condition and tested predictions of the cost‐of‐delay hypothesis in female lesser scaup Aythya affinis. Yearling females arrived on the study site in lower body condition than older females, but both age classes had similar rates of body condition gain on the breeding grounds prior to nesting. Rates of body condition gain were positively influenced by water temperature, a proxy for wetland phenology. The effect of water level was asymptotic and interacted with water temperature, with greater rates of gain in body condition occurring in years with low water levels. Our results supported the predicted response of clutch size to the rate of pre‐breeding body condition gain. After accounting for lay date, clutch size was positively related to the rate of body condition gain (= 0.08 ± 0.039). We did not find support for a predicted interaction between rate of body condition gain and intra‐seasonal decline in clutch size (= 0.01 ± 0.01). Our results indicate that local conditions during pre‐breeding influence body condition dynamics in female lesser scaup, which subsequently affects clutch size.