Murmur, a fantastic series of photographs by Richard Barnes.
I once captured a similar flock of birds flying above a hotel near my house with my video camera. They swarmed around and formed a perfect circle, and then broke off into two separate ghost-like shapes revolving around each other. Eventually they all landed in one really small tree. It was amazing. I found that tape earlier this year, and half of what I just described had been taped over.