How honeyguide birds assist human honey hunters

To explore the relationship between human honey-hunters and honeyguide birds in rural parts of East Africa, Chris Smith spoke with Claire Spottiswoode - an evolutionary biologist at the University of Cambridge’s Department of Zoology and the University of Cape Town - from her research base in Mozambique...

Claire - Cooperation between humans and wild animals is really vanishingly rare. One of the few remaining examples occurs between an African bird species, the greater honeyguide; small, rather unassuming, brown birds that are related to woodpeckers. They all share this really curious trait, which is that they love to eat beeswax, which is really rich in energy if you're able to digest it, which honeyguides have the ability to do. But the snag is that beeswax is defended by thousands of stinging honeybees. So while honeyguides are extremely good at finding bees nests, they have rather more of a problem getting access. So, to solve this, they team up with their own species and approach humans in the bush and give a strident, urgent chattering call, 'chatter, chatter, chatter,' which attracts our attention. And if we choose to follow them, they'll then fly from tree to tree, giving this chattering call as they fly in the direction of a bees nest - anything from a few metres if you happen to be standing under a bees nest without realising it, which is remarkably easy to do, up to a mile or more.

Chris - Do we know how far back in history this relationship goes? Is this a modern thing?

Claire - It's entirely possible that it actually predates our own species. And that's because the two key features that make us such useful partners to a honeyguide are, first, the use of fire to subdue these thousands of stinging honeybees, and the use of tools to chop open their nest. These are skills that humans and our hominin forebears have had for in the order of 1.5 million years in the case of the control of fire and up to 3 million years in the case of tool use. For all that time, honeyguides and honey producing honey bees have also lived in Africa, so it's entirely plausible that this relationship is extremely ancient. But of course this is just a hypothesis and unfortunately it's an unfalsifiable one: we can't test it in a way that would allow us to prove it wrong.

Chris - Do they learn this from their parents or does this appear to be bird innate? They come out of the egg and they can do this?

Claire - We can be pretty sure that the aspects of the relationship that are learned rather than innate are not learned from their parents because honeyguides, like cuckoos, are brood parasitic, meaning they're raised by other bird species and, in fact, one of a honeyguides first acts as a chick in a dark nest hole is to stab its foster siblings to death and it'll thereafter be fed by another species until it fledges. It'll never knowingly meet its own parents because they're raised by species that have had no such interaction with humans. A honeyguide hatches from the egg knowing how to cooperate with humans and we think this because even very young honeyguides would seem to be attracted to humans and attempt to guide them. And, furthermore, in parts of Africa where humans no longer recognise the call of honeyguides and no longer follow them because they have access to alternative sources of sweetness besides wild honey - either beehive honey or or sugar - honeyguides do still attempt to call us.

Chris - The human calls to the bird, the bird calls to the human. Is that the same all over Africa in the territories where these birds are? Or, in the same way that humans speak differently in different bits of the world, do the birds' conversations differ?

Claire - Yes, it's really remarkable. In fact, certain calls are used fairly consistently here in the north of Mozambique.

<Human call>

Claire - Even just a few hundred miles away, completely different sounds are used. And if we look across Africa, we see a mosaic of different human cultures using completely different sounds in exactly the same context. These can vary from beautiful whistle melodies, for example, used by the Hadza honey hunters of Northern Tanzania to various other forms of whistles often made with instruments like snail shells or hollow fruits with holes drilled in them, as well as in some places various words and other forms of trills and grunts. So we have this immense cultural variation among different honey hunting cultures across Africa that honeyguides have to contend with in order to recognise a good partner and cooperate effectively with our own species.

Chris - Do these calls work on the birds wherever you are in Africa or have the birds evolved or learned the specific calls made by the populace in those areas so they'll only respond to humans who respectively speak that language?

Claire - Yes, you've just laid out two very nice alternative hypotheses for why these calls work. And these are two of the alternatives that we considered in our new paper that we're discussing. We also considered a third hypothesis, and that's that these calls are designed to travel well through the particular habitat in which different people are honey hunting. So we know that some verticals are tuned to the specific environment in which the birds exist, they move better through the vegetation type, so it's just possible that this could be true for these human calls too. They might work not because honey guides prefer them, but simply because honey guides can hear them better.

Chris - How did you test this, then? What experiments did you do this time to try to get to the bottom of this?

Claire - We carried out field experiments here in the North of Mozambique and also in Northern Tanzania to test honeyguides' responses to the signals that people used to communicate with them in these different parts of Africa. And in each place, the three experimental treatments that we carried out were either the call given here in the North of Mozambique, the beautiful whistled melody given by Hadza honey hunters in Northern Tanzania, or arbitrary human sounds which were honey hunters shouting out their names.

Chris - And what did you find? Did the birds only respond to the local lingo as it were?

Claire - That's exactly right. So we found that honey guides were two to three times more likely to interact with humans who gave the local honey hunting call compared to the foreign honey hunting call or arbitrary human sounds. So here in Northern Mozambique, the honey guides strongly preferred the local honey hunting call, but largely ignored the Hadza whistle melody. Whereas in Northern Tanzania, the opposite was true - the honey guide strongly preferred the local whistle melody, but were not particularly strongly attracted to the Mozambican sound. So this supports the hypothesis that honey guides have learned what their local partners sound like and are attracted to those calls. We were also able to test the third hypothesis that these sounds simply travel better through the bush by directly quantifying how quickly sound does travel through each environment, and we found that both types of sounds travel equally well through the environments at both sites, suggesting that this isn't a good explanation for why honey guides are differentially attracted to the local sounds. And of course this, like our previous finding, is one that would probably not surprise any honey hunter. They know perfectly well that they use these calls because because honeyguides prefer them.

Chris - In some respects, they're using us as their tool then, aren't they? They're kind of manipulating us to feed them by doing us a favour? We cop all the stings in order to give them dinner?

Claire - That's very well put. We are their tools, but also they are our tools and in fact they're described almost that way in as many words by some of our honey hunter colleagues here in Northern Mozambique. Honeyguides are a kind of living tool to them and, as it happens, we are interested in different parts of the resource. Honeyguides want wax, we want honey, so there's no fighting over the spoils either.