A comparison of fitness components among different plumage morphs of the Australian Magpie, Gymnorhina tibicen

Abstract

Two hypotheses have been proposed to explain the distributions of back colour morphs of the Australian Magpie in eastern Australia. The widely accepted view has been that the distributions of white-backed (WB) and black-backed (BB) morphs are the result of past isolation, independent evolution and recent recontact. An alternative view is that the distributions have evolved in situ and possibly reflect different selection pressures favouring the WB morph in the south and the BB in the north. Recent molecular studies support this hypothesis. The current study examined a number of potential fitness component differences in the contact zone in south-eastern Australia, which, if supported, would add more weight to the 'primary contact' hypothesis.

Limited evidence was found to support some of the expectations of the 'primary contact' hypothesis for the evolution of the WB/BB distribution in eastern Australia. Habitat variation affected production of fledglings, with position of the nest tree in relation to level of surrounding cover a critical factor. WBs may be at a disadvantage compared with BBs in terms of nestling survival as cover around the nest tree declines. One reason for this relationship could be that WB males make nests more obvious to predators where cover is reduced. Taken together, these data add weight to the suggestion that differences in selection pressures imposed by habitat variables on back-colour variation in males may help to explain the current distributions of WB and BB Magpie morphs in eastern Australia. We had proposed that WB males may have an advantage in social interactions, but there was no evidence that WBs were more likely than other forms to own territories. Neither were they more likely to own territories with high vegetation cover.