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| Immature Red-tailed hawk standing on one foot |
Seeing the one-legged hawk on our recent Nature Ramble tweaked my
curiosity.
I knew that wading birds, ducks and shorebirds often stood on one
leg and I recalled a reason from the coursework in my distant past: heat
conservation. Birds that frequent bodies of water, especially those that stand
in it, lose a lot of body heat through their feet. Some have an arrangement of
blood vessels in their skinny legs that helps to reduce heat loss from the
body. The major artery in the leg that supplies blood to the foot runs next to
the vein that carries blood from the foot back to the body. This side-by-side arrangement allows heat to
flow from the artery to the vein, warming the venous blood as it returns to the
body. Of course the arterial blood gets cooled on its way to the feet. But by allowing the feet to cool, heat is retained in the bird's core,
reducing the amount of energy that is needed to maintain body temperature.
Birds,
like mammals, are warm-blooded, only more so. Their normal body temperature is
higher than that of mammals. A survey including birds of all sizes found that their
average resting body temperature was 102°F and their body temperature during
activity is even higher. The maximum recorded resting body temperature was 105.4°F.
A fever that high would be enough to make most humans delirious. So birds are even more warm-blooded than we are.
But back to
the feet. Despite the counter-current flow of venous and arterial blood in the
legs, birds still lose heat through their feet, especially when standing in
water below their body temperature. (And almost every body of water is below their body temperature.) So it would make sense to reduce that heat
loss by standing on one leg and tucking the other one into the fluffy,
insulating feathers to let it warm up. Perhaps our immature Red-tailed hawk is
doing some foot-warming itself, even though it's not standing in water. The tree branch it is standing on is probably cool to the touch from losing heat over night. The
Cornell Laboratory of Ornithology has a web cam on a Red-tailed hawk and has a similar opinion; click on this link and scroll
down to Question no. 54. Other
websites have noticed the same one-legged behavior in other kinds of hawks.
So maybe our young hawk friend is just getting cold feet.
Reference:
Prinzinger, R., Pressmar, A., Schleucher, E., 1991. Body temperature in
birds. Comparative Biochemistry and Physiology Part A: Physiology 99, 499–506.
doi:10.1016/0300-9629(91)90122-S
