"A duck foot is mostly tendons with very little muscle, the muscles that
help them move are located further from the feet and better insulated,
so there is less need for blood flow to the feet to keep them warm.
Ducks also have a very unique circulation system, where warm blood going
down to the feet goes closely past the cold blood coming back from the
feet. This is called a counter-current heat exchange as heat is
exchanged from the hot blood to the cold, meaning that the birds do not
lose too much heat through their feet."