Two of the busiest shorebirds are the Sanderling and the Ruddy turnstone.
The Sanderlings seem to be almost continuous motion. There is usually a small group of them and they dart back and forth. They will forage in an area and then for no apparent reason they will fly off down the beach. There is actually a purpose to it. They are seeking stranded mollusks or crustaceans which have been left behind by receding waves.
The Ruddy turnstone also generally in small flocks. Their diet is more diverse, but they quite often also seek food left behind by receding waves.
Both photos were taken at Goose Island State Park.