Here are a few photos of a flock of Dunlin at Ankeny NWR. I’m guessing the flock consisted of around 500 individuals. They circled in tight formation over the water for at least a half hour. When the flock turned, their wings made a roaring sound. Great fun. I don’t know why they did this for so long. There was no predator visible, and several birds were resting on the shore while the flock was carrying on. Perhaps getting in shape for migration?
Ethical Birder 
Wow, 500 seems like a serious underestimate. I started counting at one corner and I think you’ve easily got at least double that number there!