Bird Wave Endemic: Sri Lanka Drongo
It’s a quiet morning in the Sinharaja rainforest, as you trek along the well-worn forest trail, on the lookout for birds, but seeing mostly dense tangles of vines amid tall trees on either side.
de Silva Wijeyeratne, G. 2015. A Naturalist’s Guide to the Birds of Sri Lanka.
And then, the calls ring out. Bright, liquidy-metallic, like a bell. They’re followed by harsh, wheezy laughs — harr, harr, harrrr — along with screeches and squeaks and whistles. They’re punctuated by clear, bubbling notes — yop-po-pop — and, in response, trilling cries. You stop seeing, and begin looking, and realise we’re surrounded by birds, from high in the canopy, down to the forest floor.
You’re witnessing one of the most intriguing avian spectacles in Sri Lanka — a bird wave. The Sinharaja bird wave, in particular, is said to be the largest, most studied, and most aseasonal one of its kind (de Silva Wijeyeratne, 2015). Paying attention during a Sinharaja bird wave is a sure way of observing several endemic species as they forage for food — a number of which will be the focus of this post.
A bird wave — technically known as a mixed-species feeding flock — is a temporary gathering of a variety of species, mostly of birds, which can also be accompanied by other taxonomic groups. It could happen anywhere in a suitable habitat, but those in Sinharaja are the best known. What could cause this sort of behaviour? Perhaps it’s because of safety in numbers — or because the foraging flock tends to dislodge fruit and flush insects, increasing the efficiency. The phenomenon seems to be beneficial overall.
While a bird wave may seemingly be a chaotic mess of over 30-50 birds from dozens of different species scouring the forest for food, it has a certain organisational structure, with niches and roles for the diverse species involved.
In this series of posts, I will be describing selected species that participate in bird waves, in the Sinharaja Rainforest. Await more in the future!
The Sri Lanka Drongo (Dicrurus lophorinus), also known as the Ceylon Crested Drongo, is a nucleus species in a bird wave, known for its clear, ringing voice and its talent for mimicking the calls and songs of other birds. Gehan de Silva Wijeyeratne (2009) describes the bird as a “CEO” — and scientists have observed how it can summon flocks and individual birds and actually redirect bird waves by mimicry. By itself, the Drongo can create the impression that a flock has already gathered — giving others an incentive to join the party, and flush out insects that the Drongo can snap up (Goodale & Kotagama, 2006).
Scouting for insects in the air also gives the Drongo an added advantage. It can easily spot predators — which, in this context, are usually birds of prey, such as Shikras and forest eagles. If a predator is spotted, the Drongo can alert the rest of the flock with its loud, strident alarm calls — and it could bully the predator away from the scene.
There’s, however, a dark side to the CEO. The Sri Lanka Crested Drongo itself is a bit of a manipulator and a robber, often attempting to snatch the catches of other insectivorous birds such as flycatchers. Despite all this thievery (scientifically termed kleptoparasitism) and flock-handling, the flock at large seems to tolerate its behaviour — probably because, in the grand scheme of things, everyone gets their grub, is generally safe in the crowd, and all is forgiven. Hmm. Sounds familiar. Almost ... human.
Drawn to the summons of the Sri Lanka Crested Drongo, a number of gregarious birds form the core of the flock. They forage and travel together, crossing trails and traversing territories, scouring out the food in their path — covering all the strata, from the treetops and emergents, to the leaf litter on the forest floor.
The Sri Lanka Crested Drongo, a resident of wet zone forests, also exhibits some interesting nesting behaviour. A pair may often use the same nesting site year after year, and to reduce access for predators, they perform what is termed “tree canopy isolation” (Rajeev et al, 2018). They actively strip off leaves from the outer branches of the host tree and the surrounding trees (up to a distance of 3.5 metres), creating a bare ring around the host tree, cutting off direct contact with the surroundings. They were even observed stripping off layers of moss and lichen from the host tree’s trunk, in order to reduce traction and “footholds” for predators.
References and Interesting Links
de Silva Wijeyeratne, G. 2009. The Sinharaja Bird Wave. Hi Magazine. May 2009. Series 7, Volume 2. Page 146-147. ISSN 1800-0711.
Goodale, E; Kotagama, S.W. 2006. Vocal mimicry by a passerine bird attracts other species involved in mixed-species flocks. Animal Behaviour, 72, 471-477
Rajeev, G.; Vidanapathirana, D.R.; Wickramasinghe M.; Benaragama K.P. 2018. Host tree canopy isolation by nesting Sri Lanka Drongo Dicrurus lophorinus. Forktail, 34(1):52–57.