Crested pigeons communicate without even opening their beaks. The birds have a built-in alarm system that’s set off by fluttering feathers when flying away from danger, researchers report November 9 in Current Biology.
In animals, nonvocal sounds are not uncommon. “All animals produce sound as we move, even humans, and that sound can be useful to those that hear it,” says study coauthor Trevor Murray, a biologist at the Australian National University in Canberra.
Among birds, the go-to instruments for creating these sounds are the wings. Some birds, like Ecuador’s club-winged manakins, use wing sounds in mating rituals, while other species such as mourning doves make nonvocal sounds in times of perceived peril (SN: 7/30/05, p. 67). But whether such noises truly represent communication in the same manner that bird songs and calls do is hard to prove.
Crested pigeons (Ochyphaps lophotes) have 10 primary flight feathers on each wing. The eighth — that is, the third from the top of a bird’s extended wing — doesn’t look like a normal feather; it’s slender and oddly shaped. A 2009 study suggested that this specialized wing feather might be behind the noisy takeoffs that occur when crested pigeons sense danger.
For the new study, Murray and his colleagues used high-speed video, audio recordings and feather-removal tests to take a closer look at this feather and how it might produce sound. When air flows across the feather’s pointed tip as the bird pushes down with its wing, the feather flutters and makes a high-frequency tone. The feather below may amplify this high tone, while the feather above helps produce a low tone as the wing flaps up.
When birds are fleeing, the two alternating tones increase in tempo, “much like you make footsteps in all locomotion, but the sound of you running from a threat may be a bit louder and more rapid than in walking,” says Christopher Clark, a biologist at the University of California, Riverside who also studies nonvocal sounds in birds.
In addition to recording pigeons with and without various flight feathers, the researchers recorded takeoffs under normal conditions (top) and when escaping a fake predator at high speed (bottom). The recordings demonstrate the influence of speed on the sound so that a fast exit alone serves as a warning signal to other birds.
When pigeons heard a recording of the wing sound from a fleeing bird, they sought to flee as well. But the birds only responded to the sound when they heard the high tone, confirming that it is a true signal and that feather eight is vital to its broadcast.
There is an advantage — for both the bird producing the alarm and the birds that respond — to this strategy of flying away. “If the entire flock flees, predators are less likely to catch any prey,” Murray says.