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Bird Species Communicates by Fluttering Its Wings — And They Have Different Accents

Scientists have discovered two distinct subspecies of fork-tailed flycatchers that communicate with each other by producing a high-pitched frequency with their feathers — and exhibit regional "accents" while talking to each other.

(CN) — Scientists have discovered two distinct subspecies of fork-tailed flycatchers that communicate with each other by producing a high-pitched frequency with their feathers — and exhibit regional "accents" while talking to each other.

In this study, published Tuesday in the journal Integrative and Comparative Biology, researchers observed these birds mid-flight and recognized that those from the same migration patterns communicated most similarly. They seemed to exhibit different sounds depending on these patterns, as if speaking in “dialects” between possible subspecies.

"We already knew from past genetic analysis that the two groups are becoming different species, so we wanted to know if there were any differences in the sounds that the males produce with their wings," said lead-author Valentina Gómez-Bahamón, a researcher at Chicago's Field Museum and a doctoral student at the University of Illinois at Chicago. "We not only confirmed the way that these birds make sounds with their feathers, but that the sounds are different for the two subspecies."

Although it’s not uncommon for birds to communicate with each other by flapping their wings to make a desired noise, this was a particularly fascinating discovery because the two subspecies are in the process of separating, a phenomenon called speciation. This can occur for a number of reasons, including physical barriers like mountain ranges or human intervention, but for the flycatchers it has to do with the massive geographical area they inhabit.

The researchers believed that because of this speciation, there might be after-effects that could be seen or heard now so they set out to study the flycatchers first-hand.

This tropical bird prefers savanna ecosystems, as well as open pastures and environments with plenty of trees and bushes. They have been found all over South America, throughout Southern Mexico, and often around the Atlantic coast. They weigh about one ounce and have black heads, gray backs, and a characteristic tail that forms a foot-long scissor shape when extended. The flashy tails are useful for attracting partners and can be seen fully extended while catching insects in the air.

The male flycatchers in particular possess the feathers used to make the distinct communicative flapping sounds, which they use to attract mates.

"The other moment in which the flycatchers produce this sound, aside from fighting, is in early morning, when it's still dark, they display to the female," Gómez-Bahamón said. "They sing their song and then they are quiet for a moment and they do this little flight that we cannot see because it's still dark, but you can hear the feathers.

“So in that case, because we can't record them, we don't know how fast they're flying. But we know that it's a display in which they are not escaping predators or they are not fighting each other. It's something that is intended for the females."

Also, despite their petite size, the birds are incredibly territorial and the males often fight each other during mating season. They are also extremely protective of their nests and will fight any bird that comes near their nests, even predatory hawks.

"They produce the sounds when they fly very fast, and they fly very fast when they're fighting each other. These birds fight a lot," said Gómez-Bahamón. "They're very feisty, they're not afraid of anything."

The team suspected this high-pitched trilling was coming from the rapid fluttering of their feathers. So to test this theory, they created circumstances to make the birds produce the sound.

They would catch a bird with a mist net, a popular method for capturing live birds that consists of entrapping them in a thin, mesh fabric connected between two poles. While the bird was in reach, they documented measurements for data collection, and recorded audio and video upon releasing them, as the speed of their escape closely matched that during a midair fight. 

Additionally, to further increase their odds of hearing this sound in different situations, they planted a taxidermy hawk equipped with a surveillance camera in a field frequented by flycatchers. Upon seeing the hawk, the smaller birds would swoop down to attack, allowing them to get close observations of their sounds and movements.

Fork-tailed flycatchers are very aggressive birds, so to lure them in, the researchers set up a taxidermy hawk for the flycatchers to attack. The scientists took video and audio recordings of the birds' wing feathers to see how they produced their feather-fluttering sounds. (Photo © Valentina Gómez-Bahamón, Field Museum)

After analyzing the footage, as well as the collected closeup footage of the flycatcher’s wings, the author’s suspicions were confirmed — the high-pitched frequency did come from the flapping motions. It also confirmed that flycatchers following the same migration patterns emanated the same feather frequencies.

The findings revealed two subspecies of flycatchers: those that remain in the north region of South America year-round, and those who alternate between the south and north regions of the continent. The ones who do migrate breed down in the southern region, and fly up north during the winter, at which point both subspecies come together. 

The authors note that the two subspecies sport a few different feathers from one another. For example, males who migrate have skinnier tips at the ends of their feathers. Gómez-Bahamón suspects that this difference in feathers is likely an evolutionary development resulting from their travels. 

Furthermore, the two subspecies sound different, as if communicating in their own dialects. The team found that the migratory group makes a higher-pitched sound than the northern group. 

Gómez-Bahamón believes this difference in “dialect” could be a step towards speciation. If they evolve far enough apart from each other, they won’t be able to communicate using the wing fluttering and as a result will stop attracting mates from the other subspecies. Eventually, they would become completely separate and unable to breed with each other.

"Communication is very important for speciation. It's what determines who you're going to mate with. They get all this information from the birds of the same subspecies, and they use that information to make this choice,” Gómez-Bahamón explained. “That's the most important thing for animals, choosing a mate and having offspring. So I'm really proud of this study, because I like seeing how different ecological strategies, like migration, can indirectly affect communication signals. I think that's super cool."

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