Mystery Home

In my paddles around the eastern shore of Virginia I have encountered about 3 or 4 of these structures. They are all located in Va, on the bay side, in coastal marsh grass lands. I have not found any in Maryland, and none in the marshes on the ocean side of the Va eastern shore. They look like they are configured for some large bird. Anyone know what bird, or where I can get any information about them?

6/20/2010: Mystery solved - They are Peregrine Falcon nesting boxes!