Murmuration
A murmuration is the huge, swirling flock that starlings form in the hour before dusk, when thousands of birds, sometimes hundreds of thousands, wheel, fold and stretch across the sky as if they were a single living thing. There is no leader and no plan. The patterns ripple through the whole flock in an instant, shifting from teardrops to columns to figure-eights. The display is thought to protect the birds, since a falcon trying to pick out one bird is confused by the shifting mass.
The animation below simulates a murmuration of twenty thousand starlings in three dimensions, running entirely on your graphics card. Each bird follows three simple rules: match the direction and speed of its neighbours, stay close to them, and keep a little distance. It tracks only its seven nearest neighbours, which is how real starlings are thought to behave. Every so often a falcon sweeps through, and you can watch the flock react as the turn spreads from bird to bird. The camera drifts slowly around the flock, and the shapes it forms are not scripted.