The Chelsea Standard
A Heritage Newspaper
Weekly Publication
Guest Column: Why bird's feet don't freeze
By Charlene Harris, Special Writer
PUBLISHED: February 21, 2008
Every morning I watch the birds at our feeders while drinking my coffee. Our feeders are placed near a stand of conifers to provide sheltered perches for those waiting their turn at the feeders.
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We have the tiny titmouse, chickadee, an occasional house wren, junco, four-to-five cardinal couples, several goldfinch, doves and the bully blue jays. The downy, hairy, and red-bellied woodpeckers prefer the suet cakes that hang two-feet from my kitchen window. However on occasion the larger red-bellied woodpecker will stake his claim on the feeder, where only the small birds seem unintimidated by his presence.
As the birds hop and walk around on the frozen ground, I wonder, "Why don't their feet freeze?" And what about the geese sitting on our frozen lake in the gusty winter winds? So I went to my computer and searched the web for an explanation and found several interesting items.
First bird's feet are mostly bone and tendons, so unlike mammals there's not much to freeze. Next I learned bird's blood circulates in a network of arteries and veins called the rete mirabil that works like a mini heat exchanger.
Warm blood from the body in the artery passes next to the vein that carries the cooled blood from the foot back to the body. During this passage they exchange heat and cold. Cold blood is warmed as it travels to the body and warm blood cooled as it travels through the leg to the foot. This countercurrent exchange minimizes heat loss from the body through the bird's legs and feet, and keeps their feet just above freezing.
And birds have no sweat glands in their skin, no sweat, no moisture to freeze. On hot days birds open their mouth and breath fast, almost panting. Heat and moisture are accumulated in sacs, transferred to the lungs and eliminated through the mouth.
Birds body temperatures ranges above 100 degrees Fahrenheit; their downy feathers insulate and trap body heat in a network of air pockets. So sweating would be a big problem, much like a wet down jacket.
Bird's legs are covered with scales and they have toenails, neither of which is living tissue, so less susceptible to frostbite or freezing.
We have a 10-year-old Chestnut-fronted Macaw, Jade. She's brilliant emerald green with red and turquoise markings, about 20-inches long, half of which is tail. I've often watch her preening and picking off the scales from her feet. Both the scales and toenails regrow, like our fingernails and hair. We trim her toenails.
My original idea, when I talked my husband into getting a bird capable of mimicking voices and sounds, was that she'd learn some of the wild bird songs. But instead of learning "bird talk," she calls to wild birds from the window ledge or her perch on the side porch, "Hello, Hi, step up," and touts her virtues to them, "Pretty girl, good girl." And on occasion when they fly away, with one foot raised high, I'll hear her say, "Goodbye."
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