Geographical variation in Cinclosoma castanotum and its historical significance

Abstract

The Chestnut Quail-thrush Cinclosoma castanotum varies geographically in the amount of dorsal chestnut, the differences between extreme variants being very pronounced, specially in females. Quail-thrushes from southern Western Australia, the south-western side of the Nullarbor Plain and east of Spencer Gulf have the least amount of chestnut and those from near Shark Bay and the deserts north of the Nullarbor Plain, the greatest. Variation is strongly clinal in Western Australia, negligible in the central deserts, slightly clinal east of Spencer Gulf and apparently strongly stepped in north-western Eyre Peninsula. At the peak of the last ice age, when the climate was quite arid in southern Australia, the central deserts were extremely harsh and a wide coastal plain with mallee occurred in the Great Australian Bight, populations of quail-thrushes with little dorsal chestnut presumably ranged along the entire southern edge of the continent. As the present interglacial regime became established and the climate wetter, the species probably retracted from coastal areas and expanded from southern Western Australia into the Great Victoria Desert, recontacting the population on Eyre Peninsula, which had become isolated by Spencer Gulf and the Nullarbor Plain.