Andy and I have been taking long walks at Hammonasset Beach State Park, which is a nice place to bird in the first place, but as we walked along some of the scrubby pines behind the dunes, we looked up to find hundreds—maybe as many as a thousand—Tree Swallows hawking for insects in the air above our heads. I can't rule out that there might have been other species of swallows in the air, but every bird I got a good look at, as well as every bird I can identify in this picture, is a Tree Swallow, lacking the distinct banding on the chest that could indicate a different species.
The picture only captures a fraction of the birds and an even smaller fraction of the feeling of standing out in the open air and watching a thousand little individuals choosing a path in that huge three-dimensional dance.
I've seen maybe 20-30 at a time back in NM, but never that many. That truly must have been an incredible experience.