Being unrelated genetically, monogamous mates have some interests in common, others at odds. Where costly biparental care is provided to nidicolous young, the sexual partnership is likely to be a complex mixture of cooperation and selfishness: each individual may enhance its lifetime reproductive success by getting its partner to supply the greater investment. Recent ESS models predict that such a parent should compensate only partially for any shortfall by its mate, yet should become more generous as its own future (its residual reproductive value) diminishes. Because many factors influence life-history parameters asymmetrically according to gender, the ESS approach leads to novel predictions regarding the parental division of labor. The proposed field observations and experiments on house sparrows will explore these dynamics. Parental labor required for provisioning the brood will be artificially increased (by slightly encumbering one parent with small lead fishing weights affixed temporarily to the rectrices) and the food-delivery performances of both the taxed parent and its mate will be recorded. At the same time their daily energy expenditures will be assessed via the doubly-labelled water technique. At other nests, the costs of parental care will be artificially decreased (via food supplementations) and the impact on both male and female parents will be determined. Because house sparrows are remarkably sedentary and breed multiply within a season, it should be possible to make robust assessments of how increased parental exertion affects both future adult reproductive performance and current-brood recruitment. The overall goals of the initial support period are: (1) to examine the separate costs of male and female parental care at three levels (in terms of behavior, energy expenditure, and future reproductive success); (2) to measure the impact of variable levels of parental provisioning on whole broods of offspring; (3) to tailor ESS models of biparental care so as to account for observed differences between males and females in this particular system; (4) to test several key predictions from current ESS models about how parental effort and sex differences in effort should relate to its costs; and (5) to assess whether discontinuation of partnerships from one nesting cycle to the next is contingent on disruptions of the relative workloads of each parent.
