LISTEN: Hear birds communicate without using their beaks

Like humans using body language to send non-verbal signals, some bird species communicate without opening their beaks.

The fork-tailed flycatcher is among the birds that rustles its feathers or flaps its wings to send messages during life's biggest moments, whether it is fighting and mating.

Beyond being a quirky communication tool for the birds, this technique is also useful for researchers who want to learn more about these species.

In fact, detecting subtle differences in the sound from a rustling fork-tailed flycatcher has revealed two subspecies of the bird have different 'accents.'

The findings were published in the journal Integrative and Comparative Biology.

The fork-tailed flycatcher lives in the American tropics. As its name suggests, the bird's tail-feathers split into two long, elegant wisps. The side and shape of those wisps, it turns out, affects the sound the birds' feathers make.

Researchers recorded these sounds and charted the differences and similarities between different birds. By listening to the fluttering sounds, called sonations, researchers discovered that the two subspecies of fork-tailed flycatcher, Tyrannus savana, sound a little different.

The subspecies were previously discovered, distinguished by their different migration patterns. The subspecies are:

• Tyrannus savana savana, a migratory subspecies
• Tyrannus savana monachus, which lives year-round in the northern part of South America

Take a listen — here is T.s.monachus, the migratory subspecies:

And here is T.s.savana:

The migratory subspecies of fork-tailed flycatcher breeds in the southern part of South America, but flies north in the winter, when the two subspecies live together. But in the migratory subspecies, males' wing feathers are shaped differently to their residential counterparts: The tips of the feathers are skinnier.

Because of that, when these two subspecies flutter their feathers, the noise they make is ever so slightly different, too.

Valentina Gómez-Bahamón, a researcher at the Field Museum in Chicago, led the study. Gómez-Bahamón explains that the new findings help to confirm the differences between the two subspecies of fork-tailed flycatcher.

"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," Gómez-Bahamón said in a statement.

"We not only confirmed the way that these birds make sounds with their feathers, but that the sounds are different for the two subspecies."

When animals use structures other than their vocal apparatus to communicate, the resulting sound is known as a sonation. The fork-tailed flycatcher's sonations are typically used while the animals are fighting or calling to a mate. The sound comes from the birds' outer primary feathers — the large feathers that help them fly.

With the new finding, researchers identify a different dialect of these ruffle-based communications.

Researchers have looked at bird song dialects before. For instance, a previous study showed how a specific sparrow song variation spread across North America, shared between individuals that spent their winters together.

In the new research, it seems birds may also hew to certain dialectical characteristics without even opening their beaks.

"I like seeing how different ecological strategies, like migration, can indirectly affect communication signals," Gómez-Bahamón noted. "I think that's super cool."

Abstract: Sonations are sounds that animals produce with structures other than the vocal apparatus for communication. In birds, many sonations are usually produced with modified flight feathers through diverse kinematic mechanisms. For instance, aeroelastic fluttering of feathers produces tonal sound when airflow exceeds a threshold velocity and induces flight feathers to oscillate at a constant frequency. The Fork-tailed Flycatcher (Tyrannus savana) is a Neotropical bird with both migratory and year-round resident subspecies that differ in the shape of the outer primary feathers of their wings. By integrating behavioral observations, audio recordings and high-speed videos, we find that male Fork-tailed flycatchers produce sonations with their outer primary feathers P8-10, and possibly P7. These sounds are produced during different behavioral contexts including: the pre-dawn display, intraspecific territorial disputes, when attacking potential nest predators, and when escaping. By placing feathers in a wind tunnel, we elicited flutter at frequencies that matched the acoustic signature of sounds recorded in the wild, indicating that the kinematic mechanism responsible for sound production is aeroelastic flutter. Video of wild birds indicated that sonations were produced during the downstroke. Finally, the feathers of migratory (T.s.savana) and year-round resident (T.s.monachus) Fork-tailed flycatchers flutter in feather locations that differ in shape between the subspecies, and these shape differences between the subspecies result in sounds produced at different frequencies.