From September into October large flocks of Tree Swallows put on incredible displays just before sunset and ending some 10 minutes later. Congregating en masse, the birds resemble an enormous "science fiction cloud" of small alien life forms, as they twist and turn, hover and flutter, and generally seethe across the sky at an altitude that makes them hard to see without the aid of binoculars. They make up what looks like a rolling, thin cloud, composed of many tens of thousands of individual birds.
Counting, even estimating, their numbers is problematic: there could be 50- or 100-thousand, or half a million. The only thing for certain is there are a remarkable number of small birds, relatively far away, swirling in a very impressive mass. The birds wheel and swoop, and then at some moment that only the swallows understand they start swirling down in a tight spiral.
It resembles nothing so much as “the finger of god” that is used to describe where tornadic funnel clouds touchdown causing massive localized damage. The swallows descend in a vortex, and if you are close enough you can hear the sound of the air being ripped by their wings. It sounds like a train going by!
The birds plummet down in a dizzying funnel, and the cloud disappears in a long line that drops from the heights to the reeds on the edge of the pond. It is over in a matter of minutes! The mass of swallows has plunged from high altitude to roost cheek-to-cheek in the security of the reeds, out on an island in the middle of the pond.