This wonderful footage made the rounds here and elsewhere some time ago.
But thanks for running it by again.
In fact, this behavior has caught the eye of many people in recent years
due to its spectacular nature.
There was even a short clip of it featured in Terence Malick's* Tree of Life
* movie last year.
However, it really isn't the big mystery some people would have you
believe.
Whenever you see this type of behavior, always look for a falcon of some
sort, often a peregrine, attacking the cloud.
You may see some of the greatest flying you have ever witnessed.
Most birders have seen the same principle when watching shorebirds being
hunted over the sands by Merlins or peregrines. We called these forms
"evasion clouds".
You can see similar stuff when the corn is being harvested locally in fall
and the "field birds" (Red-winged Blackbirds, Brewer's Blackbirds, European
Starlings and Brown-headed Cowbirds) are foraging among the corn stalks.
Merlins hunt them like wolves after Caribou.
Find yourself a good starling roost with large numbers anywhere and you'll
see the same type of behavior (one of many things Starlings are good for in
my opinion).
There may not be a falcon on any given night but that is what the behavior
is for, escaping capture.
Professor Steve Herman first pointed this stuff out to me in the 1970's and
I have always been grateful to him for providing me with so many hours of
observing this stuff.
To me, it is some of the most fascinating behavior in the bird world.

Bud Anderson
Falcon Research Group
Box 248
Bow, WA 98232
(360) 757-1911
falconresearch at gmail.com
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