Geographic variation in the Weebill, Smicrornis brevirostris (Gould) (Passeres: Muscicapidae, malurinae) a sedentary species with a continuous range.

Abstract

The present paper analyses geographic variation in Smicrornis brevirostris (Gould), a sedentary species of unusual colour plasticity, and which shows marked and irregular geographic variation. Two main feather pigments are involved in this variation, a yellow one (phaeomelanin) and an olive-brown one (eumelanin). Changes in coloration correspond closely with changes in the climatic environment, and they follow definite lines. Yellow forms are found in a broad zone extending across the north of the continent, where the temperature is high and rainfall is restricted to the summer (giving a warm, dry climate). Brown forms occur across the southernmost third (cold and relatively wet climate). Pallid forms, in which neither pigment is well developed, are typical of arid areas (low rainfall, intermediate temperatures). Parts of the continent over which the average rainfall increases rapidly are associated with steep colour clines and these occur at various places around the periphery. Variation in S. brevirostris is essentially an expression of Gloger's ecogeographic rule and, in that there is a south-north size cline, Bergmann's rule also. Its interest lies in the degree of colour variation displayed, the way in which this can be explained, and the difficulty associated with fitting the variation into the trinomial system of nomenclature.