The role of competition in determining morphological differences between Victorian and Tasmanian passerine birds

Abstract

Bill length, tarsus length and wing length were measured in 107 passerine species in southern Victoria and 46 in Tasmania, and the possibility that the impoverishment of the Tasmanian avifauna elicits morphological shifts was evaluated. An hypothesis of competitor release failed to account for the main morphological features of the Tasmanian avifauna. There was no significant difference between the frequency distributions of the three variables in mainland and island species, which suggests that no character size on the mainland is over- or under-represented in Tasmania; in spite of this, in nearly all the species common to both areas all three variables are significantly larger in Tasmania. No significant difference was found between the frequency distributions of character-ratios for mainland and island congeneric species-pairs, though theory predicts the island distribution would be the more skew. In 17 pairs of congeners occurring in both areas, the bill length and tarsus length ratios are not significantly different, and though wing length ratios do differ significantly, it is the mainland pairs that have the larger ratio. The variability of the three characters in 25 species did not differ significantly between mainland and island. Many genera which are represented by several sympatric species in Victoria have only one representative (usually the larger) in Tasmania. Although the theory of competitor release predicts that the species in Tasmania should show decreased character sizes, a clear-cut trend to increased size was found. Congeneric species-pairs in Tasmania do not show large morphological differences, which cannot be necessary for successful coexistence there. The above results are discussed briefly, and an hypothesis based on the larger size of food items in Tasmania is proposed to explain them.