Lara Davidson, of Waveley, asks :-
Recently I observed a group of red-billed gulls, many of whom appeared to be standing on one leg until they moved which showed they had two good legs. Is this normal bird behaviour and why do they do it?
Phil Battley, an ornithologist with the Ecology Group at Massey University, responded.
Many birds, particularly long-legged ones, do roost on one leg a lot of the time. The other is tucked up in the plumage of the underside of the bird. The main reason birds are thought to roost like this is to conserve heat. The legs of most birds lack feathers, so are poorly insulated, and have blood vessels just beneath the skin. Heat from the warm blood can be lost easily from the legs, so tucking one leg up into the plumage should reduce the heat loss to the environment. That is the same reason why birds tuck their bills into their shoulder feathers, as bills also can lose a lot of heat.
Having said that, birds can be found roosting on one leg even when it is quite hot and conserving heat would not seem to be important to a bird. Birds are very comfortable on one leg, and may hop along rather than walk, or even fly a short way and land without even having untucked the leg they were not standing on. This can be very frustrating if a bird is colour-banded and you have to see both legs to read the full combination! It may be that roosting on one leg is habit-forming, and birds continue to do it even when there seems to be no real need to do so.
In very hot conditions on the beaches of Northwest Australia where the air temperature can reach 40ºC and the even more, I have seen shorebirds roosting with one leg hanging down but with the foot not touching the ground. We presumed that this would allow the leg to help cool the body but would keep as little of the body on the hot sand. However, we also saw birds doing the same thing at night when temperatures were much more reasonable, so again, maybe the birds just got in the habit of standing like that.