system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 How is it that birds can swim in freezing cold water and expose their feet to sub zero temperatures without suffering from frostbite?

It's something that's always baffled me, birds are everywhere in the winter with their exposed feet perched on branches, in water, or on top of ice, and they rarely seem to be affected by the cold in the same way that human skin is. How do birds not get frostbite?
gpt
Basically birds thermoregulate differently than mammals like humans do. Mammals use hair or fur to keep warm because they can't change their base metabolic rate easily. Birds usually just change their base metabolic rate to create more heat. Also their feathers are a better insulator than most hair, and they have a different blood circulation system to prevent extremely cold blood from the extremities from going straight back to the heart.