Just Because: Birds by the Long Beach Bike Path

I think these are swallows, and I think the nests are in progress rather than finished. They're under the extremely busy, extremely loud Long Beach Boulevard, and there are hundreds of them. They seem to like the scrubby area around the river and the bike path quite a lot:

Birds: Swallow Puzzle

These are darting around in the parking lot paying much less attention than usual to cars and people. There must be something attracting them, but I can't see it.