In addition to crazy cold temperatures, the recent Polar Vortex that raged down from the Arctic Circle also brought with it a lot of well-worn idioms, such as “chilled to the bone” and “frozen stiff.” Birds were, of course, neither of those. With a diet of good seed from your feeders and the amazing insulating power of feathers, birds will maintain a core temperate of 100 degrees F even on frigid sub-zero nights. But that begs the question: what about their legs? Devoid of feathers and endlessly exposed to snow, cold and wind, surely they’re susceptible to frostbite, right?

Actually, no.

And the reason is a marvel of natural engineering. To begin with, birds’ legs are mainly made of hollow bone, skin and connective tissue such as tendons. They have very little muscle and – most importantly – cells containing fluid that could freeze. But as spartan as they are, birds’ legs still require blood flow to provide nutrients, oxygen and heat.

So here’s the cool part (pun totally intended.) Birds are biological masters of something called “counter-current heat exchange.” Here’s what that means. The vessels carrying blood to and from the heart are extremely close together in birds’ legs. So as warm arterial blood travels to a bird’s toes, it’s cooled by colder blood returning to the heart. Why is this important? Because now the blood pushing into a bird’s legs is warm enough to keep these important extremities from freezing, but cool enough that the bird doesn’t bleed off too much body heat into the air. Conversely, the cold blood traveling back to the heart from the legs is heated significantly by the newly-arriving warm arterial blood, so it doesn’t cause the bird’s core temperature to drop when it arrives deep inside the body.

There you have it. That’s why birds don’t need tiny wool socks, boots…or snow days. We should all be so lucky, right?