Speckled warblers break cooperative rules: absence of helping in a group-living member of the Pardalotidae

Abstract

In the majority of cooperatively breeding bird species, subordinates help the dominant pair to provision the young, regardless of the origin of groups and relatedness between members. Within the family that includes speckled warblers, Chthonicola sagittata, there is considerable variation in social organization and parental behaviour, but the societies of other group-living species in this family are of kin, and subordinates provision the young of breeders. Speckled warblers differ in that offspring are never philopatric, and breeding groups comprise unrelated individuals. We used behavioural observations and DNA fingerprinting to examine genetic parentage and the mating system of the group-living speckled warbler in Canberra, Australia. Speckled warblers breed as pairs or in trios consisting of a female with two males. Alpha males are socially dominant to beta males and maintain dominance through aggression. Beta males never helped to feed nestlings or fledglings, even though they competed for copulations and in one case gained paternity of two of three young in a brood. The social system is therefore polyandrous but not cooperative. As well as competing for copulations with females on their own territories, beta males and single males (those that had lost their breeding partner) regularly undertook extraterritorial forays, most often when the neighbouring female was fertile. Failure by beta males to help even with broods containing their own offspring may be adaptive in this species and may result from increased opportunity for extrapair fertilizations or enhancement of prospects for future reproduction through the formation of alliances with females.