(Inside Science) -- The fork-tailed flycatcher whistles with its wings in two different accents, potentially more evidence this bird is splitting into two species, a new study finds.
Birds are known for the songs they can sing, but dozens of species also use their feathers to generate sounds. For instance, peacocks can rattle their quills together, and the crested pigeon's wings whistle when they fly.
In the new study, researchers investigated fork-tailed flycatchers -- 1-ounce birds found throughout the Americas that resemble black-and-gray swallows. The males sport foot-long scissor-shaped tails as ornaments to help attract mates, and they also spread these giant feathers to help turn sharply while hunting by using the plumes as air brakes, said study lead author Valentina Gómez-Bahamón, an evolutionary biologist at the Field Museum in Chicago.
When these birds fly -- sometimes as fast as 65 miles per hour -- they produce a high-pitched trill. Males often fly quickly when they fight each other during mating season, Gómez-Bahamón noted. The birds also fly quickly when fighting off intruders near their nests.
The scientists studied two known subspecies of fork-tailed flycatchers: a migratory one that breeds in the southern part of South America but spends winter closer to the equator, and a nonmigratory one that spends the whole year in the northern part of the continent.
The scientists first captured the birds with "mist nets" -- fine webbing stretched between two poles like a volleyball net -- and recorded audio and video of them as they flew away after they were released. The researchers also set up a taxidermy hawk in a field with a hidden camera, and when the fork-tailed flycatchers swooped in to attack, the researchers recorded how the flycatchers’ feathers moved and what sounds they made. The whole project took three years.
"Recording a fast-flying fighting bird is really hard," Gómez-Bahamón said. "It took many attempts."
The audio and video footage, as well as experiments with fork-tailed flycatcher plumes in a wind tunnel, revealed the birds create these trills with fluttering feathers. Airflow causes these plumes to vibrate with short repetitive whistles, much like the sounds one can whistle using a blade of grass.
Gómez-Bahamón and her colleagues discovered the migratory subspecies made higher-pitched trills with their feathers than their nonmigratory cousins.
The migrating males possess wing feathers with skinnier tips than those of their homebody brethren. These may have evolved to make it easier to fly longer distances. The researchers suggested a group of migratory fork-tailed flycatchers ceased to be migratory, and as their wing feathers thickened because they no longer made long journeys, they ended up sounding different from those of their migratory relatives.
"This is super-challenging work -- these birds are really aerial, and they're not tame," said evolutionary ornithologist Richard Prum at Yale University, who did not take part in this research. "I was amazed at the detail of the analysis they were capable of doing."
Aside from escapes and fights, males of both subspecies trill with their wings in the early morning when it is still dark, likely as displays to females, Gómez-Bahamón said. The birds sing songs, are quiet for a moment, and then perform a short flight where one can hear the fluttering.
Since wing fluttering may help the birds communicate during mating season, Gómez-Bahamón and her colleagues suggest the feather “accents” they found may help further drive the subspecies apart. Eventually, the two types of flycatchers may evolve into fully separate species that cannot interbreed with one another. "Differences in migratory behavior can cascade to other behavioral traits," Gómez-Bahamón said.
Future research will investigate whether related species display similar behavior. The scientists will also explore whether female fork-tailed flycatchers prefer sounds from males of their subspecies, Gómez-Bahamón said. Ornithologist Juan Ignacio Areta at the Institute of Bio and Geosciences of Northwest Argentina, who did not participate in this study, wonders how preventing the birds from making feather trills might influence mate choice. "Answering these exciting questions is difficult, and requires a lot of carefully designed field experiments," he said.
The scientists detailed their findings Sept. 22 in the journal Integrative and Comparative Biology.
Charles Q. Choi is a science reporter who has written for Scientific American, The New York Times, Wired, Science, Nature, and National Geographic News, among others. Reprinted with permission from Inside Science, an editorially independent news product of the American Institute of Physics, a nonprofit organization dedicated to advancing, promoting and serving the physical sciences.