Look a little closer
Ducks and gulls can stand on ice because they do not try to keep their feet as warm as their bodies. They protect core heat while allowing the feet to approach freezing, supplying only the blood flow needed to keep tissue functioning.
In a bird's leg, warm arteries descending from the body run close to cool veins returning from the foot. Because the blood streams move in opposite directions, heat passes from the arterial side to the venous side in a countercurrent exchanger.
Arterial blood therefore reaches the foot already cooler, reducing the temperature difference with ice and the amount of heat lost outside. Returning venous blood is warmed before it reaches the body, preventing cold blood from abruptly cooling core organs.
Birds can further limit leg heat loss by constricting vessels and adjusting flow through vascular shunts. They do not stop circulation completely. Tissues still need oxygen, and a small or periodically increased supply helps prevent freezing injury.
Feet also contain relatively little bulky muscle and are dominated by bone and tendon, reducing the amount of active tissue that must be kept warm. Birds may tuck one foot into their feathers or lower the body over both feet to shorten exposure.
These adaptations are not a magical guarantee against frostbite. Limits vary with species, health, wind, wetness and the severity of cold. The central strategy is not to heat the entire foot like the core, but to keep it cold, viable and carefully regulated while conserving the body's energy.
