Tolerance of female co-breeders in joint-laying pukeko: the role of egg recognition and peace incentives

Joint laying, where multiple females contribute eggs to a single nest and provide parental care, is a rare breeding system in the avian world. Currently, little is understood about the fitness consequences that joint laying imposes on all members of a cooperatively breeding group, and this is necessary for understanding the evolution of this unusual breeding system. Here, we combine descriptive and experimental studies to understand the costs of joint laying in the pukeko, Porphyrio porphyrio melanotus. Our study shows when total clutch size is large (as a result of two females contributing eggs to a joint clutch), a lower percentage of eggs hatch. As a result, joint-laying females were reproductively compromised relative to females that nested singly. Given this apparent cost of joint laying, it is surprising that females tolerate the eggs of co-breeding females. Thus, in a follow-up study, we tested whether female pukeko restrain from ovicide to avoid the risk of retaliation by the co-breeding female (i.e. the peace incentive hypothesis). Females did not retaliate or cease parental care in response to experimental removal of their eggs from joint nests, and thus, we found no evidence to support the peace incentive hypothesis. However, we suggest that the risk of nest desertion, most likely initiated by male partners, may be an indirect cost preventing the evolution of ovicide. This threat of nest abandonment could force primary laying females to tolerate eggs laid by secondary females.