Sex-specific costs of reproduction in Eastern Bluebirds Sialia sialis.

In species with bi-parental care, individuals must partition energy between parental effort and mating effort. Typically, female songbirds invest more than males in reproductive activities such as egg-laying and incubation, but males invest more in secondary sexual traits used in attracting mates. Animals that breed more than once within a season must also allocate time and energy between first and subsequent breeding attempts and between current and future breeding seasons. To investigate strategies of reproductive investment by males and females and the consequences of such strategies, we manipulated the size of broods of Eastern Bluebirds Sialia sialis. Pairs with enlarged first broods were less likely to produce a second clutch or took longer to initiate one than pairs with reduced broods. After rearing enlarged broods, females were less likely than males to survive to the following year. Although plumage coloration is a sexually selected trait in Eastern Bluebirds that is influenced by nutritional stress, we did not detect an effect of brood-size manipulation on female coloration. Past research, however, demonstrates that, in males, plumage colour is negatively affected by increasing brood size. We suggest that there are sex-specific strategies of reproductive investment in Eastern Bluebirds, and that researchers should incorporate measures of residual reproductive value in studies of life-history evolution.