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I snuck along the creek-side, examining the snow for any telltale signs of wildlife. I spotted some depressions in the snow along the banks. The tracks created curlicues across the white, circling a stick here, meandering to a rock there. Looking closer, the snow revealed the three front toes and single back toe of a large bird, each track about as long as my index finger — a raven’s trail. I pictured the bird stepping through the snow and marveled that it was walking barefoot, while I relied on my insulated Xtratufs. Why didn’t it get frostbite?
Frostbite is what happens when tissue freezes. When a person is exposed to deep or prolonged cold, the body constricts blood vessels in extremities such as the hands and feet to keep more blood in the core of the body to warm the vital organs. Frostbite causes damage in two ways. First, the lack of blood flowing to provide warmth and oxygen allows ice crystals to form, killing cells. The other kind of damage happens when the frostbitten area warms up again. As blood flow returns, depending on the severity of freezing, small injuries in blood vessels may leak blood into surrounding areas, causing inflammation, blood clots, and more tissue damage.
The first line of defense against frostbite for birds is the covering on their feet. Instead of skin, they have hard scales on their legs and feet. These scales are made of a type of keratin unique to birds, but similar to reptile scales or your fingernails. Blood vessels close to the surface in skin lose heat easily. The scales on birds’ feet are thicker and therefore a little more insulating than skin, so it takes longer for birds to react to the temperature of the surfaces under their feet. The raven walking in the snow wouldn’t have felt the cold as quickly as my bare feet. There is more to the story, however.
Surveilling chickadees at a feeder outside my window, they seem to have no trouble clinging to the chain holding the feeder up, or perching anywhere, even when it’s snowing. Their delicate legs and feet look like tiny twigs, and for good reason. Birds have very little in the way of muscle in their feet and lower legs.
The backward bending part of a bird’s leg you see is actually its ankle. The knee and thigh are hidden up in its feathers. The muscles that move a bird’s toes are located high on the leg, kept warm by body feathers. The muscles attach to long tendons and ligaments that move the lower leg and toes — the scaly parts of the leg.
Muscles need warmth and a sufficient blood supply to work properly, but blood and muscle lose heat rapidly in the cold. Since tendons and ligaments don’t need as much warmth or blood supply as muscle to function, a chickadee’s toes can work just fine even hopping through snow. The need for less blood in exposed parts of the leg means losing less heat to cold temperatures. It also means less liquid that could freeze into frostbite. This trick of anatomy also keeps the chickadee warm overall, since it can move its feet without worrying about muscles losing too much heat.
In early spring, try staking out a lake or pond as the ice starts to melt. Returning ducks, geese, and gulls will often make use of the open water. They seem completely unconcerned about dangling their toes in near-freezing liquid. Sometimes they may even stand around on ice or snow.
These birds keep their toes from freezing, just barely, by a special setup of their veins and arteries. Blood flowing in arteries from the heart to the foot is warm. Blood returning from the foot to the body in the veins is chilled from the outside temperature.
The arteries and veins in a bird’s leg pass close together, and in the case of waterfowl, are interwoven in a small tangle at the top of the leg. The bird’s body can divert blood through this mesh of arteries and veins. The outgoing blood loses heat to the incoming cool blood in the nearby veins. This warms the incoming blood so it doesn’t chill the bird’s body. The outgoing blood has already given up much of its heat by the time it reaches the foot, so the bird doesn’t lose much body heat from standing on the ice. Because of this heat exchange method, the top of the scaly part of a duck’s leg may be close to 90 degrees F while its foot could nearly be a chilly 32 degrees F.
Another trick birds have is the ability to constrict the arteries to the foot. This lowers the amount of warm blood flowing through the feet, conserving body heat. Using a mixture of these methods allows birds to keep their feet functioning at just above freezing.
On a particularly cold morning, I detected a junco bopping around on the ground under a feeder. It was so fluffed up it was almost completely round and its toes nearly hidden. The fluffy down under their outer feathers is another way birds keep their feet warm.
Unlike other feathers, down feathers usually don’t have a central shaft, although the down of waterfowl may have a short one. Most feathers have little branches called barbs coming off the main shaft. Each barb has miniscule hooks that help it attach like Velcro to its neighbor, creating the familiar smooth shape of a feather. Down feathers lack the Velcro-style hooks, so they stay fluffy, trapping small pockets of air that act as insulation. When you put on a down coat and feel that immediate warmth, your body heat is warming the air trapped in the down. That layer provides a barrier between cold air and your body.
When birds fluff their feathers, they create more air pockets, increasing the insulation. If the air temperatures become very cold, you may see fluffed up songbirds sitting on their feet. For them, it’s like putting on a pair of down slippers. And after reading so much about cold feet, you may want to do the same.
• Beth Peluso is a freelance writer and illustrator and avid birder. She enjoys spying on wildlife across Alaska.