When birds make noise, it's not always with their throats. In hummingbirds and manikins, for example, special feathers flutter and vibrate to produce tones and whistles, which impress potential mates and scare off competitors. Now researchers have found that pigeons use wing noise to warn the flock about approaching enemies--the first example of a nonvocalized alarm call in birds.
Not all birds make vocal calls to alert companions, so behavioral ecologist Robert Magrath of the Australian National University in Canberra wondered whether they use flight sounds instead. He reasoned that because a bird takes off faster and at a steeper angle when startled, these different wing mechanics might produce a unique sound that other birds recognize as distress.
Magrath and his student decided to study the crested pigeon (Ocyphaps lophotes)--a more handsome relative of the street pigeon--because it produces a distinctive fluttering whistle on takeoff. They set up 13 feeder stations and recorded normal takeoff sounds. To collect sounds of a startled ascent, the researchers waited for a solitary bird to start feeding and then threw a glider that looked like a hawk to scare the pigeon. Compared with the sounds of a pigeon's normal, leisurely takeoff, this whistle was louder and faster. (Hear both, in order, here .) "You get this sense of urgency when you listen to it," Magrath says.
The researchers then played the two types of sounds for pigeon flocks at their feeders. None of the 15 flocks did anything when they heard the normal takeoff noise. In contrast, 11 of 15 flocks immediately flew off after hearing the startled takeoff noise.
Because the alarm sound is naturally louder than the normal takeoff sound, the researchers also adjusted the volume of both recordings. When they made the normal takeoff sound louder, the birds just kept eating; when they turned down the volume of the startled takeoff sound, three of 15 flocks fled and the rest perked up and looked around for danger. Thus, the startled whistle's faster tempo, not its volume, encodes the alarm call, the team reports  online today in the Proceedings of the Royal Society B.
The source of the alarm noise may be a narrow outer feather on the pigeon's wing, Magrath says. He hypothesizes that this feather may have shrunk over time to accentuate the sound the birds make during a startled takeoff. But because the whistle's role hasn't been fully characterized, the alarm could just be an additional benefit of some other evolved signal, such as a call to attract mates or to tell the flock which way to turn during flight, Magrath says.
The findings are "the best demonstration yet that such a nonvocal signal functions as an alarm," says behavioral ecologist William Searcy of the University of Miami in Florida. Kimberly Bostwick, an evolutionary ornithologist at Cornell University, says that about half of all pigeon and related dove species have a similarly modified feather: "[These wing whistles] are probably widespread across doves as an alarm call."