In the early '80s there was a crow roost at Ft. Cobb, Oklahoma, that attracted literally millions of crows. They roosted on a peninsula that juts out into Ft. Cobb Reservoir, and there were so many of the dang things that they literally broke the branches off all the oak trees on that peninsula, as well as on neighboring farms all around there. A bunch of my artillery school buddies and I found a farmer who was happy to have us come and hunt them because they were destroying his woodlots and fields, and we had a great time shooting them on the wing in the evenings as they returned to the roost. On a good evening each of us would shoot around 50 birds.
As far as I know nobody ever figured out why the crows gathered there, and I don't know whether or not they still They do. I've seen piles of dead crows 10 ft high there.