On Nicobar Island, in the Indian Ocean, a most unusual hunting party is searching for food. Through the branches of the forest, the tiny Nicobar treeshrew scuttles about searching for insects. They're followed by the racket-tailed drongo, a small bird that picks off juicy morsels flushed out by the foraging treeshrews. So far, this isn't unusual - many distantly related animals forage together, either because they net more food or because they can watch out for predators.
But this alliance has a third an altogether more surprising member - a sparrowhawk. This bird of prey is five times larger than either of the other two species and can easily kill treeshrews. But it doesn't - instead, it only deploys its talons on other prey that are disturbed by its partners.
Predator and prey species don't usually fraternise with each other outside the confines of animated films, but this alliance is unique. The drongos and sparrowhawks will actively seek out the treeshrews and stay with them in consistent formations over long periods of time. If you spot a treeshrew, there's a 71% chance that there's also a drongo around, and a 43% chance that a sparrowhawk is too.
Meera Anna Oomen and Kartik Shanker discovered this strange coalition and they suspect that it's engineered by the drongos. Certainly, sparrowhawks aren't traditional companions for treeshrews. On their own, these mammals keep their distance from the raptors for their own safety and are extremely vigilant. But with the drongos around, they tolerated the hawks and allowed them to get a couple of metres closer. The hawks, meanwhile, appear to use the drongos as a way of finding treeshrews.
Birds of prey are unusual participants in foraging groups. When they join another species, as is the case with double-toothed kites and capuchin monkeys, it's usually one relatively big enough that it couldn't be a possible meal. That clearly doesn't apply on Nicobar Island.
Nonetheless, the hawks, and certainly the drongos, seem to get a good deal out of their partnership. Oomen and Shanker think that they find more food when they follow treeshrews (although the data on this are still limited). Why a bird of prey should stray from its typical hunting technique is unclear, but it may be that the treeshrews simply aren't a possible target with the drongos around.
It's unclear how the treeshrews benefit from this coalition, or whether they simply can't avoid the pursuit of their partners. Certainly, Oomen and Shanker found that in the drongos' presence, their foraging rates fell by around two-thirds. Drongos are notorious food thieves, and this was abundantly clear if there were two of them around - in these situations, the treeshrews often turned on them and chased them away.
Ironically enough, the treeshrews may get protection from predators by having the drongos act as intermediaries. These small birds are incredibly vigilant and especially so when hawks are around. Even though a hawk is actively following the treeshrew, the drongo may be able to warn it if the predator partner decides to pull a double-cross, or if another predator flies in from outside the group.
Reference: Biology Letters doi:10.1098/rsbl.2009.0945
Images: Drongo by Nimesh M
More on natural alliances:
- Leafcutter ants rely on bacteria to fertilise their fungus gardens
- Ants herd aphids with tranquilisers in their footsteps
- Social spiders do better when hunting with relatives
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Wonderful summary, as usual, Ed! Such inter-species alliances are so intriguing...
That's pretty remarkable. I wonder what the double-cross rate actually is in arrangements like this. It obviously can't be too high, or the shrews would evolve to shun the birds altogether. I imagine the amount of other prey flushed out must be reliably substantial enough to dissuade betrayal, especially if the hawk is not otherwise very good at finding it in eg. dense brush.
It doesn't seem as if the shrew has any choice in the matter. This seems more like opportunism, on the part of the sparrowhawk, than mutualism. The interesting phenomenon, then, is that it has given up trying to attack the shrew because the drongo makes such attacks a waste of time. Without the drongo, the sparrowhawk might mooch about anyway, watching equally for the shrew to make a mistake, or for anything the shrew might disturb.
We might compare the squirrelhawk's behavior to a cop shadowing a drug addict, hoping to net a dealer. The drongo corresponds only to knowing the addict isn't holding.
If this occurs on the Nicobar isles but not on the mainland it may be a result of island diversity of prey species being comparably reduced, with any slightly symbiotic relationship being better than none at all. Perhaps during the stormy monsoons, flying birds which stay under the top canopy close to tree shrews get blown away less often than those that don't, similar effect in flying insects losing out to wingless insects on windy islands.
(The drongos tail feathers are cool! I can't figure out why the tree shrew (tupaia) has rounded ears like apes while squirrels have pointed ears, maybe it relates to this 'alliance'.)
I've seen spangled drongos steal food from kookaburras, which requires a bit of nerve on the part of the thief. Pretty sure that's opportunism, though, rather than a neatly orchestrated relationship.
Drongos are notorious thieves and mimics. In South Africa, I spent a morning with a meerkat researcher, following live meerkats. He said that he had anecdotal evidence that the fork-tailed drongo would sometimes mimic the predator alarm calls of meerkats while they were foraging and then swoop down to nick their unearthed morsels.
Wow. Straight out of Panchatantra. What a cast and what roles. Mahesh
Very interesting observations! Should check this out in the peninsular treeshrews as well!
Here is another narration of a participation of a raptor in mixed species flocks, which in itself is quite a curious finding.
http://www.indianbirds.in/Archives2007_3.1.html
Hari Sridhar. (2007) Participation by Black Baza Aviceda leuphotes in mixed-species bird flocks in rainforests of the Anamalai hills, Western Ghats, India. Indian Birds
Just two days back my colleague saw Tree shrew amongst jungle babblers and only when i showed this article he realized it was part of hunting party. This is at Kanha National Park, Central India.