An 18-month study shows that hornbills and Diana monkeys warn each other of approaching predators. This study represents the first investigation of its kind and reveals how birds interpret relevant predator alert calls made by mammals.

The Science News Online article, Hornbills know which monkey calls to heed, provides online recordings of both monkey and hornbill warning calls. Scientists have noted that the two warning calls monkeys make are quite similar (one call warns of leopards, the other warns of eagles). The hornbills must be very adept listeners, careful to distinguish between the two calls (and responding to the warning of eagles, their predators, not leopards, a predator not able to catch the tree-dwelling hornbill). The hornbill's skill at distinguishing between similar monkey calls further emphasizes how amazing it is that a bird species and mammal species are able to understand each other's cries of caution.