Small wading birds (shorebirds), mostly dunlin, have been pushed from their feeding grounds on tidal mud by the rising tide, and collect on the remaining shingle beach.
They take off and fly in restless patterns, waiting for signs that the tide is receding. As they fly they work as a team to make it difficult for birds of prey to isolate an individual target.
The white flashes from the underside of their wings which are visible from time to time in this short clip help to achieve this defensive effect of confusion.