I'm guessing they are really young ones, and maybe a Texas variant -- the black collar is very pronounced. Not very big yet. They acted differently a few minutes ago, so I'm wondering if there's a nest of fledged ones hanging out together. They chase a local bluejay occasionally.

First pics I find are by a photographer I know here in the area, CJ Mills' BIL!

http://www.greglasley.net/barnswallow.html