Conflict over parental care in house sparrows: do females use a negotiation rule?

Ho do parents resolve their conflict over parental care? The classical sealed-bid model of biparental care suggested that parents use a fixed best effort given the partner's effort. Alternatively, parents may negotiate their actual effort until the efforts of both partners settle down to limiting values, but in this case, the resulting efforts will not be the best responses to one another. Consequently, under the best response scenario, the response of 1 parent to the removal of its mate can be predicted from the response to a reduction in its partner's effort, whereas the negotiation model predicts that such an extrapolation will underestimate the effort of a parent caring alone. We tested this prediction in free-living house sparrows (Passer domesticus). We experimentally manipulated the males' parental care as follows: males' care in group 1) was reduced by using a capture-handling-release stress protocol, 2) stopped by removing the male, and 3) left as control. In response to these manipulations, control females kept their feeding rate constant, whereas male-stressed-released females showed a moderate increase of feeding rate. When this response was extrapolated to zero male effort, their effort was still significantly lower than the observed effort of male-removed females. These results suggest that females may use the negotiation rule to determine their actual parental effort.