The enigmatic mating system of White-winged Fairywrens ( <i>Malurus leucopterus</i> )

Abstract Group-living social systems are common in nature, but the adaptive benefits of these social systems and how they relate to mating systems have not been well studied. White-winged Fairywrens have one of the most complex and enigmatic avian social systems. Group territories are occupied and defended by up to 20 adults in brown plumage and one adult male in ornamented plumage (vivid blue and white on mainland Australia; black and white on two islands off the west coast), with up to five pairs breeding on each group territory, often with assistance from helpers. The adaptive benefits of this social structure and how it relates to the species’ mating structure have not been studied. Here we present a field study of two populations (one island, one mainland) during two consecutive breeding seasons in which we documented the social structure of adults and quantified the genetic relationships between 205 adults, 34 juveniles, and 158 nestlings. First- and second-order kin relationships between adults were more commonly observed across group territories than within, and although more than 70% of nestlings were sired by extrapair males, most extrapair sires were from outside of the social group territory. When social mates were first- or second-order kin, extrapair males were almost always unrelated, and extrapair males tended to be in better condition than the pair males they cuckolded. Our results do not clearly support previously suggested hypotheses (kin selection or cuckoldry) for this species’ complex social structure. The functional benefits of this social system remain enigmatic.