Demographic consequences of sexual differences in age at first breeding in Cooper's Hawks (Accipiter cooperii)

Life history theory predicts individuals should breed as soon as they are able to reproduce, but many long-lived birds delay breeding. In the Accipitriformes, delayed breeding is the norm, and age when breeding begins is influenced by competing selective pressures. In most Accipitriformes, reproductive roles of males and females differ; males do most of the foraging and females tend eggs and young. Thus, sexual differences in age at first breeding might be expected, but these differences, possible causes, and implications for individual fitness have received little study. We investigated sexual differences in age at first breeding in a marked population of Cooper's Hawks (Accipiter cooperii) from 2011 through 2018 in central New Mexico, USA. We predicted: (1) a lower mean age at first breeding for females than males because male provisioning ability is thought to improve with experience, and (2) that expected individual fitness of early-breeding males would be lower than for early-breeding females. Consistent with our predictions, we found that 79% more females than males bred in their first year (HY), and expected individual fitness of HY-breeding females was 21% greater than for HY-breeding males. HY males that attempted to breed settled on nesting territories with exceptionally high prey abundance, nevertheless they experienced 37% lower second-year survival than males that delayed breeding. Females competed for mates based on male age. HY females that paired with relatively older males had 33% higher second-year survival and 16% higher expected individual fitness than HY females that paired initially with relatively younger males. The observed annual rate of growth (λ) of our study population was 1.08, midway between λ predicted by male (1.02) and female (1.19) demographic models. Delayed breeding by males thus had important ramifications for λ, highlighting the need to consider sexual differences in age at first breeding in demographic models