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COMPUTING SCIENCE

Flights of Fancy

How birds (and bird-watchers) compute the behavior of a flock on the wing

Brian Hayes

Avian Democracy

For early students of bird flocks, the most pressing questions had to do with decision making and communication. When hundreds of birds all take to the air at the same instant, how do they synchronize their liftoff? When the flock suddenly veers left or right, who decides which way to turn?

Human experience suggests one solution: Appoint a leader—a conductor, a choreographer, a dictator. Leadership might be the social role of a specific individual. (Tennyson wrote of “the many-winter’d crow” that leads its flock home.) Or leadership might shift from moment to moment, as a flock on the wing follows whichever bird is currently at the front of the formation.

The trouble is, biologists have found no clear evidence of either kind of leadership in flocks, and the whole idea has some deep conceptual difficulties. How do cues from the leader reach the rest of the birds? In large and dense groups, both sight and sound would be unreliable. And there’s another issue: A flock attacked by a falcon responds instantly with evasive maneuvers; it’s implausible that the threatened birds await instructions from a leader.

A more democratic vision of flock governance was proposed early in the 20th century by Edmund Selous, an intrepid English bird-watcher who kept careful notes on flocking behavior for more than 30 years. Selous rejected the notion of designated leaders as “well-nigh unthinkable—it is too ridiculous”; his alternative, however, prompts the same kind of dismissive response. Selous found the birds’ synchronized movements so uncanny that he could explain them only as a product of “thought-transference” or “simultaneous collective thinking.” In other words, the birds are telepathic.

Selous was led to this extraordinary hypothesis from his assumption that flocks make decisions by some form of global consensus. The decision-making protocol is almost as hard to fathom as the spooky communication medium. Does each bird transmit its intention or preference to every other bird? From an algorithmic point of view, this process would be highly demanding: The N members of a flock would have to exchange almost N2 messages. Thus the agility of the flock would depend on N, as larger flocks would necessarily take longer to make up their collective mind.








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