Whatbird.com

What kind of bird is this:  http://www.youtube.com/watch?v=E0dZH-iKz48 ?  I found it on the beach in Pensacola Beach, Fl, and have seen them a few times.  They kind of flop up on the shore and just lay there like they are injured, but they aren't actually sick or injured at all.  Their legs are really far back too.

Forgive the long post but I wanted to be as detailed as possible.   Since colours are subjective it was about 3pm in the spring, fairly bright day but was behind an apartment building so seemed more overcast. the bird was seen on a grey concrete porch with black wrought iron balustrade. There were two of them but they seemed identical so ...