Should you walk through the lush rainforests of Australia’s Atherton Tablelands, you might stumble upon one of nature’s greatest shows. Among the dense verdure is a small opening with a conspicuous perch. A grating call emanates nearby; then, the showman appears. Jet black, with feathers that absorb more light than any other natural material, a small bird lands on its narrow stage. Though mostly a void of darkness, its lower breast, tail, and throat scintillate with an otherworldly iridescence—a swirl of cerulean and emerald that glows like liquid metal. Opening its mouth to call, its interior—called the gape—shines a neon yellow. A female arrives, landing on the edge of the perch, and the true spectacle begins. Posturing itself, the male raises his wings in the shape of a disk and opens his mouth, flaring his throat as he bobs up and down. Once his potential mate has come close enough, he whips his head back and forth along the backside of his wings, producing a soft clapping sound as he further extends one wing and slightly retracts the other in rapid succession.
However unbelievable, this is no fever dream. This is the display of the Victoria’s riflebird. From his bewildering acoustics to his radiant flamboyance and masterful choreography, the male riflebird has fine-tuned the art of performing over generations to achieve one goal: attracting a mate. His unique features have been carefully selected for by sustained mate choice driven by the unostentatious female, who makes up for her bland plumage with terrifically high standards.
Riflebirds are members of the family Paradisaeidae, more commonly referred to as the birds-of-paradise, which inhabit eastern Australia, New Guinea, and parts of Indonesia. Many of the forty-four known species boast resplendent plumage and courtship displays that remain unrivaled in the animal kingdom. They have captivated humans since their discovery, influencing Aboriginal culture—which prizes their feathers—and mystifying the first Western viewers.
Within Paradisaeidae, four species of riflebird exist: the Victoria’s riflebird, the paradise riflebird, the magnificent riflebird, and the growling riflebird. Apart from slight differences, however, all male riflebirds share similarly impressive color schemes, practice polygyny, and beguile females with their intricate displays. But despite a global fascination with birds-of-paradise, very little is understood about the mechanics of their performances.
Thomas MacGillavry, a PhD student at the Konrad Lorenz Institute of Ethology in Vienna, hopes to change that. . MacGillavry was initially fascinated by insects and reptiles, but around age sixteen, he became enraptured by the wonder of birds. In Germany, the first species that caught his eye were classic backyard birds known as blue and gray tits, which are related to America’s chickadees. “Quite gradually, I became more interested in birds going on from there,” MacGillavry said.
Following his master’s degree, MacGillavry realized he wanted to focus his future work on behavior. “It was actually Rick’s book, The Evolution of Beauty, that got me really interested in the evolution of courtship and mate choice,” he said, referring to Yale professor Richard Prum. Then, MacGillavry landed on his quarry.
“I was looking for a group of animals to study the courtship displays of, and then I ended up being intrigued by birds-of-paradise because they seemed so mysterious. So I was looking at potential species to study, and I ended up getting really interested in riflebirds,” MacGillavry said. He soon got to work, planning research in the heart of riflebird territory to unravel the mystery of their performances: not merely what they do, but also how they do it.
Focusing mainly on Victoria’s and magnificent riflebirds, MacGillavry conducted his field work in the Atherton Tablelands and Kutini-Payamu National Park in Australia. His work was split into two seasons, each about four months in length, and his colleagues Marco Sensi and Jack Rees-Elford accompanied him for one season each. Still, MacGillavry had his hands full.
After a tumultuous first season focused on finding lodging, acquiring permits, and locating riflebirds, his second season found him at the School for Field Studies, which has a site in the Gillies Range—within the Atherton Tablelands—where he was able to work and live among the riflebirds. “My cabin there was in the shade, in the rainforest, where I could often be woken up in the morning by riflebirds calling. So that was much more exciting,” MacGillavry said.
Using nearly 1,200 hours of camera trap footage and in-hand measurements of riflebirds, MacGillavry’s research clarified the murky waters of riflebird display. Of highest importance is the newly defined ability of wrist hyperextension, a capacity previously unknown in the avian world. To perform their displays, riflebirds must hyperextend their wrists to nearly 240 degrees. For context, when most birds open their wings to fly, the wrist is extended between 110 degrees to 180 degrees from its resting position. Riflebirds exceed the known extension maximum by 60 degrees. For MacGillavry, this realization was a foundational moment in his research. His discovery was one of complete serendipity since no definitive authorities on riflebirds had ever noted wrist hyperextension previously.
“The moment I caught one and [it] did a wingspread, the wrist just kept extending, and I was like, ‘Oh my god!’ It was kind of an aha moment, where I was like, ‘Oh this is how they do it!’” MacGillavry said.
These findings contrast with the wingspread of the monogamous trumpet manucode, one of the few other birds-of-paradise on the Cape York Peninsula. The manucode does not have a remotely comparable ability. All in all, the data suggest that wrist hyperextension may be globally unique to riflebirds. Intriguingly, MacGillavry found wrist hyperextension to be present in female riflebirds despite their lack of displays, provoking further questions regarding the genetic advantages behind this trait.
Hyperextension directly connects to MacGillavry’s second major discovery: the cause of the sonation, or sound production, in Victoria’s riflebird displays. Previous research assumed the “wing-clap” sound was produced by the collision of primary feathers, which are the same feathers responsible for flight. Now, thanks to detailed video footage, MacGillavry has revealed the true mechanism behind the clap. As the wrist—and outer half of the wing—are hyperextended during display, the bird’s beak scrapes against the stiff central stem of the primary feathers, known as the rachis. The rapidity of the beak being dragged against the keratinous rachises creates the classic snap noise, representing a brand new modality of bird sonation.
The footage also yielded evidence of a mechanical trade-off. When the Victoria’s riflebird first postures with its yellow mouth agape, the beak is open, tilted towards the female, and on the underside of the wings. Once wing snaps commence, the beak is closed, set backward, and above the wings, meaning both displays cannot occur concurrently.
MacGillavry’s third major find relates to sexual dimorphism between male and female riflebirds. Analyzing a dataset of museum specimens of riflebirds worldwide, he tentatively concluded that riflebirds exhibit extreme differentiation between male and female wing length—more so than any other bird-of-paradise. In the growling riflebird, male wing size is independent of body size, so MacGillavry believes wing size—and by extension wing surface area—is instrumental in courtship signaling and almost certainly a product of sexual selection.
Having whet his appetite with riflebirds, MacGillavry hopes to further explore the fine-tuned mechanisms of their extraordinary performances. His paper has broached as many questions as it has answered, and he has already planned his next target research: the temporal organization of these displays—in essence, contextualizing the variable of time in riflebird courtship.
MacGillavry’s results have great consequences in the world of birds and beyond. “It adds so much more detail to what we know about these birds and how sexual selection through mate choice can produce phenotypes that involve the whole body—turning the whole body into a signaling machine,” MacGillavry said.