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Wild Turkey standing on one leg to conserve heat. This image was taken on a day when the afternoon high was 9 below zero and the morning low was 12 below zero.

Excerpt from birdnote.org:

Birds' legs have an adaptation called "rete mirabile" that minimizes heat loss. The arteries that transport warm blood into the legs lie in contact with the veins that return colder blood to the bird's heart. The arteries warm the veins. Because the veins also cool the arteries, the bird’s feet are closer to environmental temperature and thus don’t lose as much heat as they would if they were at body temperature. And by standing on one leg, a bird reduces by half the amount of heat lost through un-feathered limbs.