In the previous post, we saw how birds can withstand bitter winter weather by puffing up their feathers, huddling, choosing favorable roost sites and violent shivering through the night. I want to continue with a couple of other remarkable adaptations.
Have you ever wondered why most birds lack feathers on their lower legs? A narrow, cylindrical leg has a high surface-to-volume ratio so quickly loses heat. It turns out those naked legs are designed to lose heat of a bird when it is flying.
When a bird flies, it increases its resting metabolic rate up to eight-fold. That high metabolism produces a huge amount of heat that would raise a bird’s temperature to lethal temperatures if that excess heat was not dumped. So, a flying bird pumps a lot of blood to its legs where the heat is lost to the surrounding air.
The downside is that a standing bird will lose heat to cold air that the bird cannot afford to lose. One trick that birds use is to stand on one leg with the other tucked into the body feathers. Other birds may sit, minimizing the exposure of naked legs.
Just as a flying bird increases blood flow to the legs, a sitting or standing bird decreases the blood flow to the legs. The result is that the temperature of the legs falls well below the central body temperature. The coldest part of the leg is the foot, the most distant part of the leg from the warm blood pumped into the leg.
A bird carefully controls the amount of blood pumped into the legs. The temperature of the underside of a Herring Gull foot can get as low as 32° Fahrenheit but does not freeze because just enough heat is provided. More blood has to be pumped into the legs as the air temperature falls.
Mechanical engineers use a clever idea to make heating systems of large buildings efficient. The furnace is situated in the center of the building. Air is brought into the furnace in an intake pipe that is half the length of the building. The exhaust pipe is erected adjacent to the intake pipe.
Here’s what happens in the winter. Cold air enters the intake pipe and is pumped into the furnace. The air in the exhaust pipe is at room temperature. By putting the pipes next to each together, heat is exchanged all along the pipes from the warm exhaust air to the cold air in the intake pipe. By the time air gets to the furnace, it is already warm and needs relatively little extra heating by the furnace. Meanwhile, the air exiting the building has given up most of its heat so little heat is lost to the outside. Genius!
Engineers call this particular arrangement a counter-current exchange system. Birds beat the engineers to this principle by tens of millions of years. The arteries and veins of a bird’s leg are arranged as a counter-current, helping the bird lose as little heat as possible as it pumps blood to the cells of the bird’s leg to provide nutrition and oxygen.
We know that the rate of heat loss between the air and a solid structure is proportional to the differences between the two temperatures. Some birds gain a marginal decrease in heat loss by allowing their body temperature to fall at night. Black-capped Chickadees have a normal temperature of 108° F but allow their temperatures to fall as low as 86° F at night. So the difference between air temperature and body temperature is a bit lower, resulting in less heat transfer. Like the regulation of leg temperature, this nocturnal hypothermia is tightly controlled. Death can result once the body temperature dips below 86°.
Given a bird’s arsenal of adaptations to tolerate the cold, the limiting factor for a bird is finding sufficient food. If food is adequate, a bird can scoff at the Maine winter climate.