Birds are capable of recognising warning calls from other species, according to scientists.
The hornbills were found to monitor warning calls
University of St Andrews experts claim certain species are able to tell one warning call from another, ignoring those which do not concern them.
A study found that the wild hornbill and the Diana monkey warned each other of predators nearby in their shared West African habitat.
It is thought to be the first time any such relationship has been observed.
Biologists Hugo Rainey, Professor Peter Slater and primatologist Dr Klaus Zuberbühler spent 18 months studying the animals.
Diana monkeys are a brightly coloured, extremely observant species and are excellent look-outs for predators.
The scientists noticed the hornbills were often present in their vicinity when they studied the monkeys' alarm calls.
The duo share a predator - the crowned eagle - and it was found the birds only responded to the monkeys' specific warnings for the eagles.
"We presume the hornbills did not respond to the Diana monkey's warning call about the leopard because the leopard is not a threat to them," explained Mr Rainey.
The monkeys are on the look-out for eagles and leopards
"This demonstrates how capable birds are of obtaining relevant information about predators from the calls of other species."
The hornbills responded by increasing the number of calls they made when the eagles were present which are thought likely to act as a warning to fellow hornbills.
Some mammals can distinguish between and respond appropriately to the alarm calls of other mammal and bird species.
However, the ability of birds to distinguish between mammal alarm calls had not been previously investigated.
The paper will be published in the April edition of Proceedings of the Royal Society.