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A number of widespread species vary morphologically throughout their range. Ultimately such geographic variation appears to be adaptive and represents local evolutionary responses to the pressures of natural selection. Detailed knowledge of size, shape and color variation in species such as the Red-winged Blackbird (Agelaius phoeniceus) may eventually allow the identification of breeding localities of autumn and winter flocks simply from morphological evidence. Knowledge of variation in behavioral and physiological parameters also will be useful in assessing the effect of various control devices.
The present study stems from an attempt to elucidate the adaptive significance of geographic variation of redwings in the Great Plains states of the U.S. and the Prairie Provinces of Canada. Two characters are considered here, the surface/volume ratio of the bill and wing length as measured from the bend of the wing to the tip of the longest primary feather. Character means were calculated for a maximum of 54 localities throughout the study area. All represent breeding season samples, and both males and females are treated.
In order to assess the relationship between character variation and multiple environmental factors, a multiple regression model is used. Independent variables are locality values for longitude, latitude, altitude, mean breeding season temperature, April minimum temperature, July maximum temperature, July wet-bulb temperature, and breeding season precipitation. The partial regression coefficients in a multiple regression analysis allow us to examine the relationship between character variation and a particular independent variable while variation accounted for by all other independent variables is held constant. In this way, we can correct for correlations among independent variables. Although a number of statistically significant partial correlation coefficients were found, only those for which both males and females showed a similar relationship and for which a physiological interpretation is suggested will be discussed.