I saw a flock of robins like that once in Colorado. It was the strangest thing. The ground of our local park was literally covered with them. There probably wasn't a square foot of the grass without a robin on it. Haven't seen anything like it since. I did have quite a few in the yard at once a couple of weeks back, but nothing like that.
It was interesting watching them fly. There would be hundreds of them flying in the air. I would stand at the window at 6:10 pm and watch them come back. Our daughter was here visiting March 5 and she watched them. She had never seen anything like it.