Complementary Descriptive and Experimental Studies of Clinal Variation in Birds

Open Access
Publisher Website
Google Scholar
Abstract
In this paper I argue that the general concordance among species of birds in theirclinal (gradual) patterns of geographic variation in size and color of plumage is so high that the primary regulatory process must be physiological adaptation to climate or its correlates. Size variation in the American Robin, a widespread species that shows the pattern of clinal variation typical in North American passerines, can be well predicted by measures of summer climate that include the effects of both temperature and humidity. July mean noon absolute humidity is a good predictor of size variation in the robin within the United States. Geographic variation in the color of the plumage of the robin, which involves interactions among several pigments, is independent of its size variation but in accord with color variation in other species. Both Bergmann's and Gloger's ecogeographic rules are important empirical models for which the physiological basis needs further study. To establish that functional differences are subject to natural selection requires showing that geographic differences have a genetic basis and that these differences are related to the survival and reproduction of individuals at different localities. Laboratory and field experiments with Red-winged Blackbirds show that environmentally induced effects on development probably contribute to clines in measurements of adults in this species. Further work is needed to clarify the generality of the relationship between clinal variation and environmentally induced variation and to determine the functional significance of clines and their relationship to survival and reproduction in different localities. Environmental plasticity itself is geographically variable. It may have a genetic basis and be an object of selection.

This publication has 0 references indexed in Scilit: