Helping behaviour and parental care in fairy-wrens (Malurus)

Cooperative breeding among birds was first discovered in the genus Malurus (Maluridae), the fairy-wrens. Cooperative care arises because male, and sometimes female, offspring remain in their natal territory and help the adults rear offspring. Early uses of data from Superb Fairy-wrens (Malurus cyaneus) to illustrate how kin altruism can explain helping behaviour were based on flawed assumptions. Most importantly, high rates of extra-group mating mean that the helpers often assist adults to which they are unrelated. However, measuring the costs and benefits of altruism has also proved difficult. Helping behaviour and its outcomes among species of Malurus are surprisingly diverse, despite similar founding conditions for cooperative breeding. First, species differ in whether help provides fitness benefits, in the recipients of those benefits, and whether benefits are immediate or deferred. Second, species vary greatly in whether females are philopatric and the extent to which female auxiliaries (supernumeraries), when present, provide care. Finally, male auxiliaries are much less sensitive to the needs of the brood than females. In this review we show that these three aspects of helping behaviour lack compelling explanations. We develop hypotheses to explain each phenomenon. Distinguishing among these hypotheses will greatly enhance our understanding of the remarkable social system of Malurus, and inform the study of cooperative breeding and sexual conflict in general.