COSTS AND BENEFITS OF EXTRA-GROUP PATERNITY IN SUPERB FAIRY-WRENS

One of the most extreme examples of conflict between the sexes occurs in the Superb Fairy-wren {Malurus cyaneus), a cooperatively breeding bird in which 72% of nestlings were produced by extra-pair fertilizations. Males that gained extra-pair paternity were almost exclusively dominant breeding males out- side the group and not helpers. Group size had a significant effect on the number of young sired by males on their own territory. Males breeding in a pair (no helpers) sired more young (41%, 71/175 young) and provided more parental care (42% of feeding visits) than dominant breeding males that lived in groups with helpers (19%, 56/292 young; 25% of feeding visits). This pattern of extra-group paternity was a result of female choice and control of fertilization. When females lived in a group, helpers compensated for the lower parental assistance of the dominant breeding male, so all offspring were provisioned fully and there was no cost to the female or male in terms of reduced nestling survival. Thus, the mate choice of females with helpers was not constrained by the importance of male parental assistance. In unassisted pairs, females had no alternative source of pa- rental assistance and may have allowed their mates greater paternity to ensure the assistance of the mates in providing parental care. The reduced workload of dom- inant breeding males in groups did not increase their survival; rather, it allowed them more time to engage in courtship displays to females on other territories. This increased display rate of males in groups was associated with a greater likelihood of gaining extra-group fertilizations, and it may have compensated males in groups for their lower within-pair paternity. The release of females from the constraint of male parental care was due to helpers, whereas the reduction in male parental care was most likely the result of a paternity cue and not simply the presence of helpers per se. After controlling statistically for group size, males provided less parental care when they had lower paternity. Thus, a detailed knowl- edge of the alternatives available to females (other sources of parental assistance) and males (opportunities for extra-pair matings) may be necessary to understand male and female reproductive strategies.