Batman Returns (1992) has swarms of bats and an army of penguins that nobody animated one by one. In 1987 Craig Reynolds, at Symbolics, wrote that "scripting the path of a large number of individual objects using traditional computer animation techniques would be tedious." A leader everyone follows can't split; a pull to the middle collapses into a ball. So "the birds choose their own course": each sees only its nearby flockmates and obeys three rules, "in order of decreasing precedence": avoid collisions with them, match their velocity, stay close to them. A flock splits around an obstacle and joins again with nobody in charge. Give every bird the whole flock and it stops being one: flocking "depends upon a limited, localized view of the world." In 1987, 80 of his birds took about 95 seconds a frame. Batman Returns used "modified versions of the original boids software", in Reynolds's own account.

The birds, the flocks and the tower are our own simulation (2D, our own weights); no frames from any film.

Ever coded a flock yourself? Tell us which rule broke it.

Sources: Craig W. Reynolds, "Flocks, Herds, and Schools: A Distributed Behavioral Model", SIGGRAPH 1987; Reynolds, "Boids: Background and Update" (red3d.com).
Sounds: Apple iMovie and iLife sound effects.