House Sparrows: Evolution of Populations from the Great Plains and Colorado Rockies

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Abstract
This study of House Sparrows was undertaken to determine the nature and degree of differentiation in color and morphology along a 1000-mile transect of the Great Plains and montane Colorado, and to relate these patterns of variation to climatic gradients encountered along the transect. The study is based upon examination of 785 specimens collected at 12 different localities in Illinois, Kansas, and Colorado. The results confirm earlier reports that North American House Sparrows have undergone conspicuous adaptive differentiation in color and morphology. Interpopulational variation in tarsus length is clinal and conforms with Allen's ecogeographic rule. Wing length of House Sparrows varies geographically, but variation of this character is not a function of altitude or isophane. Interpopulational variation in bill length of House Sparrows is thought to reflect differences in wear consequent to differences in abrasive action of food at the respective localities. There seems to be no relation between bill length and annual precipitation or ambient temperature. Partitioning of the food niche has not occurred in North America. Color of cryptic plumages of adult and subadult male House Sparrows varies clinally in accord with Gloger's ecogeographic rule. Auricular display plumage of adult males also varies in color, but the clinal pattern is opposite to that which obtains in cryptic plumages. No such trend in coloration is apparent in auricular plumage of subadult males. Clinal variation in color in accord with Gloger's rule is manifest in adult female House Sparrows, but not in subadults. Local differentiation has occurred in no more than 90 years. Slow rates of evolution thought to obtain for birds cannot be attributed in every instance to rigid restraints upon the genetic system; in some cases barriers to dispersal or relative constancy of the environment over long periods of time must have dictated rates of differentiation.