Birds’ feet are so narrow and fragile, and they grab onto cold branches in winter. Why don’t they get frostbite?
Because of a network of blood vessels called* rete mirable* (miraclulous network).Oops. I left the “i” out of “mirabile.”
OK, their feet have been explained, at least to some degree. What about birds’ eyes? I’d think their eyes would freeze quickly at -20 fahrenheit, especially in flight when the wind chill factor is an important consideration.
Birds have a nictitating membrane, or third eyelid, which helps to protect the eyes. In any case, the eyes are at body temperature and hence will not freeze. The moisture on the surface will be close to body temperature, and will be maintained that way by the nictitating membrane.
As an aside and FYI, it’s quite “beautiful” that the same so-called heat-saving countercurrent mechanism that operates in birds’ feet is employed by our kidneys to hold on to water while excreting salt. Here is an overview with reference to the kidney. Here is one fairly detailed reference. And another detailed one in pdf format.