It's the time of year when Tree Swallows head for the coasts to assemble for their migrations. I have seen them flocking by the hundreds on Cape Cod in August (even saw an albino among them a couple of years ago), and by the thousands in places like Jamaica Bay NWP.
These fine birds bother me in the spring, when they successfully compete for the 25 Bluebird nest boxes I have gone to much trouble to clean etc.
Bugs and berries are their food, and they like to breed near water. More about them at the new, and less informative, CLO.
Photo is a male. The females are brown.