The relationships between winter climate and selection on body size of house sparrows

Abstract

Overwinter modification of the morphology of house sparrows was examined in relation to the intensity of proximate wintertime climatic conditions. Samples were a posteriori for four of six comparisons for males and three of five for females. Two comparisons were collected specifically (one recently) to examine overwinter changes. Comparisons were of multivariate assessments of skeletal measurements for subadult fall (males only) and total fall versus total spring samples. There were significant differences in PC1 (overall body size) between fall and spring groups for 3 of 11 comparisons. For males, the overwinter change in size was significantly correlated (p < 0.01) with the proximate level of precipitation. For females, overwinter change in size correlated best with the deviation of a winter's temperature from the all-time mean winter temperature (p < 0.05). Only for females did changes in variance correlate with climate (temperature deviation, p < 0.05). This suggests that increasing severity of winter climate results in larger males, but smaller (and less variable) females. A combination of physiological (energetic), behavioral (dominance relations), and ecological pressures may cause the observed size changes.