Why Birds Don’t Get Cold Feet
Bunny slippers in hand, I approach the little bird. “You must be cold,” I say, “Here. Put these on. They’ll keep your feet warm.”
“What is it? Don’t you want them?”
“Well, okay. I’ll leave them here anyway. Maybe you’ll want them later…”
Somewhat disappointed, I toddle back indoors.
Why birds need no fluffy bunny slippers
If you’ve ever chewed on a chicken foot, you’ll have noticed that it doesn’t have much meat. It’s pretty much all tendons and bones. So, unlike your human foot, which contains plenty of moist muscle tissue, a bird’s foot contains only very little fluid in its cells. This means that in a bird’s foot there simply isn’t much that could freeze. Sure, blood circulates through Tweetie's foot, but it’s unlikely to turn into ice. Given a bird’s frantic little heart, blood simply rushes by way too fast to freeze.
But without freezing, piercing ice crystals can’t form, which in turn means that no tissue damage and frostbite can occur. So it’s not that birds don’t get cold feet. They do. It’s just that their feet are unlikely to suffer much damage from the coldness.
Birds, however, being warm-blooded animals, still need to keep the rest of their body, and particularly their core, at a toasty temperature of somewhere between 34–44 degrees Celsius (93–111 degrees Fahrenheit). Otherwise, they’ll risk turning into dead feathery popsicles with chicken feet for a handle.
To prevent heat loss, birds get the equivalent of goosebumps: their feathers puff up and trap air within them. Their bodies then warm this air up in much the same way as your body warms up the air trapped in a sleeping bag when you’re out camping in the cold. Being thusly surrounded by a shell of warm air, birds can then simply crouch down and surround their cold feet with that warm plumage.
But birds also use another nifty trick to prevent heat loss through their feet, namely, a mechanism called countercurrent heat exchange. This mechanism is illustrated below:
Warm blood is pumped down a bird’s leg through an artery and returns back to the heart through a vein. That’s nothing unusual and it’s how it works in your legs as well. However, in your case, if you were to stand barefoot on snow, the warm blood would reach the soles of your feet and transfer much of its heat to the snow, cool down, and then flow back to your heart having lost most of its warmth. Over time, as you foolishly keep standing on that snow, you would lose more and more body heat through your soles and eventually risk experiencing hypothermia.
In birds, meanwhile, this is much less likely to happen because the arteries going down to the feet and the veins going back up are very close together, almost hugging each other. They are in fact so close that most of the warmth going down to the feet is transferred to the cold blood going back up. So by the time the blood reaches the sole of a bird’s foot, it has already transferred most of its heat to the blood going up and so not much heat is lost to the snow. Instead, most of the heat is preserved internally.
And if you’ve ever wondered why birds like to stand on one foot, here’s one answer: by alternating the foot on which they stand and by pulling up the other one into their warm puffed-up feathers, birds reduce how much of their body is in contact with the snow and thus preserve even more body heat.
Finally, if all that doesn’t help, I hope a bird will be smart enough to eventually stand on something other than the cold snow.
As much as I pity little birds when they’re out in the cold, there’s really no reason to worry about them. Evolution has provided them with down feathers that are so good in trapping heat that they’re still the material of choice for many outdoor enthusiasts. Moreover, with their tendony feet and the countercurrent heat exchange mechanism, not much heat dissipates through their feet.
So don’t worry. There really is no need to equip our feathery friends with warm fluffy bunny slippers. Their feet are cold, but they probably don’t mind.
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