Australian readers – and enthusiastic bird-watchers who have visited Australia – may well be familiar with the crested pigeon (Ocyphaps lophotes), an attractive little bird common across the southern half of the continent.
There are two things distinctive about the species. The first, not surprisingly, is the prominent crest on its head. The second is the loud, high-pitched, repetitive whistling sound it makes when it flies.
Ornithologists have long been perplexed about the origin of the noise, which is audible from a considerable distance away, because it is does not emanate from the bird’s beak.
Now however, a trio of researchers from the Australian National University in Canberra have solved the mystery. The noise is produced by a highly modified wing feather.
In a paper published in the journal Current Biology, Trevor Murray, Jochen Zeil and Robert Magrath reveal that the crested pigeon has a very unusual eighth primary wing feather which produces high note when the bird starts to fly.
The researchers acknowledge that noise-making feathers are not uncommon among the world’s bird species, and some research estimates they evolved independently more that 70 times.
However, Murray and his colleagues are the first team to demonstrate that the adaptation, in crested pigeons at least, is specifically a means of non-verbal communication.
The pigeons use the noise produced by the modified feather as an alarm signal, warning other birds of the presence of predators. The sound changes in accordance with wing beat frequency, increasing in pitch as the flight urgency develops.
To make their finding, the team recorded the sound made by the oscillating eighth primary feather – a tone at 2.9 kilohertz – and played it back to birds in conditions where no other possible danger cues were present. When the sound was made, the crested pigeons all took flight.
A control experiment using noises produced by the seventh and ninth flight feathers did not produce the same results.
Murray and his colleagues note that non-verbal noise by birds was mentioned by Charles Darwin in his book on sexual selection. He called it “instrumental music” but made no suggestion about its possible purpose.
“Our results therefore indicate, nearly 150 years after Darwin’s book,” they write, “that modified feathers can be used for non-vocal communication, and they reveal an intrinsically reliable alarm signal.”