This is certainly a heat-conservation mechanism. Birds such as gulls and shorebirds that spend a lot of time in the water have the arteries and veins going into the legs arranged in a countercurrent distribution system; this can take the form of a rete mirabile, or "marvelous net," in which arteries and veins are close together so that heat escaping from the blood in an artery is largely transferred into the blood returning to the body in the vein. With such a system, the blood in the body can be at a normal temperature of, say, 32 degrees C, and the blood in the foot is very cold, around 0 to 5 degrees C. (Gill's text, Ornithology, has a diagram of this on p. 136.) So if a bird stands on only one leg, with the other tucked up near the body, it's conserving a lot of heat. What surprises me about this whole matter, though, is that birds seem often to have the opposite problem of getting rid of excess heat. They have incredibly high metabolic rates, higher on average than those of mammals, so they commonly maintain temperatures up in the 40s (C), while we have body temperatures in the 30s. I'm sure some of this has to do with the problem of surface/volume ratios; for endothermic animals, both birds and mammals, the smaller the overall size, the higher the metabolic rate must be to maintain a given temperature because of the ratio of surface area--through which heat is lost--to volume, in which heat is produced, and the smaller the object, the larger its surface/volume ratio. But I don't know why birds should maintain much higher temperatures on average--maybe because they lose so much heat through structures like legs and wings, and maybe because they expend so much energy in flight; Gill suggests that the high temperature gives them more rapid reflexes and powers, so they can be very fast-moving animals. But he also notes that as the body temperature rises, one gets close to the point where typical proteins are denatured--that is, they change shape and are no longer functional--so a bird has to have mechanisms for dumping heat to keep itself from getting overheated. The blood flow into the legs is apparently regulated, so a bird can either allow it to happen or reduce the flow, depending on its heat and temperature needs. A complicated and fascinating business! Anyway, I think it's clear that standing on one leg is for heat regulation.

Burt Guttman
The Evergreen State College
Olympia, WA 98505 guttmanb at evergreen.edu
Home: 7334 Holmes Island Road S. E., Olympia, 98503

________________________________

From: tweeters-bounces at mailman1.u.washington.edu on behalf of Lee Rentz
Sent: Sat 10/28/2006 10:30 PM
To: tweeters
Subject: [Tweeters] Shorebirds resting on one foot



I am curious: after years of seeing all sorts of shorebirds resting
on one foot, I realized that I really don't know why they often rest
that way. The one explanation I ran across suggested the behavior is
for heat regulation-important for birds that spend a lot of time
wading in cold water.

While recently watching the Marbled Godwits in Tokeland for a long
time, I noticed that when a godwit is resting on one foot, head and
bill tucked into its back feathers-but with eyes open-it can rotate
its body around in an arc and check on its surroundings. I had
always assumed they were essentially asleep when in this position,
but clearly they can be very aware. In fact, I wonder if the
rotating ability might make resting safer on one foot than on two feet.

Any ideas or observations?

Lee Rentz
lee at leerentz.com
Shelton, WA_______________________________________________
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