Parental provisioning in house wrens: effects of varying brood size and consequences for offspring

In biparental species, conflict between the sexes is expected when optimal levels of parental care differ between males and females. We studied food provisioning in house wrens (Troglodytes aedon) to test whether male provisioning is more strongly affected than female provisioning by brood size, because this should influence the marginal return on a male’s investment. To test this, we created broods of reduced and enlarged size, and found that per-nestling food delivery varied more strongly for males than for females in association with brood size and nestling age; males delivered more food per nestling to enlarged broods, and less to small broods, as nestlings grew and approached asymptotic size. Although parents delivered a greater amount of prey to larger broods, the amount of food per nestling was initially higher for nestlings in small broods, and the amount of food delivered early in nestling development, particularly by males, affected offspring body mass, survival to fledging, and recruitment to the breeding population. However, provisioning rates were negatively associated with prey size, suggesting a trade-off between prey quantity and size. Finally, parents provisioning at a high rate to broods of reduced size were less likely to return to breed, suggesting that they assessed the low return on their investment and dispersed. Although potential for sexual conflict in house wrens remains, our data suggest that its resolution does not involve negotiation over provisioning, possibly because selection for offspring viability favors heavy investment by both parents.