Many creatures are bigger where it’s colder.

For instance, a hairy woodpecker from Central America is noticeably smaller than a hairy woodpecker from Canada.

Birds of a wide-range species that live in the cold end of the range tend to be bigger than birds that live in the warm end. That’s because larger birds are able to conserve heat better than smaller ones. Over many generations smaller birds born in cold regions have been more likely to die from mid-winter cold, leaving the larger siblings to carry on the species — in larger size.

The process of size variation doesn’t take all that long, either. The house finches, introduced into North America in the mid-1800s, have already developed smaller versions in the southern United States than are found in northern states and Canada.

The tendency of a species in cold climates to be larger than its fellow members in warm climates applies not only to birds, but all warm-blooded animals. In fact, it is a scientific principle called Bergmann’s Rule, named for 19th Century German biologist Carl Bergmann, who developed it.

Bergmann did not address dieting, however, so it’s not likely those of us who want to lose weight in the new year will do it more easily if we move south.—J.S.