Morphological changes in isolated populations of some passerine bird species in Australia

Abstract

Variation in bill length, bill depth, bill width, tarsal length, hallux length and wing length was studied in isolated versus nonisolated situations in Australia with 14 species or superspecies of passerine birds. Most of these variates are of obvious ecological importance. None is prone to change more than any other in the seven isolates considered (South-western Australia, Eyre Peninsula, Fteurieu Peninsula, Kangaroo Island, King Island, Flinders Island and Tasmania), so presumably none is more advantageous to change than any other. Few shifts were found in the first four isolates, in contrast to the last three in which all populations showed marked change in dimensions. In the isolated populations, shifts most often involve increases in dimension. Whether selective processes other than competition are important in explaining changes in bill length, tarsal length and wing length was analysed with multiple regression. Changes in number of competitor species explains variation in bill length in eight out of 11 species. Variation in tarsal length and wing length are explained about equally by latitude, competitors, and other factors. Bergmann's and Allen's rules probably apply much less generally than is realized. A study of coefficients of variation of bill length, tarsal length and wing length was inconclusive in that most cases could not be explained by either latitude or competitors. When a correlation between coefficients of variation of these dimensions and latitude exists, it is nearly always positive.