Birds are well-known for their vocalizations, but this isn’t their only way to produce noise. A new study on crested pigeons finds that the birds’ wings produce distinctive high and low notes during take-off. A low note takes place during each upstroke, and a high note is heard during the downstroke. A major source of the noise is the highly modified P8 feather. When airflow over the feather is fast enough, it sets off twisting and torsion in the feather through aeroelastic flutter. It’s this vibration that causes the noise. By playing back the notes at different speeds, researchers found that the crested pigeons use the notes’ timing as an alarm. When the cycle of high and low repeats in quick succession, they respond by taking off to escape the perceived danger.
Other bird species are also known to use aeroelastic flutter to make noise. Check out these hummingbirds, which use flutter in their mating displays. (Video credit: Science; research credit: T. Murray et al.)