How it turns
The shapes over the reed bed come from thousands of small decisions, each made by one bird about the few birds next to it.
01
Studies that filmed flocks over Rome with high-speed cameras found that a starling's pull towards the others works on its nearest five to ten neighbours, and that it does not depend on how far away they are. A thin flock and a dense one hold together the same way.
The pull is stronger to the sides than ahead or behind. A starling's eyes face sideways, so those are the birds it sees best.
02
In 1986 Craig Reynolds wrote Boids, short for bird-oid objects, and published it at SIGGRAPH in 1987. Each boid follows three rules: steer away from crowding, steer towards the average heading, steer towards the average position. Ichiro Aoki had published a similar model of fish schools in 1982.
Nothing in the rules describes a flock. The flock appears anyway, and it can split and rejoin around an obstacle without being told to.
03
Flocking is generally believed to be protection against predators. In winter starlings gather in flocks of hundreds to many thousands, and birds from the north-east of the range migrate south and west to join the resident ones.
A falcon has to pick one bird out of a cloud that keeps changing shape around it.