Brian Hayes in American Scientist:
A thousand starlings rose in unison from trees along a riverbank. The ascending cloud of birds took the form of a teardrop, then transformed itself into a butterfly, then a twisting vortex narrowing to a sinuous, quivering rope of birds stretched across the twilight sky. The flock had all the synchronized precision of a marching band, but none of the rigid, rank-and-file geometry. Instead the movements were smooth, fluid, organic, as if the flock were a single organism rather than a collection of individuals. The show went on for 10 minutes, then the birds swooped low over my head with a breathy rush of wing beats and returned to the same row of trees—only to rise again moments later for another performance.
The graceful aerial displays of starlings and other flocking birds have long inspired admiration and wonder. Lately they have also inspired serious work in mathematics, computer science, physics and biology. A theoretical framework for explaining the behavior of tightly clustered flocks emerged in the 1980s. The key idea, which came from computer simulations, is that purely local interactions between nearby birds are enough to hold the group together. Similar mechanisms are thought to operate in schools of fish, herds of grazing animals, swarms of insects and even crowds of people.