So-called “seet” calls, peeps produced by many small songbirds in response to a raptor on the wing, are well-known to ornithologists. Conventional wisdom held that the calls dissipated quickly and were produced only for other birds nearby. However, that’s not what Dr. Greene noticed: chatter sweeping across the hillside, then birds diving into bushes.

Studying the phenomenon, he documented a “distant early-warning system” among the birds in which the alarm calls were picked up by other birds and passed through the forest at more than 100 miles per hour. Dr. Greene likened it to a bucket brigade at a fire.

The information rippled ahead of a predator minutes before it flew overhead, giving prey time to hide. Moreover, while raptors can hear well at low frequencies, they are not very good at hearing at 6 to 10 kilohertz, the higher frequency at which seet calls are produced. “So it’s sort of a private channel,” he said.

Dr. Greene turned to chickadees, which are highly attuned to threats. When one sees a perched raptor nearby, it will issue its well-known “chick-a-dee” call, a loud, frequent and harsh sound known as a mobbing call because its goal is to attract other birds to harass the predator until it departs.

In 2005, Dr. Greene was an author of an article in the journal Science that demonstrated how black-capped chickadees embed information about the size of predators into these calls. When faced with a high-threat raptor perched nearby, the birds not only call more frequently, they also attach more dee’s to their call.

Raptors tend to be the biggest threat to birds nearest their own size because they can match the maneuverability of their prey. So a large goshawk might only merit a chick-a-dee-dee from a nimble chickadee, while that little pygmy owl will elicit a chick-a-dee followed by five or even 10 or 12 additional dee syllables, Dr. Greene said.