Waterfowl's feet during winter, why to worry less about the cold.

Heat exchange in waterfowls feet/legs.

Waterfowl have a counter-current heat exchange system between the arteries and veins in their legs and feet. Warm arterial blood flowing to the feet passes close to cold venous blood returning from the feet. The arterial blood warms up the venous blood, dropping in temperature as it does so. This means that the blood that flows through the feet is relatively cool. This keeps the feet supplied with just enough blood to provide tissues with food and oxygen, and just warm enough to avoid freezing. By limiting the temperature difference between the feet and the ice, heat loss is greatly reduced for the bird. Scientists who measured it calculated that Mallards lost only about 5% of their body heat through their feet at 0 C (32 F).

In the photo below, without counter-current heat exchange, warm blood makes it all the way to the foot. This keeps the feet considerably warmer than the ice the duck is standing on. Remember that heat flow is roughly proportional to the temperature difference. With a large temperature difference, there is a large flow of heat from the foot to the ice. This would also cause the ice to melt more and increase the risk of frostbite because of the moisture created.

(photo credit- unknow)

In the photo below, you’ll see that multiple branches of the artery are in close contact with branches of the vein. This intertwining of arteries and veins is called retia. Because heat flows from the arterial blood to the venous blood, the arterial blood becomes colder and the venous blood becomes warmer. In short, less warm blood gets to the foot, keeping the foot cooler and reducing the temperature difference between the foot and the ice. This in turn reduces the flow of heat from the duck to the ice.

(Photo credit- unknown) Birds feet are relatively free of soft tissue and the muscles that operate the foot are actually located higher in the leg and connected to the bones of the feet with long tendons. Due to the lack of soft tissue in the lower legs and feet there is less need for warm blood in these areas.

There is evidence that some birds have valves in their leg arteries that control the blood flow and can even pulse the blood to the foot. Every once in a while, warm blood flows to the foot to make sure it doesn’t suffer from freezing. When you see your waterfowl standing on one foot in the cold, this serves to protect tissues and reduce heat loss even further, because the one leg that is tucked up under insulating feathers gets a chance to really warm up and recover from any potential cold damage. With only one foot on the ice, the surface area in contact with a cold surface is cut in half!

Waterfowl are well adapted to withstand the cold temperatures. And while they may seem cold they are actually well suited for cold weather. When we humans get cold we remove ourselves, going to a warmer area. For our birds this would be their coop, if they are laying out in the open on snow or ice they are not stupid they do know they could get to the warm coop just like us humans would. We always lock our birds up at night, to protect them from the coldest nights but also predators. Many people leave their birds out in cold climates overnight, but I always suggest locking them up at night or have good predator control in place. The saying "A sitting duck" came from somewhere.