When honey-hunters in Mozambique call out to birds in the hopes that their feathered companions will lead them to honey, the birds, in fact, recognize and respond to these specialized calls, a new study confirms. The results reveal how birds are able to attach a specific meaning to the human's call for cooperation, representing a rare case of mutualism between humans and a wild animal. Indicator indicator is a species of bird that's known to act as a honeyguide for humans, flitting from tree to tree to indicate where beehives are hidden above. Alone, the bird is unable to crack open a beehive to enjoy the beeswax within, but when humans harvest a hive for honey, they leave the wax behind - a delicious treat for I. indicator. In Mozambique, the Yao tribe uses a distinct call to attract I. indicator as a honeyguide, producing a loud trill followed by a grunt (see audio). Claire Spottiswoode and colleagues interviewed 20 Yao men who use the 'brrr-hm' call, all of whom say they learned it from their fathers and that it's the best way of attracting a honeyguide. To confirm the efficacy of this mutualistic relationship, as well as the call itself, the researchers trailed honey-hunters. They found that 75% of guiding events led to the successful discovery by humans of at least one bees' nest. Next, to test whether honeyguides associate 'brrr-hm' with a specific meaning (a higher payoff from cooperation), they made recordings of this call and two kinds of 'control' sounds. Then, an author and two local honey-hunters walked while playing back one of the three acoustic cues every 7 seconds over 15 minute intervals. Birds were much more likely respond to the 'brrr-hm' call made to attract them than they were to the other sounds, the researchers found. The traditional 'brrr-hm' sound increased the probability of being guided by a honeyguide from 33% to 66%, and the overall probability of being shown a bees' nest from 16% to 54% compared to the control sounds.