Abstract

Gerygone fusca, G. levigaster and G. mouki are considered to compose a complex that speciated after a common ancestral form was split by severe aridity in the Pleistocene into isolated populations respectively in south-western, north-western and eastern Australia. Subsequently, fusca spread eastward along the southern part of the continent into inland eastern Australia, then northward to the Carpentarian lowlands and finally westward into the semi-deserts of central Australia, while levigaster, which had become adapted to mangroves, spread eastward to coastal eastern Australia and southern New Guinea. The allospecies fusca and levigaster are now in secondary contact without interbreeding along the southern Kimberley, head of the Gulf of Carpentaria and lower Hunter River. G. mouki, being largely confined to rainforest, has not spread from eastern Australia. The various populations of fusca have diverged behaviourally as well as morphologically: the western population tends to migrate inland and northwards in winter and is known to breed only in the south-west; the south-eastern plus central-eastern population is partly migratory and apparently breeds throughout most of its range: the Eyre Peninsula and Carpentarian populations are sedentary; and the desert population is partly nomadic. An analysis of characters in Gerygone indicates that Australian species fall into three distinct groups and that G. sulphurea and probably G. inornata-dorsalis are not particularly close to the G. fusca complex.