Why do some birds stand on one foot?

Birds will often stand on one foot to minimize heat loss. Some birds with fleshy feet, such as doves, have relatively short legs and can hunker down so their warm belly is pressed against their feet while perched, but accipiters, such as this young Cooper’s Hawk, have longer legs that make this far more difficult. These birds will often hold one foot inside their belly feathers to minimize heat loss, and often switch back and forth between their feet.