The evolution of cooperative and pair breeding in thornbills Acanthiza (Pardalotidae)

Most birds breed in pairs but at least 3% of passerine species are cooperat ive breeders, whereby more than two adults help to raise the young. The gen eral rarity of cooperative breeding has led to the assumption that cooperat ive behaviour has evolved from the ancestral trait of pair breeding. Howeve r, it has been suggested that pair breeding may be the derived state in som e taxa. The primary aim of this research was to Lest this suggestion using the genus Acanthiza, which contains examples of both cooperatively and pair breeding species. Mitochondrial DNA sequences were used to construct a phy logenetic hypothesis for the tribe containing Acanthiza, the Acanthizini. T he breeding behaviour of the species sequenced was determined from records in the literature; where there were no such data the frequency of another s ocial behaviour, flocking, was used as an indicator of breeding behaviour. The mapping of breeding systems onto the phylogeny led to the conclusion th at cooperative breeding is the ancestral state in the Acanthizini, with pai r breeding evolving twice in the genus Acanthiza. Models explaining the occ urrence of cooperative breeding in terms of broad environmental factors or life history do not appear to be applicable to the genus Acanthiza. The pai r bleeding Acanthiza species cluster into two clades, suggesting some influ ence of phylogenetic history on the occurrence of the different breeding sy stems. Combining the results of this study with other data suggests the ten dency to breed cooperatively could be ancestral in the superfamily Meliphag oidea.