Hornbills know which monkey calls to heed.

The yellow-casqued hornbill, an African forest bird, can tell the difference between the alarm call that monkeys make in response to an approaching leopard, which is not a threat to the birds, and the monkey alarm triggered by a crowned eagle, which is a threat to the hornbill, researchers have found.

This makes the hornbill the first bird known to distinguish between alarm calls given by another species, says Hugo J. Rainey of the University of St. Andrews in Scotland.

Some animals make different noises when alarmed by a menace approaching on the ground versus a menace from the air. Some primates have been shown to respond appropriately to another species' calls, peering at the sky after the heads-up alarm and dashing up a tree in response to "look out below." Other researchers have found that a downy woodpecker responds to alarm calls from chickadees, but Rainey and his colleagues wanted to see whether a bird could distinguish between different calls from another species.

The scientists identified a fine opportunity to do so in Ivory Coast. There, leopards and crowned eagles prey on Diana monkeys, which give different barklike calls for each threat.

Rainey monitored hornbills before and after playing recordings of monkey alarm calls. After the eagle-related call, the hornbills got noisier, making more calls than before the recording, and more than 70 percent of the birds approached the sound, a typical defensive measure that presumably indicates to the intruder that it has been seen. After the leopard-related alarms, however, the birds didn't make more noise or fly toward the sound.

The findings will appear in the Proceedings of the Royal Society of London B.--S.M.