Clutch size and the costs of incubation in the house wren

Trade-offs in the allocation of finite resources among different stages of a breeding attempt as well as between different reproductive events should shape the evolution of life-history traits. To investigate the effects of incubation effort on within-brood and between-brood trade-offs in house wrens (Troglodytes aedon), we manipulated the clutch size that females incubated. We isolated effects of incubation by reversing the manipulation at hatching to allow all parents to provision their natural brood sizes. Females that incubated enlarged clutches had longer incubation periods than control females, both early and late in the season, suggesting that the experimental treatment increased incubation effort. Contrary to predictions, however, increased incubation effort did not adversely affect the allocation of effort to nestling provisioning. Rather, in the early season, but not in the late season, females that incubated enlarged clutches appeared to allocate more effort to nestling provisioning, producing heavier and larger fledglings than control females. Although females with enlarged early-season clutches consequently lost more mass than control females, this was likely an adaptive response to reduce wing loading in anticipation of high provisioning demands. There were no treatment-related differences in fledgling mass or size, or in female mass loss, in the late season. Thus, elevated incubation demands negatively affected a fitness-related trait (duration of incubation) that may constrain clutch size but not the allocation of resources to subsequent stages of the same breeding event or to subsequent breeding events. We suggest that environmental conditions may mediate clutch-size effects on trade-offs in allocation of resources between incubation and nestling provisioning. Copyright 2006.