High levels of extra-pair paternity in Superb Fairy-wrens in South Australia despite low frequency of auxiliary males

The Superb Fairy-wren (Malurus cyaneus) is a model system in which to study cooperative breeding and extra-pair paternity. Previous studies in this species have shown that 61–76% of nestlings were extra-pair young and 92–95% of nests contained at least one extra-pair young. Furthermore, the probability of extra-pair paternity was higher when auxiliary males were present. We studied group size and extra-pair paternity in South Australia where, unlike other study sites, auxiliary males were relatively uncommon: 16% of nests had one auxiliary male compared to 37–80% with at least one auxiliary per nest in other studies. We predicted a lower incidence of extra-pair paternity, given the shortage of auxiliary males. Our results did not support this prediction: we found extra-pair paternity among 67% of nestlings and 83% of nests.