Two Posts from Badger Run Wildlife Rehab
Ever wonder why birds can land on freezing metal in winter and their feet don't get stuck to it?
Our fingers will get stuck on cold metal ice cube trays when pulling them out of the freezer. Our tongues freeze to cold metal ala A Christmas Story. That's because the moisture on our skin freezes in contact with the icy metal.
Birds' feet are covered with dry scales so there is no moisture to freeze to frigid metal. Birds have no sweat glands and essentially no secretory glands (not zero) so the skin does not secrete moisture through the skin on their feet.
The photo ~~below~~ of an Osprey's foot shows these scales in the extreme.
Frigid temps can be hard on wildlife. How do they keep those bare feet from freezing? Countercurrent heat exchange.
Basically, the arteries carrying warm blood down to the feet are very close to, if not intertwined with, the veins carrying cooled blood back up to the body and the heart. So, the warm blood in the artery essentially rewarms the blood coming back up the leg's vein so it does not cool the body's core temperature.
And birds aren't the only ones that use countercurrent heat exchange in their extremities to conserve body temperature. Other animals like arctic foxes and wolves use it, too. Deer species, as well. Also, beaver, muskrat, otters, and sea mammals.
not oc, but I think the question was why the feet can survive with 22c blood (from the graphic)
I went back to read some more things and watch some videos, and most stuff just seems to boil down to "it just isn't and issue for them", which doesn't feel very helpful. This article snip isn't much more technical, but seems to say, there just isn't much actual tissue or nerve in a bird foot to freeze and damage cells, reducing the issue of frostbite, and between the heat exchange, high rate of bloodflow, and vasoconstriction, there is negligible heat loss to the environment, so the temps in their tippy toes stay just warm enough to keep the overall temps of their digits where they may not be happy, but they're not actually being damaged.
AllAboutBirds
that's very interesting, thank you!
You're quite welcome! You guys asking questions helps me learn a lot too!