[chris (OP)]

Anna Stejskalova has written to ask:

Why do birds stand on one leg when they rest and sleep?

What do you think?

[chris (OP)]

This needs @SquarishTriangle or @atrox

[jeffreyH]

Minimising heat loss. Legs without feathers lose heat. Tucking one up into feathers helps to keep the bird warm.

[atrox]

There have been two possible explanations for the resting on one leg.
One is, as jeffrey said, thermoregulation. This assumption is supported by several observations, like that herons, flamigos and suchlike stand more often on one leg when they are in water (good thermal conductor) as when they are on land. Also if it is really hot those bird will more often not retract their leg completely into the feathers. On the other hand they more often retract the legs up into the plumage if it is really cold.

Another explanation that has been widely discussed is whether the retracted leg helps to reduce muscle fatigue, similar to humans or other mammals (horses, elephants..) that usually put the bodyweight on one (hind-) leg when standing. However, this seems not very likely as retracting the leg rather seems to require the use of muscles that will get fatigued.

[SquarishTriangle]

I wonder if it's also related to some birds being able to sleep half of the brain at a time. Perhaps, if one leg is going to undergo sleep paralysis and cause you to potentially fall over as soon as you nod off, it is better to tuck the leg away in a balanced position, before you attempt to sleep.

Other thoughts: Possibly to temporarily allow increased circulation in the rested leg. We know that some birds that bear abnormally large amounts of body mass on one or both legs (eg. due to being overweight, frequently standing on unusually hard ground, or having an injury of the opposite leg) can be at increased risk of inflammatory disease of the feet (i.e. pododermatitis). A domestic chicken that loses one leg, for example, is highly likely to develop pododermatitis in the foot of the remaining leg due to its large body weight (and probably a severe change in the way it has to get around). Relieving one leg from bearing weight may allow blood to flow to the extremities of the foot with less effect of gravity and pressure, although it does imply that the other leg would have to bear more weight while this happens.

Lastly, maybe it's just a security mechanism, like an alarm, to wake the bird up to any disturbance that causes it to become unbalanced.

Lots of possible reasons, but I don't think we know the definitive answer yet.

[chris (OP)]

Right on cue, this paper has an answer:

http://rsbl.royalsocietypublishing.org/content/13/5/20160948