This is in accordance with Bergmann's rule, which predicts that the largest birds should be found higher elevations or in colder and more arid regions.
Among extant penguins, larger penguins inhabit colder regions, while smaller penguins are generally found in temperate or even tropical climates ("see also" Bergmann's rule).
The species does not conform to Bergmann's rule, with the pattern of size variation seeming to follow a trend of size increase along an east-west axis.