Double Crested Cormorant.

This cormorant was trying to cool off on a hot day with it's leg raised and eyes closed.I liked the vulture like look!!
According to Evans & Heiser (2004), birds tuck one foot close to their bellies to keep warm on a chilly day. After all, the legs of most birds are not covered with feathers and they have been shown to be important sites of heat exchange (Steen & Steen, 1965). Other ways of conserving heat include tucking the bill into the feathers of the shoulder, fluffing their feathers to increase the air space and roosting close together.

However, the fact that birds also stand on one leg under warm conditions means that thermoregulation is not the only function.

And while standing on one leg, these birds continue to rest, preen or sleep.http://www.besgroup.org/2010/02/26/why-do-birds-stand-on-one-leg/