Published: Saturday, January 5, 2008
Birds have tricks humans don't to stay warm in snow and ice
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U.S. Fish and Wildlife Service
A fluffed-up bunting stands in snow. A bird's circulation system is adapted to protect its feet and legs from the cold.
If the winter weather is too cold for comfort, the remedy is simple: go inside. Humans can regulate their body temperatures, typically 98.6 degrees, by turning up heat sources or, if playing outdoors, adding layers.
Birds don't have those options to keep their temperature at an average 105 degrees. Down jackets are made for humans, not tweeters. When the wind-chill factor drives the temperatures into the teens and 20s, year-round or winter residents depend on biology for survival.
Winter birds can search out insects in bark crevasses or in grasses and bushes, unless freezing rain or too-deep snow takes away those options. Those that also eat seeds have another source of food, both in the wild and at our feeders.
But a food supply that generates energy and heat isn't the only defense. Tree cavities provide space for chickadees to huddle together or woodpeckers to hunker down out of the wind, heating the space with body warmth.
Feathers are another barrier against bitter cold. Small downy feathers next to the body create air pockets that insulate the body by trapping body heat. When we fluff up a down jacket or comforter, we create air pockets that trap our warmth.
Birds can also twitch, or shiver, their flight muscles, to help keep the blood circulating; stand on one leg and pull the other under feathers, or cover both legs by squatting. Marine birds also have a layer of fat under their skin to help conserve heat.
But why don't those skinny legs and feet get frostbite? Why doesn't all the heat escape through their legs? How can ducks calmly walk around on a frozen pond?
Some species' legs and feet don't seem to be affected by the cold, thanks to a heat-exchange system for cooling the blood before it reaches the foot and warming it as it returns to the body.
This two-way system is a slick adaptation that humans don't have. In birds, arteries carry warm blood toward the feet and veins return cooler blood; because the arteries and veins are interwoven so close together, some arterial heat warms the returning venous blood.
By the time the blood gets to the feet, it's almost as cold as the temperature outside, but it's reheated in the veins on the way back to the body. Since the blood is already cooler in the legs and feet, it has less heat to lose to the cold air.
This network of veins and arteries is called a rete mirabile, a countercurrent mechanism that keeps much of the heat in the bird rather than losing it to the cold, providing just enough heat to keep the legs and feet from freezing.
Anna's hummingbirds, which periodically stay year-round in the lowlands, have another way to stay alive in cold weather. They can lower their body temperatures to the point of torpor, a deep sleep-like state that slows down body functions to conserve energy and thus loss of heat.
It's not the same as hibernation, in which animals stay "asleep" for weeks or months, but for a few species of birds, torpor can be a life-saver.
Iron Goat Trail: About three weeks ago, a hiking guide author recommended in this column some winter hikes, including Iron Goat Trail. But as caller Eric, who owns a cabin in that area, so aptly pointed out, the trail runs by sheds that protect the tracks from avalanches.
"There's five feet of snow out there, so it's not a matter of if (there will be an avalanche) but when it will hit," he said.
Good point. Fortunately there are many other places to hike or snowshoe that do not cross such an obvious avalanche zone.
Columnist Sharon Wootton can be reached at 360-468-3964 or www.songandword.com.
Birds don't have those options to keep their temperature at an average 105 degrees. Down jackets are made for humans, not tweeters. When the wind-chill factor drives the temperatures into the teens and 20s, year-round or winter residents depend on biology for survival.
Winter birds can search out insects in bark crevasses or in grasses and bushes, unless freezing rain or too-deep snow takes away those options. Those that also eat seeds have another source of food, both in the wild and at our feeders.
But a food supply that generates energy and heat isn't the only defense. Tree cavities provide space for chickadees to huddle together or woodpeckers to hunker down out of the wind, heating the space with body warmth.
Feathers are another barrier against bitter cold. Small downy feathers next to the body create air pockets that insulate the body by trapping body heat. When we fluff up a down jacket or comforter, we create air pockets that trap our warmth.
Birds can also twitch, or shiver, their flight muscles, to help keep the blood circulating; stand on one leg and pull the other under feathers, or cover both legs by squatting. Marine birds also have a layer of fat under their skin to help conserve heat.
But why don't those skinny legs and feet get frostbite? Why doesn't all the heat escape through their legs? How can ducks calmly walk around on a frozen pond?
Some species' legs and feet don't seem to be affected by the cold, thanks to a heat-exchange system for cooling the blood before it reaches the foot and warming it as it returns to the body.
This two-way system is a slick adaptation that humans don't have. In birds, arteries carry warm blood toward the feet and veins return cooler blood; because the arteries and veins are interwoven so close together, some arterial heat warms the returning venous blood.
By the time the blood gets to the feet, it's almost as cold as the temperature outside, but it's reheated in the veins on the way back to the body. Since the blood is already cooler in the legs and feet, it has less heat to lose to the cold air.
This network of veins and arteries is called a rete mirabile, a countercurrent mechanism that keeps much of the heat in the bird rather than losing it to the cold, providing just enough heat to keep the legs and feet from freezing.
Anna's hummingbirds, which periodically stay year-round in the lowlands, have another way to stay alive in cold weather. They can lower their body temperatures to the point of torpor, a deep sleep-like state that slows down body functions to conserve energy and thus loss of heat.
It's not the same as hibernation, in which animals stay "asleep" for weeks or months, but for a few species of birds, torpor can be a life-saver.
Iron Goat Trail: About three weeks ago, a hiking guide author recommended in this column some winter hikes, including Iron Goat Trail. But as caller Eric, who owns a cabin in that area, so aptly pointed out, the trail runs by sheds that protect the tracks from avalanches.
"There's five feet of snow out there, so it's not a matter of if (there will be an avalanche) but when it will hit," he said.
Good point. Fortunately there are many other places to hike or snowshoe that do not cross such an obvious avalanche zone.
Columnist Sharon Wootton can be reached at 360-468-3964 or www.songandword.com.
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