It was around 0900 am on a beautiful morning in February 2006. Winter lingered around in the slight nip in the air, but spring announced its arrival with the first red blooms of the Indian Coral Tree (Erythrina suberosa in Latin and Kinchaga in Soliga). I had been walking the forests of the BR Hills for just over a year, recording bird songs for my Master’s research project. I was always accompanied by my field assistant, Madha, on these walks. He is a Soliga, and these forests are his home.
As we walk along the game road, our attention is drawn to a black bird with a long tail that is calling loudly from its perch on a dry tree branch. We stop and listen, and I switch on my recorder and microphone. It makes a few loud, metallic calls…the same note repeated over and over again. And then it flies lower, and suddenly we hear it make a Flameback Woodpecker’s call. Then it calls like a Crested Serpent Eagle. We stand mesmerised by this accomplished vocal display, as the bird imitates several other species. It is a Greater Racket-tailed Drongo, or Dodda Karali to the Soliga. As it gracefully sallies after an insect, we walk on, intrigued by our experience. Madha jokes, “Nivu ide hakki bagge PhD maadi – yaakendre adu yella hakki bagge PhD maadtaide!” (“you should do your PhD on this bird since it is doing a PhD on all the other birds”). At that time, neither of us could have guessed at the profundity of his amused observation.
It is now 2015, and I have spent the past seven years in pursuit of the Racket-tailed Drongo.
Greater Racket-tailed Drongos are found across south-east Asia in deciduous and evergreen forests. Regularly found in mixed species flocks throughout their range, these are conspicuous birds – glossy black with a prominent crest on the head, and two long and thin tail feathers with spatulate tips. But more than their looks, they are renowned for their ability to mimic other birds, often with an accuracy that can confuse the bird watching veterans. We all know of birds like parakeets that can “talk”. But these birds only do so in captivity, where they mostly hear human speech, and not other birds of their own kind. These species never mimic in their natural habitat. Perhaps more intriguing, then, are those species, like drongos, that do so in the wild.
Greater Racket-tailed Drongo (Dicrurus paradiseus)
Since I was familiar with the birds of BR Hills (and their calls), it made sense to study Racket-tailed Drongos in the same area. The first years were all about collecting baseline data – selecting suitable sites where the drongos were abundant and the terrain not too steep, documenting the time and duration of their breeding season, nesting behaviour, etc. I recorded the racket-tails mimicking 35 species of birds, 3 mammals, at least 2 frogs and even an insect! A very important tool in the study of birdsong is something called a spectrogram (or sonogram). This is nothing but a graph of frequency (in kHz) with time, and there are several softwares available that allow us to view sound in this manner. This graphical representation also allows us to perform further analyses on our recordings. Here are some examples of spectrograms of racket-tailed drongo’s own calls, without any mimicry.
If you look carefully, you will see that each note has a unique shape or motif in the spectrogram. These are like signatures that aid in identifying and classifying the many different notes in a bird’s vocal repertoire. And of course, are very useful in identifying mimicry as well. When a note produced by the drongo matches the signature of a note from another species’ vocal repertoire (known as a “model”), it is very likely to be a mimicked one. Here are few examples of some mimicked notes made by Racket-tailed Drongos in BRT.
Model of a Fairy Bluebird Call
Mimic of a Fairy Bluebird Call
Asian fairy-bluebird (Irena puella)
Model of a Common Tailorbird Call
Mimic of a Common Tailorbird Call
Common Tailorbird (Orthotomus sutorius)
Most species get along fine with their own song, so why mimic? How did this phenomenon of vocal mimicry evolve in birds? Turns out, as happens so often in nature, there is no simple answer. In Satin Bowerbirds, for example, females prefer to mate with males that are accurate mimics and have relatively larger mimicry repertoires. So the more species a male Satin Bowerbird mimics, the more females he can attract. Fork-tailed Drongos in the Kalahari in Africa, use mimicry to deceive other species into dropping their food, which the drongo then steals. The chicks of several species of cuckoos, which are brood parasites that lay their eggs in other species’ nests, are known to imitate the begging calls of their host chicks. This stimulates the host parent to feed them, and also possibly aids in reducing the chances of the host rejecting these chicks, which often look very different from its own chicks!
For most mimics however, many fundamental questions remain to be answered. In order to understand the functions of mimicry in any species, we need to know the contexts in which the mimicry is produced. What is the Racket-tailed Drongo doing when it is mimicking? And who is the mimicry directed at? Several studies on Racket-tailed Drongos in Sri Lanka by Eben Goodale and his group have shown that drongos are regular participants in mixed species flocks, and that their mimicry could potentially facilitate the formation of such flocks. They also found that alarm mimicry produced by Racket-tailed Drongos could induce a flee response or even mobbing behaviour from the species that were being mimicked.
We found some very interesting patterns in our observations of Racket-tailed Drongo behaviour. The species that the drongos mimicked when they were in the presence of other racket-tails were very different from the species they mimicked when they were with other species in mixed species flocks. We believe that Racket-tailed Drongos use these different sets of mimicry, embedded within a unique singing pattern to communicate with different audiences – their own species, or other species.
The truth is, birds and their voices are part of a complex communication network that we are only now beginning to comprehend. Cross talk and even eavesdropping between species is no more the subject of speculation, but a well accepted fact.
The author, Samira Agnihotri, on the field at BR Hills.
I have frequently been asked, “What is the application of your research?”. Most often, I say that there is no “application”, but all studies of animal behaviour increase our understanding of the species we share the planet with, and help us comprehend how and why certain behavioural strategies have evolved. And lately, I am intrigued by the similarities between human language and animal communication; and horrified by how little we want to know of the effects of sound pollution on ourselves. And I want to ask back, “Do you remember a time when the only music we heard was the birds in the trees, the frogs and the crickets in the bushes at night, and the sighing of the leaves in the breeze? Do you remember how happy you were then?”
The time has come that we think about not just the conservation of endangered animals, but also about the preservation of acoustic landscapes.