Is it at all possible that the gulls have chosen the parking lot to “loaf” in because it is simply warmer than any surrounding “loaf” area? (this is D.C. in the winter, right?) Presumably the parking lot is asphalt which will retain more heat (when warmed by the sun) than a grassy area or body of water. As to the question of why not closer to a food source…these birds aren’t stupid (as birds go), they’ve been conditioned to understand that people congregate there…and where’s there’s fat, slovenly, carnivorous, Americans, there’s food…regardless if there’s a McD’s anywhere in sight. (please no flames…I happen to be one of those fat, slovenly, carnivorous, Americans)
Well, that could have something to do with choice of a particular lot, but they do exactly the same thing on pale concrete parking lots in winter, and also do it in the summer.