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 resolve this problem. To test this idea, we studied the behaviour of great tit parents raising chicks together. We found that parents speed up their feeding rate after their partner has visited the chicks, but slow down again once they have visited in turn, promoting alternation. We conclude that conflict over parental investment in this species is partly ameliorated by a simple form of reciprocity.