Most of the time, animals have a special way of warning other members of their groups if danger is approaching. Usually, they alarm other by issuing special calls or screams, but a certain species called crested pigeons do it without using their vocal skills. Instead, the warning signal is produced by their feathers as they fly.
Crested pigeons produce high-pitched sounds when they fly
Crested pigeons have a bigger feather which produces a special kind of high-pitched call when air frictions with it. Whenever they feel threatened by a predator and they start flapping their wings more rapidly, this call becomes more alert. At first, researchers thought this is only the result of a regular flight.
However, they noticed an interesting behavior in other crested pigeons. They observed them while a specimen got chased by a predator and, upon hearing the alarm, all the other pigeons in the group flew away. Therefore, they perceived the signal and hid from danger.
The first one to suggest that birds could send alarms by non-vocal means was Charles Darwin. However, it was hard to prove it. The fact that crested pigeons flew noisily was not a mystery and, finally, researchers observed that other pigeons did pay attention to these sounds.
The loud call alarms other pigeons that danger approaches
For the study, researchers shot high-speed videos which then they could slow down, and identified two feathers which produced high and low sounds, respectively. As soon as the birds started flapping faster, the sounds went even higher in pitch. Then, they removed the feather responsible for the high call, and recorded the resulting sound.
When the other crested pigeons heard the high-pitched sound made by the respective feather, the birds fled. When they heard the sound made without the feather, they just looked cautiously around. Therefore, this mechanism of loud flying has a clear purpose, and is truly effective in helping other specimens in the group escape danger.