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Tropical trickery: birds sound false alarm.

Tropical trickery: Birds sound false alarm

When a hawk soars over a flock of feeding birds, a cry of alarm triggers a dash for cover. In a flock of birds made up of various species, as in the Amazonian rain forests, birds of just one of the species almost always sound this warning. These "sentinel' birds, which are especially skilled at spotting flying objects, sit attentively while the other birds rummage through leaves or study nearby bark and foliage in search of insect prey. But often the sentinel sounds the alarm falsely, just to snatch for itself an insect flushed from hiding by another bird.

While there are many examples of deception between a predator and its prey, there are few examples of trickery between animals of the same species or those living cooperatively. The sentinel birds' use of an alarm call to distract other birds and so increase the sentinel's chance of capturing prey is reported in the Jan. 9 NATURE. "[It] suggests that deception among animals may be more widespread than is generally assumed,' says Charles A. Munn of Wildlife Conservation International, located at the New York Zoological Society in Bronx, N.Y.

The deceptive behavior is part of a specialization of labor and an apparently unwilling sharing of resources within the bird flock. The rain forest that Munn studied in southeast Peru has two layers of bird flock territories. Different bird species are found in the canopy and the understory flocks, but each of the two levels has a sentinel species. And both the sentinel species issue false-alarm warning cries, Munn reports.

Each sentinel species relies on the insect-hunting abilities of other birds for most of its food. The sentinels position themselves below a group of foraging birds and wait for an insect to flee the predators. Then, both the foraging bird and the sentinel fly after the insect, and the fast-flying, highly maneuverable sentinel often wins it. But just to make sure, the sentinel may give its alarm cry as the birds dart through the air. The other bird hesitates, and the sentinel grabs the prize. Munn observed that the whitewinged shrike-tanager, the sentinel for the canopy flocks, sounded an alarm four times as often when it and another bird were chasing an insect as when it was chasing an insect alone.

About half the categorizable alarms are false, Munn reports. He recorded the alarm cries of the shrike-tanager and of the bluish-slate antshrike, the sentinel species of the understory flocks. True and false alarm cries were similar in a spectrographic analysis, although the false alarm cries tended to be shorter. When Munn played back alarms to a flock, the birds reacted to both true and false alarms as true alarms.

Why don't the foraging birds recognize that the sentinel is "crying wolf'? They do, Munn says, but they usually still lose the contested insect. When a bird hears an alarm while darting after an insect, it does not immediately go for cover as it would if it heard an alarm while foraging. Instead, it glances at the sentinel--which dives for cover after a true alarm cry, but which soars out toward the insect after a false alarm. By taking its eyes off the insect momentarily, however, the forager usually loses the prey.

In the long run, that may be a small price to pay for sentinel protection. Each bird seems to lose only a small fraction of its food to the sentinel, Munn says, wheras the sentinel is extremely effective in protecting the flock from hawks. "The potential penalty . . . for ignoring even one alarm call might be death,' Munn says, "so it is not surprising that flock birds seem to take all alarm calls seriously.'
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Copyright 1986, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Author:Miller, Julie Ann
Publication:Science News
Date:Jan 18, 1986
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