For the Birds Radio Program: Frostbite
In the dead of winter, when mercury in Northland thermometers is lucky to see the sunny side of zero, people often sit inside their toasty warm kitchens drinking hot coffee or cocoa and looking out the window, passing the time by wondering why birds never seem to get frostbite. After all, you never see a bird in a pair of Sorels or the latest parka from LL Bean. Of course, birds don’t have to worry about frostbitten fingers or lips, but why don’t they get frostbitten toes?
Frostbite is the result of frozen tissue. If a piece of celery is placed in a freezer for a short time, the water within the cells will expand, bursting the cell walls. Then as it thaws, the celery will get soft and limp. If living animal cells are frozen under natural conditions, they, too, die. Because blood circulates in a warm-blooded animal in the coldest of conditions, carrying warmth to the extremities from the heart, the animal is vulnerable only where tissue loses its heat quicker than the blood can replace it. Most mammals would succumb to hypothermia before frostbite, though a lot of abandoned cats lose their tails to frostbite and we probably lose a lot of squirrels with mange and other skin conditions during bad weather. Humans are more vulnerable than most other mammals because we don’t have a fur coat covering every square inch of skin the way, say, a moose or a mouse does. Most of a bird is also covered with insulation, and ounce for ounce, feathers make an even better insulation than fur. But Northland birds have large patches of skin where no feathers grow, called apteria. These featherless tracts of skin, which stay hidden under the outer contour feathers, probably prevent birds from getting overheated when flying and allow them to cool off in summer since they can’t sweat. In winter, the apteria aren’t too vulnerable, because they remain hidden.
The parts of birds that seem most susceptible to frostbite are the beak, legs, and feet. Fortunately, the beak is safe–it’s made of similar tissue to our fingernails, and has such a small blood supply that there is simply no danger of frostbite. But the legs and feet of some birds are vulnerable, especially pigeons and Mourning Doves. Their fleshy, muscular toes have enough of a blood supply that there exists a very real danger that the blood will freeze in their capillaries, bursting them. The way these birds protect themselves is actually in their stomachs. They have a crop–a big pouch separate from the actual stomach–where they can store large quantities of food. At first light, a Mourning Dove wakes up and hurries to its favorite eatery, where it pigs out until its crop is stuffed. Then it spends the day perched in a sheltered spot, preferably facing the sun, keeping its feet tucked against its warm tummy while it digests. By late afternoon, the crop is emptied and the dove returns to its feeding area for a bedtime snack. As long as its food supply is safe, a Mourning Dove can stay safe all winter, but this is one bird that definitely depends on the feeders it visits, and can actually die if people suddenly stop filling their feeders.
Songbirds like chickadees and Blue Jays have a special shunt to their toes that sends circulating blood back up the legs without going through the toes when the bird is cold. The tiny amount of blood in the toes, along with the hard, fingernail-like tissue of the toes, keeps them safe from frostbite. Songbirds also have very few nerve cells in their feet, so they don’t feel particularly cold or uncomfortable no matter what. One Gray Jay actually perched on a hot camp stove for several minutes without apparent discomfort–something a barefoot person could hardly do.
Mallards have very fleshy-looking feet, but like songbirds, they can reduce the amount of blood circulating to their feet in cold weather, and so very seldom get frostbite. No, except for doves and pigeons, the bodies of birds are a lot more adapted to Northland winters than ours are.