Bergmann's Rule in House Sparrows (Passer domesticus)

House sparrows 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 pattern) than more southerly animals (light patterns). This is consistent with Bergmann's Rule, which states that members of the same species in colder climates at higher latitudes will be larger.