Let's start with the obvious.ALL birds have air retaining filaments called...feathers! As well as assisting in flight, feathers are excellent insulators and in the terms of thermal dynamics, keep the birds warm, or cool, as the case may be. So what has that to do with birds standing on only the one foot? Obviously, as their feet do not have the feather cover that their body does, there is a greater heat loss through the exposed skin of the legs/feet. To compensate for this the bird conveniently tucks one leg up into its body feathers, alternating each leg as required. Of course this refers only to those birds that find themselves of such a species that are water/mud/other cold area bound. ie ducks geese, swans etc.. oh! and the flamingo. For those birds that spend their time in the trees? these share one particular trait with their ground roosting cousins, they all sleep with their unfeathered beaks tucked into their body feathers. So in conclusion, flamingo's stand on only one leg at times for much the same reason as we would plunge an ungloved hand into the warmth of our pockets on a cold day.

i think this might be to do with blood pressure. i wondered why it is more relaxing to sit with your feet up on something and have decided it is because it is easier to pump blood around if you are more flat. it must be worse if you have long legs like the bird in the picture, so maybe standing on one leg burns up[ less energy?thanks,nick

It has been proposed that the reason that flamingos stand on one leg is so ducks don#39;t swim into them as often! The most likely answer, though, has to do with energy conservation. In cold weather, birds can lose a lot of heat through their legs because the blood vessels there are close to the surface. To reduce this, many species have a counter-current system of intertwined blood vessels so that blood from the body warms the cooler blood returning from the feet. Keeping one leg tucked inside their feathers and close to the warm body is another strategy to reduce heat loss.I imagine the converse is true in hot climates - blood in the legs will heat up quickly, so keeping one leg close to the body will reduce this effect and help the birds to maintain a stable body temperature.Another factor in long-legged birds is that it may require significant work to pump blood back up the leg through narrow capillaries. Keeping the leg at a level closer to the heart may reduce this workload.It is also worth remembering that birds#39; legs are articulated differently to ours - what looks like the knee is in fact more like our ankle. Many birds have a mechanism to quot;lockquot; the leg straight, so for them it is much easier to stand for hours on end on just one leg - on numerous occasions I have seen birds take off, and even land, on one leg.Rob Robinson, Senior population biologist, British Trust for Ornithology, Thetford, Norfolk, UK

The illustration shows a captive bird in a park - it#39;s on cut grass not its natural desert habitat. Hiding one leg causes park visitors to hang around gossiping about the curators chopping off one leg to stop the birds clambering out of their pen, and summoning them to explain themselves. Thus the birds keep marauding foxes, rats etc one step further away all day.

Standing on one leg makes a bird more take-off ready: by shooting the other leg out it can gain bodily momentum faster; if attacked from below it can pull one leg in as it takes off with less loss of bodily momentum.