Bergmann's Rule in House Sparrows (Passer domesticus)

House sparrows (Passer domesticus) were introduced to North America in the late 19th century. The map shows trends in size (not color) from a composite of measurements. More northerly animals are larger (darker patterns) than more southerly animals (lighter patterns). This is consistent with Bergmann's Rule, that members of the same species in colder climates at higher latitudes will be larger. Note that the geographic trend has developed within a species derived from relatively few founders, in under 200 years.