How far does each bird need to see for flocks to form? Randomly headed arrows line up into bands that travel together. Flocks split, merge and wrap around the edges.
Try this While it runs, drag vision down to 1, the same as minimum-separation. Every bird a flocker can see is then too close, so it only ever separates, and the flocks break up. Raise vision past 2 and they form again.
About this model Where it ends up
Craig Reynolds' boids (1987) showed that flocking needs no leader: each simulated bird steers by a few rules using only the birds near it. This is a port of Uri Wilensky's NetLogo Flocking model (1998), which applies the rules in a fixed priority.
Each tick, every flocker looks at the others within vision. If the nearest is closer than minimum-separation, it turns away from it by at most max-separate-turn degrees and does nothing else. Otherwise it turns toward the flockmates' average heading (at most max-align-turn degrees) and toward their centre (at most max-cohere-turn). Then it moves forward one patch. The world wraps on both axes.
