Most birds breed in pairs but at least 3% of passerine species are cooperative breeders, whereby more than two adults help to raise the young. The general rarity of cooperative breeding has led to the assumption that cooperative behaviour has evolved from the ancestral trait of pair breeding. However, it has been suggested that pair breeding may be the derived state in some taxa. The primary aim of this research was to test this suggestion using the genus Acanthiza, which
contains examples of both cooperatively and pair breeding species.
Mitochondrial DNA sequences were used to construct a phylogenetic
hypothesis for the tribe containing Acanthiza, the Acanthizini.
The breeding behaviour of the species sequenced was determined from
records in the literature; where there were no such data the frequency of
another social behaviour, flocking, was used as an indicator of breeding
behaviour. The mapping of breeding systems onto the phylogeny led to the
conclusion that cooperative breeding is the ancestral state in the
Acanthizini, with pair breeding evolving twice in the genus
Acanthiza. Models explaining the occurrence of cooperative
breeding in terms of broad environmental factors or life history do not
appear to be applicable to the genus Acanthiza. The pair breeding
Acanthiza species cluster into two clades, suggesting some influence of phylogenetic history on the occurrence of the different breeding systems. Combining the results of this study with other data suggests the tendency to breed cooperatively could be ancestral in the superfamily Meliphagoidea.