Differences in Sexual Size Dimorphism and Body Proportions between Adult and Subadult House Sparrows in North America

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Abstract
North American samples of House Sparrows (Passer domesticus) collected by R. F. Johnston and R. K. Selander (see Johnston and Selander 1971, Johnston 1973) were separated into adult (19 localities) and subadult (21 localities) groups. Significant clines were generated for all age and sex combinations through regression of locality means for body size against environmental seasonality. The strong body-size clines for subadult House Sparrows suggest a large genetic component to geographic variation in House Sparrow body size. There were no significant differences between the male and female clines for either age group. Therefore, there was no consistent relationship between sexual size dimorphism (SSD) in adult House Sparrow populations and environmental factors. Adult levels of SSD were higher than subadult levels at localities with distinct seasonalities. This resulted from selection against relatively large subadult females in areas with cold winters. Adult males were larger than subadult males, but whether this represented growth or selection is equivocal. The ratio of body core to limb variables was correlated poorly with the winter temperatures of the collecting localities. The relationship improved if samples from coastal areas were excluded. Adult levels of this ratio were higher than subadult levels at localities with low January temperatures. Overwinter changes in both House Sparrow size and shape can be correlated with environmental variables. Although selection for efficient thermoregulatory design is implied, it is difficult to account for the mortality of large subadult females over cold winters and to completely discount the effects of growth.