flocks

I've been noticing the autumn flocking of birds (though not as spectacular as this, which looks like one of the Danish gatherings of starlings, called sort sol, or "black sun"). The movement is fascinating.

Much of it seems to be the simple behavior of each bird maintaining a certain proximity to those around it. But there's also a freedom or randomness of individual motion that sets off the unique patterns. I wonder why they do it. Flocking has clear benefits, but why the occasional mass displays like this?