"Brrr-hm!"

When a human makes that sound in Mozambique's Niassa National Reserve, a wild bird species instinctively knows what to do. The greater honeyguide responds by leading the human to a wild beehive, where both can feast on honey and wax. The bird does this without any training from people, or even from its own parents.

This unique relationship pre-dates any recorded history, and likely evolved over hundreds of thousands of years. It's a win-win, since the birds help humans find honey, and the humans (who can subdue a beehive more easily than the 1.7-ounce birds can) leave behind beeswax as payment for their avian informants.

While this ancient partnership is well-known to science, a new study, published July 22 in the journal Science, reveals incredible details about how deep the connection has become. Honeyguides "actively recruit appropriate human partners," the study's authors explain, using a special call to attract people's attention. Once that works, they fly from tree to tree to indicate the direction of a beehive.

Not only do honeyguides use calls to seek human partners, but humans also use specialized calls to summon the birds. Honeyguides attach specific meaning to "brrr-hm," the authors say, a rare case of communication and teamwork between humans and wild animals. We've trained lots of domesticated animals to work with us, but for wildlife to do so voluntarily — and instinctively — is pretty wild.

Here's an example of what the "brrr-hm" call sounds like:

"What's remarkable about the honeyguide-human relationship is that it involves free-living wild animals whose interactions with humans have probably evolved through natural selection, probably over the course of hundreds of thousands of years," says lead author Claire Spottiswoode, a zoologist at the University of Cambridge.

"[W]e've long known that people can increase their rate of finding bees' nests by collaborating with honeyguides, sometimes following them for over a kilometer," Spottiswoode explains in a statement. "Keith and Colleen Begg, who do wonderful conservation work in northern Mozambique, alerted me to the Yao people's traditional practice of using a distinctive call which they believe helps them to recruit honeyguides. This was instantly intriguing — could these calls really be a mode of communication between humans and a wild animal?"

Honey hunters use smoke to subdue bees while they work. They face no competition over food from honeyguides — humans take the honey, and the birds eat the beeswax. (Photo: Claire Spottiswoode)

To answer that question, Spottiswoode went to Niassa National Reserve, a vast wildlife refuge larger than Switzerland. With the help of honey hunters from the local Yao community, she tested whether the birds can distinguish "brrr-hm" — a sound passed down from generation to generation of Yao hunters — from other human vocalizations, and if they know to respond accordingly.

She made audio recordings of the call, along with two "control" sounds — arbitrary words spoken by the Yao hunters, and the calls of another bird species. When she played all three recordings in the wild, the difference was clear: Honeyguides proved much more likely to answer the "brrr-hm" call than either of the other sounds.

"The traditional 'brrr-hm' call increased the probability of being guided by a honeyguide from 33 percent to 66 percent, and the overall probability of being shown a bees' nest from 16 percent to 54 percent compared to the control sounds," Spottiswoode says. "In other words, the 'brrr-hm' call more than tripled the chances of a successful interaction, yielding honey for the humans and wax for the bird."

The researchers released this video, which includes footage from their experiments:

This is known as mutualism, and while lots of animals have evolved mutualistic relationships, it's very rare between humans and wildlife. People also recruit honeyguides in other parts of Africa, the study's authors note, using different sounds like the melodious whistle of Hadza honey hunters in Tanzania. But aside from that, the researchers say the only comparable example involves wild dolphins who chase schools of mullet into anglers' nets, catching more fish for themselves in the process.

"It would be fascinating to know whether dolphins respond to special calls made by fishermen," Spottiswoode says.

The researchers also say they'd like to study if honeyguides learned "language-like variation in human signals" across Africa, helping the birds identify good partners among the local human population. But however it began, we know the skill is now instinct, requiring no training from people. And since honeyguides reproduce like cuckoos — laying eggs in other species' nests, thus tricking them into raising honeyguide chicks — we know they don't learn it from their parents, either.

This human-honeyguide relationship isn't just fascinating; it's also threatened, fading away in many places along with other ancient cultural practices. By shedding new light on it, Spottiswoode hopes her research can also help preserve it.

"Sadly, the mutualism has already vanished from many parts of Africa," she says. "The world is a richer place for wildernesses like Niassa where this astonishing example of human-animal cooperation still thrives."