Geographic Variation in Mid-Winter Body Composition of Starlings

Abstract

-Starlings (Sturnus vulgaris) collected during mid-winter from a 2,800-km longitudinal range in the eastern United States showed significant geographic variation in body weight, feather weight, wing length, culmen length, tarsus length, dry weight, lean dry weight, lipid weight and lipid index. Total lipid reserves and indices were greatest at middle latitudes. Starlings from the central part of the study area were significantly larger than those at either northern or southern extremes as measured by wing length, body weight and lean dry weight. Insulation, as measured by weight of body feathers per unit of surface area, increased with isophane, an index of regional temperature, but morphometric measures were generally less interpretable. Geographic variation within a species generally reflects ecotypic adaptation or phenotypic response to differences in environment. Earlier in this century, the morphometric aspects of such variation received much attention partly because of the ease of measurement, availability of museum specimens, and also because the theme of most research was systematic or taxonomic rather than physiological or ecological. In the past two decades, biologists have increasingly paid attention to the functional and adaptive significance of geographic variation. However, the analyses seldom have provided data useful in the study of the relationships among morphology, physiology and environmental conditions. To date, studies of the relationships between variation in body composition and environment in endothermic animals are rare, have dealt entirely with sedentary species, and have produced differing results. Hayward (1965), in an analysis of six geographic races of North American deer mice (Peromyscus), found large individual and interracial variability in body fat, but discerned no regular interlocality variation in any gross component of body composition either in winter or summer. Blem (1973, 1974), working with North American House Sparrows (Passer domesticus), demonstrated regular geographic variation in several body components and insulation, which appeared to be related to mid-winter temperature and adaptive for winter survival. Although numerous studies of seasonal differences in body composition or geographic variation in metabolism are available (e.g., see Hudson and Kimzey 1966, Helms et al. 1967, Barnett 1970), few other data reflect upon geographic variation in body composition or energy reserves. I studied the mid-winter body composition of a partially migratory species, the Starling (Sturnus vulgaris). In eastern North America, individual Starlings may migrate several hundred kilometers along a generally north-south axis, but resident flocks are found throughout the range sampled in the present study (see Kessel 1953). Although migratory individuals cannot be distinguished from non-migrants, I know of no evidence that mid-winter populations are moving great distances (if at all) in response to weather extremes. Therefore my working hypothesis was that interlocality variation at least partially reflects adaptation for survival of mid-winter environment at the collec-