A wild African bird that will famously lead people to trees filled with honeycomb seems to somehow learn the distinct whistles and calls of the human foragers who live near them.
Scientists have long puzzled over this unusual cooperative relationship between humans and a wild animal. This bird species, the Greater Honeyguide, is not domesticated at all, and no one trains them.
Yet in Tanzania, Hadza foragers can use a special whistle to attract this bird, which will then flutter down and start chattering away to lead them to honey.
In Mozambique, meanwhile, honey hunters with the Yao community will attract these birds with a trilling sound followed by a low grunt, which sounds like brrr-humph.
The birds in each place are more responsive to the sounds made by the local culture, according to a new study in the journal Science.
It found that birds were far more likely to appear and lead a person to honey when they heard recordings of the kind of call made by their usual human partners, compared to the sounds made by honey hunters from a different country.
"This is a very strong result which supports the idea that there's a learning process involved," says Brian Wood, an anthropologist at UCLA who did this new work in collaboration with Claire Spottiswoode of the University of Cambridge and the University of Cape Town.
Human honey foragers learn how to communicate with these birds from their elders, according to the researchers, and those foragers say they stick to their communities' traditional calls because it's the way to get the most honey.
But how do the birds learn? It's still unclear.
What is clear is that this human-animal communication seems to benefit both parties, and it may go back many thousands of years. It's a rare example of humans and wild animals collaborating, with another example being dolphins and people fishing together in Brazil.
In Africa, the honeyguide birds have far more information about what the bees are doing than humans ever could. The birds act as the eyes in the sky, keeping track of trees that have honey hives hidden within.
Then, when a forager calls to them, a honeyguide will fly down and, twittering loudly, lead the way.
"The bird is very conspicuous. You know, it'll fly to the Hadza with its chattering sound," says Wood, adding that the bird basically seems to be saying, "Hey, I'm here and I know where there's some honey, so follow me."
Once the bird arrives at a tree with honey inside, it will perch near the beehive and go silent. "That's the signal to the Hadza to really start looking," says Wood. Pretty soon, the foragers will locate the hive and hack open the tree trunk.
When the honeycomb is out, the birds get some of the discarded beeswax, which they love to eat.
And the human foragers get the honey — which is an enormously important food for the Hadza. Wood has calculated that the Hadza get about 10% of the calories in their annual diet with the help of honeyguides.
"And so we have this nice partnership where birds are sharing their knowledge of the location of these bee trees, and honey hunters are sharing their exceptional skill and bravery and talents to be able to access the honey inside," says Wood. "The bird really does provide important assistance to them."
He says a report on the human-honeyguide relationship that was published decades ago described how the Boran people of Kenya used hollowed out shells to make shrill whistles that attracted the birds.
But he and Spottiswoode independently realized that the communities they worked with in Tanzania and Mozambique used different kinds of signals.
"Both of us thought it would be great to collaborate on a project where we investigate whether honey guides in our respective field sites had learned to identify and differentially respond to those culturally varying signals," he says. "It was incredibly fun to do this research."
He and Spottiswoode used recording equipment to collect examples of the signals that honey hunters use.
The next year, they returned to each of their research sites with a standard set of recordings: the whistles from Tanzania, the trills from Mozambique and also recordings of people just saying their names, as a control sound that would simply alert birds to the presence of a person.
At each location, they did an experiment that involved a honey hunter just walking through the landscape. A researcher followed close behind, holding a speaker that was calibrated to broadcast the audio samples at a specific volume (measured in decibels). Any time a honeyguide bird appeared, the researcher would take note of it.
What they found is that the birds were more likely to show up when the familiar signal used by the locals was being broadcast.
In Tanzania, honeyguides appeared 82% of the time that Hadza whistles were being played. But the birds appeared only 24% of the time when the researchers played the kinds of trills traditionally used in Mozambique.
The opposite was true for birds living in Mozambique. They responded to trills 73% of the time, but only 26% of the time when whistles from Tanzania were broadcast.
This makes it clear that the birds had to have learned what their human neighbors do when they want to partner up and go honey-hunting.
The fact that these birds can respond to culturally-specific cues is fascinating, says Steve Nowicki, a bird communication researcher at Duke University who co-wrote a commentary that accompanied the researchers' report on their findings.
He points out that these birds come from a group of birds that isn't known for vocal learning, unlike parrots or songbirds.
"That's why I think it's kind of remarkable that the honeyguides are cueing in on these population-specific cultural differences," says Nowicki.
The birds couldn't have learned how to do this from their parents, because honeyguides lay their eggs in the nests of other species which then raise the offspring.
But Wood notes that when foragers hack open a tree and expose honeycomb, typically more than one honeyguide flies in to enjoy the treat.
"There's often several honeyguides, up to half-a-dozen or even more," says Wood. That means it's possible that younger birds watch how the older ones interact with people and then start to copy that behavior.
If that's the case, then honeyguide birds would learn about these special calls from their elders, just like their human partners do.
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