Cold Feet

When Aldo the American Kestrel sits comfortably inside on cold winter days, part of me worries about Carson the Red-tailed Hawk still perched outside. But then I hear my mentor's voice in my head saying reassuringly, "she's fine - they're built to be here year-round."

One adaptation that allows Carson to tolerate cold weather is one that she shares with all birds: cold feet. Birds can let their foot temperature drop to near-freezing to conserve their core body temperature. This is achieved through an efficient system called countercurrent heat exchange. Arteries carrying warm, oxygen-rich blood travel down the leg closely intertwine with veins. The proximity of the descending arteries and ascending veins allows heat to transfer across the blood vessels. Cold blood from the toes is passively warmed by the arteries as it heads back up to the body. How efficient!

This system maintains the toes above freezing with a continued blood supply to avoid frostbite. Birds also avoid tissue damage because their feet are made mostly of tendon and bone and not so much delicate muscle or nerve tissue. 

Countercurrent heat exchange in bird legs allows for passive heat exchange between arteries and veins to maintain core body temperature. Diagram adapted from graphics by Cornell Lab of Ornithology.