Humble Origins: When a Cry for Help Becomes a Song-and-Dance Number
By Emma Greig
January 15, 2012
The Gray Butcherbird of Australia is a frightful-looking thing, the size of an American Kestrel with a long, hooked beak it uses to rip apart locusts, lizards, and occasionally birds. But for some reason its cries don’t send nearby, bite-sized fairywrens scrambling for cover. Far from it— when a male Splendid Fairywren hears a butcherbird sing, he jumps into the open, puffs up his cheek feathers, and lets loose a trilling song of his own.
At the heart of this act—the so-called “predator-elicited trill”—is something that has puzzled many a human female: Why should a male make a spectacle of himself when danger is near? Guppies do it, lizards do it, magpie-jays do it, even guys outside of bars do it. These tiny, gorgeously colored birds called fairywrens do it, too, and finding out why led me to remote parts of Australia during my dissertation research. Now, as a postdoctoral researcher at the Cornell Lab of Ornithology, I am delving into the evolutionary lineage of fairywrens to find out how this strange behavior came about.
Why sing in the face of danger?
Could a trilling male be warning his family? Perhaps, but the butcherbird’s loud song should do that all on its own. Could this trill be a display in which a male shows off how fit, vigilant, and bold he is? Maybe, but all the males do it, from glittering blue adults to wimpy brown youngsters. Could males be seizing an opportunity to show off while the butcherbirds have heightened everyone’s attention, safe in the knowledge the predators are otherwise engaged?
It took me five field seasons and six experiments to find out, but this last possibility seemed to be the case. Using recorded calls of butcherbirds and male fairywrens, I found that females did perk up at a butcherbird’s song. If a male’s song followed on its heels, females tended to be more responsive than when the male sang alone. I had solved the mystery of the predator-elicited trill, or so I thought.
In fact, all I had done was explain the trill’s current function. I still didn’t know how this weird behavior came to be. Had the trill been around for eons before males started using it in response to predators? Or was it the other way around: Was the trill once a simple alarm call that became more elaborate after other fairywrens started paying attention?
To answer this question, I needed to see how the ancestors of these wrens behaved. Though I didn’t have a time machine, I did know the evolutionary lineage of the fairywrens. If related species sang trills of their own, maybe they could yield clues about when this puzzling behavior evolved. It was time to head to the field! I packed up some speakers, an iPod, and some recordings from the Macaulay Library and headed to the outback.
My crash course in fairywrens
Nine species of fairywrens live in Australia, but some are easier to find than others. Splendid and Superb fairywrens inhabit city parks and backyards, so finding them was a breeze. Red-backed Fairywrens flit through many an overgrown highway shoulder, so I was able to study them while topping off gas in the rental car.
But species such as the Purple-crowned Fairywren can only be found in remote, streamside forests of Pandanus spiralis, a large palm-like tree. To find this species I flew by mail plane, along with sacks of oranges and cases of Victoria Bitter, to the Mornington Wildlife Sanctuary in Western Australia.
Red-winged Fairywrens, though common where they occur, live only in a small area of southwestern Australia. The old karri forests they call home were a grand sight, and as I had hoped, Red-winged Fairywrens twittered busily and fed young around the bases of the giant trees.
White-winged Fairywrens occur all over Australia, but might as well be invisible. In open heaths that stretched to the horizon, these wary wrens could see me coming from a long way away. Each time I would take a few steps in the direction of their revealing chirps, they would lift off and zoom another 100 yards away.
One thing became clear as I played predator songs to fairywren after fairywren: lots of species responded. Blue-breasted Fairywrens in the dry eucalyptus mallee sang sharp notes when they heard Gray Shrike-Thrushes. Buzzy trills of Lovely Fairywrens rang through the Queensland rainforest in response to Black Butcherbirds. In the karri forests, Red-winged Fairywrens stopped what they were doing to give a series of long whines at the sound of a Gray Butcherbird.
The songs of these fairywren species were all very different from the trills of Splendid and Superb fairywrens, but they were being used in the same manner. What was particularly exciting was that a few species also sang these songs during dawn chorus displays, even with no predators around. This might mean the sounds evolved first as a predator response and then males of some species may have started using them as full-on singing displays—a fascinating case of a cry for help evolving into a song-and-dance routine.
As a scientist, I hasten to say we don’t know the full story yet. But my colleagues and I are starting to look farther into the evolutionary tree, to see if more-distant fairywren relatives such as emuwrens also sing in the face of danger. We are beginning to understand just how many secrets these little Australian birds have, and we hope to uncover more.
Originally published in the Winter 2012 issue of BirdScope.