Reciprocity and conditional cooperation between great tit parents

When individuals invest in a common good, an efficient outcome is hard to achieve, because each can free ride on others' efforts. This problem can lead parents that raise their young together to reduce their investment in care, with negative consequences for offspring. Here, we present a mathematical model to show that a strategy of conditional cooperation, in which parents take turns feeding their young, can help to resolve this problem. To test this idea, we studied the behavior of great tit parents raising chicks together. We found that parents alternated visits to the nest more than would be expected by chance, speeding up their feeding rate after their partner had visited the chicks, but slowing down again once they had visited in turn. This effect was not mediated by visit-to-visit changes in offspring begging intensity, although females (but not males) were influenced by mean begging levels across broods. We conclude that conflict over parental investment in this species is partly ameliorated by a simple form of reciprocity.