Smaller birds often drop on the ground to cover both legs with their fluffed up bodies. To minimize heat loss from their legs, the arteries and veins in the legs of many birds lie in contact with each other and function to retain heat. Arterial blood leaves a bird's core at body temperature while venous blood in the feet is cool. As cold blood returns to the body core, heat moves byconductance from the warm arteries to the cool veins. Arterial blood reaching the feet is already cool and venous blood reaching the core is already warm.
Waterfowl have fleshy feet with little blood circulation so they are less sensitive to cold.
Constricting blood vessels reduces the amount of blood flow to the feet at low temperatures. All of this may seem of little importance as you bundle up in multiple layers to face another wintry day. But just imagine how the dull the landscape would be without these hardy souls. Winter songbirds, waterbirds and waterfowl are often the only wildlife one sees at this time of year. And Americans do love their birds.