I would guess there are probably between 5,000 and 10,000 blackbirds in the marsh. One of our favorite activities on summer evenings is to sit up on our deck in the Sparkill Gap (the first break in the basalt dike of the Palisades north of Manhattan on the west side of the Hudson River) and watch the birds come back to the marsh; never in the huge flocks that they leave it in, but always in tight clusters of 10 or 20, sometimes 30 birds. On rare occasions a larger flock comes back; but never the masses that leave together in the morning.
One of the phenomena that isn't recorded in the video above is a strange and striking moment that takes place just before the flock takes off in the morning. When one walks along the marsh in the darkness, say, about 5:00 AM, the birds are dead silent; I can verify this, because I've done it many times. But as soon as the first light starts to show in the sky, some few birds begin clattering — at first softly, but then in increasing numbers. As the light intensifies, the clattering gets louder and louder, until — after perhaps 20 minutes or more of anticipation — it reaches a crescendo. At this point, it often sounds like a huge, rushing waterfall in the immediate vicinity. If you come upon it and don't know what it is, you will certainly think it is the sound of a large river rushing over stones.
At this point, an extraordinary event takes place. All of the birds instantly stop making all the noise, at the same time. There is no tapering off; it's as though someone turned off a light switch, and the power went out.
Moments after that, they take off.
Listening to this phenomenon, I've wondered exactly what it is that triggers the behavior. Whatever it is, all the birds instantly sense it at exactly the same time, as though they were psychic; but of course, birds make relatively imperfect mediums. There must be a natural cue.
I think what it is is the total volume or decibel level. There's a point, a threshold, that's reached; and when this threshold is reached, it triggers the flight behavior.
I've never heard of anyone studying this before, although it's possible someone has. In any event, it's one of those miracles one has to be there to fully appreciate.
This morning, my wife and I were there just to take off, once again. We watched impressive masses of birds wheel and dive over the marsh, circling while more and more birds peeled up out of the phragmites into the dervish black cloud above it. They behave as though they were a single organism; and this kind of behavior, where large groups of organisms take collective critical behavioral cues from a threshold of chemical signaling, is well known in the microbial world. The chemical signaling is one thing; but this audio signaling is another story, a signaling of vibrations.
One wonders whether there are bacteria who do something like this as well.