Sept. 22 (UPI) -- Scientists have added another species to the list of birds that use sounds made with their feathers to communicate.
The male fork-tailed flycatcher, a passerine bird species native to the American tropics, creates unique sounds by fluttering its feathers at high frequencies, according to research published Wednesday in the journal Integrative and Comparative Biology.
"Back in the 1960's, scientists noticed that they produce a distinctive sound only during a particular flight display," researcher Christopher Clark, told UPI in an email.
"And those species of flycatcher in the genus Tyrannus, those that make the most distinctive sounds have the most distinctly shaped outer primary feathers," said Clark, an associate professor of evolutionary biology at the University of California Riverside.
For the latest research, scientists conducted field studies to better understand both the mechanics of the feather fluttering and its communicative utility.
"We found that the birds don't produce sounds every time they fly, but only under specific behavioral contexts," lead study author Valentina Gomez told UPI in an email.
"One is during the pre-dawn display, after waking up. They begin displaying by singing and then they include feather songs. They also produce these sounds during territorial displays," said Gomez, a doctoral student at the University of Illinois at Chicago.
To study the fluttering's acoustic qualities, scientists captured males with mist netting, and used cameras and microphones to record the sounds the birds made as they escaped and retreated.
Fork-tailed flycatchers are quite territorial and aggressively defend their nests. Males are especially aggressive, regularly engaging in aerial battles with other males over mating opportunities and territory. They're also more than willing to attack larger birds that stray too close to their nests.
Researchers used a taxidermy hawk outfitted with microphones and a camera to measure the movement of the fork-tailed flycatcher's feathers during displays of aggression. The recordings revealed a difference in the sounds made by two subspecies, one that migrates long distances and another that is more stationary.
"Differences in migration likely influenced the shape of feathers and this affects the frequency at which they flutter," Gomez said.
In effect, the two subspecies have developed dialects. Researchers suspect this phenomenon might help drive speciation, or species divergence.
"The evolution of different movement behaviors promotes the initial trigger of the speciation process," Gomez said. "Through time, correlated evolution of morphological traits affects how they communicate."
In other words, the difference in sound-making didn't jumpstart the speciation process, but Gomez and her research partners hypothesize that the development of feather-flapping dialects works to reinforce the divergence.
Similarly, while the sound-making feathers of fork-tailed flycatchers may have initially evolved in response to pressures unrelated to communication, researchers claim the birds now utilize their sound-making abilities with intentionality. They've harnessed the power of their fluttering features for the purposes of communication.
"The birds alter how they are flapping their wings when they produce sound; their wingbeat frequency goes up by quite a bit," Clark said. "The altered kinematics is another clue that this is 'intentional.'"
While non-vocal communication has been observed in a variety of bird genus and species, scientists suspect the prevalence of the practice is underestimated.
Researchers hope that future investigations of feather-based communication among flycatchers will offer new insights into why so many birds have evolved non-vocal communication over and over again.
While the latest findings suggest the fork-tailed flycatcher relies on feather-based communication for pair bonding and displays of aggression, many questions about the fluttering's utility remain.
"We still need to learn a lot about bird acoustic perception," Gomez said.