The chatter of the forest is full of hidden meanings. Animals send messages back and forth all the time. Usually, the messages are unique to a species. A robin talks to other robins. Monkeys talk to other monkeys.

Now, it seems, a type of bird in Africa can understand monkey language, too. The yellow-casqued hornbill is the first example of a bird able to decipher the calls of another species well enough to recognize two different kinds of warnings.

Previously, researchers had seen primates respond to calls of other species of primates, and certain birds respond to calls of other species of birds.

Scientists from Scotland wanted to see whether a bird could distinguish between different calls from another species. They chose to do their study in Ivory Coast, where Diana monkeys make one alarm call when they see a horned eagle and another one in response to an approaching leopard. Eagles eat hornbills. Leopards hardly ever bother the birds.

After monkeys made their eagle calls, the researchers found, hornbills got much noisier. More than 70 percent of them also moved closer to the sound, probably to show the intruder that it had been seen. When monkeys sounded the leopard alarm, on the other hand, the birds continued their normal activities.—E. Sohn