Whistling feathers sound predator alarm
An Australian species of pigeon produces a whistling sound with its wings to prompt others in the flock to flee from danger, researchers have found.
Birds hang around in a flock rather than on their own because there are more individuals looking out for predators, says Magrath: "There are more eyes looking for danger."
But if one individual in a large group spots a predator, how does it communicate this to the rest?
While some bird species use vocal alarms to warn the rest of the flock, not all do.
One that doesn't is the Australian crested pigeon, Ocyphaps lophotes.
The bird has a very curious whistling sound when it flies, which seems to change when it flies off in alarm, says Magrath.
"You can close your eyes and know when a crested pigeon is flying around," he says.
Magrath says the whistling sound is so loud some might suspect it was made vocally, but it is actually made by the pigeons' wings.
"In fact one of their common names is the whistle-winged pigeon," he says.
Magrath and Hingee hypothesised that the crested pigeon might be using its whistling feathers as an alarm.
To test this, they made model hawks which they used to scare flocks of crested pigeons.
They recorded the whistling sound made by pigeons taking off in alarm and compared this to a recording of the whistling sound made by birds taking off routinely.
Magrath and Hingee then analysed the characteristics of the different wing whistles on computer.
They found that each downbeat and upbeat of the wings produced different tones but the alarm signal was faster and louder than the routine flight sound.
The next step was to independently test whether the birds responded differently to these two different sounds.
Magrath and Hingee recorded the alarm and routine flight sounds onto CD and then played them back to flocks of birds.
"If you played back the routine whistle they just kept on feeding as if nothing had happened," says Magrath.
"If you played back the alarm whistle they just shot off - 80% of the flocks just went for the hills. Clearly the birds are playing attention to these whistles."
The researchers now want to look more closely at how the wings produce their alarm sound.
Magrath says there is a special feather on the wings that appears to be key to the whistles.
This feather amplifies the differences between slow and fast wing flapping, turning a routine whistle into an alarm whistle, he says.
Magrath says future tests will involve the use of high speed photography to try and associate particular wing strokes with particular sounds.
Other tests will place individual feathers in a wind tunnel to see if the alarm whistle can be recreated, he says.