A hundred birds, and none of them is in charge
Every bird here obeys the same three urges, and only ever looks at the few neighbours near it: don't crowd them, head roughly the way they're heading, drift towards them. Nothing tells the flock to be a flock — the shapes, the splitting, the turn that ripples through it all come out of those three lines. Drag to look around.
birds —
neighbours each —
average speed —
Turn "stay together" down and the flock dissolves into loners. Turn "don't crowd me" down instead and they collapse into a knot. Somewhere between the two is the setting where it looks alive — and that band is narrower than you'd expect.