Superb fairy-wrens: making the worst of a good job

Fairy-wrens have always played a pivotal role in the study of cooperative breeding. So far as we are aware, the first recognition that more than two birds could combine to rear a single brood of young comes from John Gould’s (1841) depiction of the superb fairy-wren Malurus cyaneus in his treatise on the birds of Australia, many decades before Alexander Skutch’s explorations of the biology of Neotropical birds (Boland and Cockburn 2002). Indeed, experiments on cooperative breeding were reported for the superb fairy-wren M. cyaneus as early as 1910, and in the 1950s this was also the first cooperatively-breeding species to be studied using color-rings to distinguish between individuals (Rowley 1957; Bradley and Bradley 1958). Since then there have been three intensive demographic studies of Malurus species lasting more than fifteen years (Russell and Rowley 1993, 2000; Cockburn et al. 2008b). Comparative studies between populations also have access to additional data from 13 populations of 7 congeneric species (van de Pol et al. 2013). It is certain that early detection and sustained interest in cooperative breeding in fairy-wrens reflects the comparative ease with which they can be studied. At least some species are abundant, adapt to the presence of humans, and build nests conveniently close to the ground. In addition, in contrast to many cooperatively breeding species, the sexes are easily distinguished, as adult males are among the most brilliantly-colored of birds, while females and juveniles in most species are much drabber. Species can therefore be instantly identified as cooperative breeders if two males are seen carrying food to the nest (Buchanan and Cockburn 2013). While early work on fairy-wrens repeatedly informed theory concerning cooperative breeding (Margraf and Cockburn 2013), the current explosion of interest in this group stems from the surprising discovery that most young in most species are not sired by the males that provision and defend them (Brooker et al. 1990; Mulder et al. 1994; Cockburn et al. 2013). Instead, parentage is dominated by extra-group fertilizations initiated by the female. Such rampant infidelity undermines the expectation that helping behavior by subordinate males will be directed toward full-siblings, immediately reducing the potential for indirect fitness benefits at nests with helpers