Why do birds stand on one leg?

Flamingo, great egret, oyster catcher, American robin.  All photos taken by me.

You've seen it.  A bird, perching on a single leg.  But why are they doing it?  It's not something we do, or any other bipedal organisms for that matter.  Ever see a kangaroo standing on one leg?  What about primates, pangolins, certain rodents (kangaroo rat), or even dinosaurs (many theropods were bipedal, not that we have evidence that they ever stood on one leg)?

There are two main reasons that birds stand on one leg.  The first has to do with maintaining body heat.  Legs have no feathering (mostly, there are a few species that do have feathery legs), and the bare skin will experience significant heat loss.  By tucking a leg up into the lower breast feathers, a bird can cut the heat loss from the legs by half.

The second reason has to do with resting.  It takes energy to stand.  The birds can rest one leg while the other does the work.  They will switch legs from time to time.  The burden is not too much for a single leg, as birds are very light with their hollow bones and air sacs within their bodies.

An important part of this is balance, and it is important to note that the center of gravity is pretty much right over the legs.  If it were too far forward, the bird would have to use the second leg to move forward to prevent tipping over.  When egrets, herons, and the like lean forward to hunt, using the second leg is necessary to prevent tipping.

Another interesting fact is that many wading birds can rest on a single leg with surprising efficiency.  They have a leg-locking mechanism that enables them to rest on a single leg.  Their knee, hip, and back work together to orient the body's center of gravity over the leg.  There is an interesting video you can watch on the subject here.

Thanks for reading.

Eric Svendsen     www.ericspix.com