Increased opportunities for cuckoldry may be why dominant male fairy-wrens tolerate helpers

The highest known rates of extra-pair fertilization (76%) occur in the superb fairy-wren (Malurus cyaneus), a bird that lives in both breeding pairs and in cooperative groups where 2-5 males assist a single female. Males living in groups are cuckolded more often than males in pairs, apparently because females can rely on helpers as an alternative source of care, and so do not need to allow their mate fertilizations. It is therefore unclear why dominant males tolerate helpers. Here we show that dominant males with helpers provide less parental care during the nestling period, and use this reduced workload to make extraterritorial forays which are used to court extra-group females. DNA fingerprinting suggests that this increased display rate provides them with an advantage in obtaining extra-group copulations. These data suggest that within-pair and extra-pair paternity will not always be positively correlated.