A female mallard duck standing on ice at Green Lake two Friday mornings ago made me wonder how her feet didn't freeze. I did a little research and this is why mallards and other ducks are so tolerant of frigid ice temps: "Birds such as gulls and ducks endure long periods of standing on ice via regional heterothermy, or maintaining a core body temperature while allowing the temperature of extremities to deviate from the core temperature.
Keeping an entire foot warm requires a tremendous energy cost. Instead, these birds allow the foot to approach freezing temperatures. Blood is still supplied to the foot, however, so the birds use a countercurrent heat exchange system—cool blood coming back from the foot travels through veins grouped around arteries that are sending warm blood from the body to the foot. Heat is transferred from the warm arteries to the cool veins.
This countercurrent heat exchange system is very efficient at maintaining heat in the core. Periodic increases in blood flow allow a little heat to reach the foot and prevent it from freezing."
