Imitation of other species is one of the most intriguing and mysterious aspects of birdsong. Often — as in the case of mockingbirds — there seems to be little connection between what the bird is imitating and what it is doing at that moment. But in the rainforests of Sri Lanka, a bird that travels in mixed-species flocks has learned to use the calls of other species in the same contexts that those species use them.
The behavior of drongos was documented by Eben Goodale, who completed his Ph.D. in organismic and evolutionary biology last May at the University of Massachusetts Amherst. His article about drongo behavior, based on his doctoral thesis, was published last month online in the Proceedings of the Royal Society: Biological Sciences, a British scientific journal. The study, “Context-dependent vocal mimicry in a passerine bird,” is co-authored by Sarath W. Kotagama of the University of Colombo in Sri Lanka.
Goodale’s discovery of the drongo’s mimicking ability is the culmination of years of studying communication among various bird species in Sri Lanka.
“I still remember vividly the moment that I first observed this behavior,” he says. “I was following through the rainforest a mixed-species flock of birds. Mixed-species flocks in this rainforest are large and noisy, averaging 12 species and nearly 40 individuals. From the back of the flock, one bird, a greater racket-tailed drongo, swooped down and approached me to within three meters at my head height. The drongo was clearly mobbing me—a behavior that birds use to notify other individuals of the presence of a stationary predator.”
The familiar scene quickly changed, according to Goodale. “The drongo did an extraordinary thing: it began to mimic the mobbing-specific note types of other species. It kept rotating through the mobbing notes of other species, in addition to its own notes. I wouldn’t have understood what was happening if I hadn’t just completed a study on the alarm-associated calls of all the species in the flock system.”
Further study by Goodale revealed that drongos’ repertoire is varied. They imitate other species’ notes in the same contexts as other birds and also mimic the calls of predators, which they use in alarm situations.
“What’s fascinating about this behavior is that it’s reminiscent of what we know some birds—notably parrots—are able to do in the laboratory: learn to use other species’ signal in a context-dependent manner. But such context-dependent mimicry has not yet been demonstrated in the wild.”
“It’s very significant,” says Bruce E. Byers, a UMass biology professor and one of Goodale’s thesis advisors, “The idea that signals could’ve evolved beyond species boundaries hasn’t really been demonstrated before.”
Byers says Goodale’s paper also reveals “the pretty extraordinary cognitive abilities of the drongo,” an area that his former student is continuing to investigate.
Other aspects of Goodale’s research were published earlier this year in Auk and the Journal of Tropical Ecology. He is planning further fieldwork in Sri Lanka, India and Papua New Guinea.
From UMass Amherst