Geographic Variation in the Surface/Volume Ratio of the Bill of Red-Winged Blackbirds in Relation to Certain Geographic and Climatic Factors

Abstract

Much circumstantial and experimental evidence exists to relate logically intraspecific and interspecific variation in bill morphology with functional adaptations in diet and feeding behavior (e.g., Beecher 1951; Pitelka 1951; Betts 1955; Morris 1955; Hinde 1959; Bowman 1961; Kear 1962; Myton and Ficken 1967). Although geographic variation in bill shape and size is often explained in this way, it is also important to consider the fact that more than one environmental factor may exert a selective influence on a given character, and that multiple environmental factors may therefore be responsible for the pattern of geographic variation. The present analysis considers one aspect of bill morphology, the surface/volume ratio (s/v), of Red-winged Blackbirds (Agelaius phoeniceus) from central North America and examines geographic variation in this character in relation to eight geographic and climatic factors. The results suggest that variation is the consequence of natural selection by more than one factor and also show what appears to be a previously unrecorded relationship between bill s/v and minimum temperature during the breeding season. Earlier (Power 1970), geographic variation in bill length, bill depth, lower bill width, and upper bill width was examined statistically with over 50 samples of males and 48 samples of females, all of breeding season birds, from throughout central North America. The study area (fig. 1) includes the prairie provinces of Canada and most of the Great Plains states of the United States. In both sexes, bills are longest in the northwestern and central Canadian plains and parts of the northern U.S., become shorter in the central plains and states of Wyoming and Colorado, and are long again in the southeastern plains, including Iowa, eastern Oklahoma, Illinois, and Missouri. Bills are thickest in Northwest Territories and