What is this flock?

What is this flock?

Observation - What is this flock?. Description: Taken from a distance they swooped in very fast flight, the sun made their undersides look all white and they disappeared as fast as they had come. I thought they might be Dunlin or Knot but maybe too big for these birds? Help!

Taken from a distance they swooped in very fast flight, the sun made their undersides look all white and they disappeared as fast as they had come. I thought they might be Dunlin or Knot but maybe too big for these birds?
Help!