SummaryIn their zone of geographical overlap in Western Australia,Malurus pulcherrimusandM. lamberti mastersibehave as distinct species by rot interbreeding despite occupying seemingly identical habitats. WhereM. elegansandM. pulcherrimuscome into close geographical contact, competition is avoided by the ecological unsuitability of each other's habitat. Contact betweenM. dulcis rogersiandM. lamberti mastersiin the south Kimberleys is unknown.No geographical variation apparently occurs inM. elegansandM. pulcherrimusin the south-west, whileM. lamberti mastersitends to be smaller in inland and northern areas, the trend being correlated with the temperature gradient. The Gloger and Bergmann effects occur at the interspecific level in theM. lambertisuperspecies in south-western Australia. On Bernier Island the distinctive subspeciesM. lamberti bernierihas taken at the most 8,000 years to attain its present status. On the basis of morphological criteria,M. dulcisis considered worthy of specific rank, and is divisible into two subspecies—rogersiin the Kimberleys anddulcisin the Northern Territory.