When birds squawk, other species seem to listen
May 19, 2015
Scientists believe bird calls are a sophisticated early-warning system alerting birds and other prey over wide areas to the presence and size of predators.
In the backyard of a woodsy home outside Missoula, Montana, small birds – black-capped chickadees, mountain chickadees, red-breasted nuthatches – flitted to and from the yard’s feeder. They were oblivious to a curious stand nearby, topped by a curtain that was painted to resemble bark.
Erick Greene, a professor of biology at the University of Montana, stepped away from the stand and stood by the home’s back door. He pressed the fob of a modified garage-door opener. The curtain dropped, unveiling a northern pygmy owl preserved by taxidermy. Its robotic head moved from side to side, as if scanning for its next meal.
The yard hushed, then erupted in sound. Soon birds arrived from throughout the neighbourhood to ornament the branches of a hawthorn above the mobbed owl, calling out “yank-yank” and “chick-a-dee”.
As a recorder captured the ruckus, its instigator grinned with delight. “For birds, this is like a riot,” Greene said afterward, adding that he heard “a whole set of acoustic stuff going on that’s just associated with predators”. The distinctions are subtle – “even good naturalists and birders can miss this stuff,” he added.
Studies in recent years by many researchers, including Greene, have shown that animals such as birds, mammals and even fish recognise the alarm signals of other species. Some can even eavesdrop on one another across classes. Red-breasted nuthatches listen to chickadees. Dozens of birds listen to tufted titmice, who act like the forest’s crossing guards. Squirrels and chipmunks eavesdrop on birds, sometimes adding their own thoughts. In Africa, vervet monkeys recognise predator alarm calls by Superb starlings.
Greene wants to better understand the nuances of these bird alarms. His hunch is that birds are saying much more than we ever suspected, and that species have evolved to decode and understand the signals. He acknowledged the obvious Dr Dolittle comparison: “We’re trying to understand this sort of ‘language’ of the forest.”
At his laboratory on campus, Greene, 57, plugged the recording of the pygmy owl fracas into a computer that he likened to an “acoustic microscope”. The calls appeared as a spectrogram – essentially musical notation. On the screen, they looked like a densely layered cake fallen on its side. One call may last only a second, but can have up to a dozen syllables. Parsing one of myriad encounters with a pygmy owl or other robo-raptors, even with the help of a computer, will take the researchers hours.
“It’s cutting-edge stuff,” said Jesse Barber, an assistant professor at Boise State University who studies animal acoustics. Greene is looking at communication “across large swaths of habitat, and this is really where the field has yet to go,” Barber said. “It’s a new frontier for animal communication work.”
Greene developed his fascination with birds and sound early on, growing up around Montreal as a “total nature nerd,” he said. As a boy, he listened to and played classical, jazz and Renaissance music. He recalled being “a harpsichord-playing, hockey thug, bird nerd.”
As a teenager, he met Peter and Rosemary Grant, then at McGill University in Montreal, whose studies of Darwin’s finches in the Galapagos Islands were groundbreaking. Offered a year-long job as their field assistant, he dropped out of high school and never returned.
That experience helped him gain admission to Dalhousie University in Nova Scotia. There he spent much time playing obscure Renaissance instruments like the crumhorn – “which sounds like a pig being slaughtered,” he said – before attending Princeton for his doctorate in ecology, evolution and behaviour.
“What I’m doing now is really a natural marriage of those sorts of interests,” Greene said of his interest in animal communication. “It’s nature’s music, in a way.”
He and his wife, Anne, met before college while studying birds 800 miles north of the Arctic Circle. Theirs is a science family: Anne teaches science writing at the university, and the couple has two grown daughters working in the field – one teaches at a charter school in Brooklyn that has an environmental-sciences theme, and the other is working towards a master’s degree in aquatic biology.
Greene has spent much of his career at the University of Montana studying the pas de deux of predator and prey. As part of this dance, most animals, including birds, have evolved alarm signals to warn of danger. Greene’s interest in the subtlest bird alarms developed several years ago while studying lazuli buntings.
The buntings occasionally stopped responding to the artificial calls he broadcast and instead dived into the bushes. “And then maybe four, five minutes later, a Cooper’s hawk” – a major predator of small birds – “would cruise by,” he said. Clearly, some signal was spreading among them.
So-called “seet” calls, peeps produced by many small songbirds in response to a raptor on the wing, are well-known to ornithologists. Conventional wisdom held that the calls dissipated quickly and were produced only for other birds nearby. However, that’s not what Greene noticed: chatter sweeping across the hillside, then birds diving into bushes.
Studying the phenomenon, he documented a “distant early-warning system” among the birds in which the alarm calls were picked up by other birds and passed through the forest at more than 45km/h. Greene likened it to a bucket brigade at a fire.
The information rippled ahead of a predator minutes before it flew overhead, giving prey time to hide. Moreover, while raptors can hear well at low frequencies, they are not very good at hearing at six to 10 kilohertz, the higher frequency at which seet calls are produced. “So it’s sort of a private channel,” he said.
Greene turned to chickadees, which are highly attuned to threats. When one sees a perched raptor nearby, it will issue its well-known “chick-a-dee” call, a loud, frequent and harsh sound known as a mobbing call because its goal is to attract other birds to harass the predator until it departs.
In 2005, Greene was an author of an article in the journal Science that demonstrated how black-capped chickadees embed information about the size of predators into these calls. When faced with a high-threat raptor perched nearby, the birds not only call more frequently, they also attach more dee’s to their call.
Raptors tend to be the biggest threat to birds nearest their own size because they can match the manoeuvrability of their prey. So a large goshawk might only merit a chick-a-dee-dee from a nimble chickadee, while that little pygmy owl will elicit a chick-a-dee followed by five or even 10 or 12 additional dee syllables, Greene said.
The researchers next showed that red-breasted nuthatches, which are chickadee-size and frequently flock with them in the winter, eavesdrop on their alarm language, too.
Greene, working with a student, has also found that “squirrels understand ‘bird-ese’, and birds understand ‘squirrel-ese'”. When red squirrels hear a call announcing a dangerous raptor in the air, or they see such a raptor, they will give calls that are acoustically “almost identical” to the birds, Greene said. (Researchers have found that eastern chipmunks are attuned to mobbing calls by the eastern tufted titmouse, a cousin of the chickadee.)
Other researchers study bird calls just as intently. Katie Sieving, a professor of wildlife ecology and conservation at the University of Florida, has found that tufted titmice act like “crossing guards” and that other birds hold back from entering hazardous open areas in a forest if the titmice sound any alarm. Sieving suspects that the communication in the forest is akin to an early party telephone line, with many animals talking and even more listening in – perhaps not always grasping a lot, but often just enough.
Greene says he wants to know not only the nuances of that party-line conversation, but also how far it stretches across the landscape – and who else is listening.
If chickadees indeed issue alarm calls that indicate the size and thus the danger of their predators, how many other species of birds – robins, crows – hear and evaluate those alarms based on their own body size? Perhaps a big Steller’s jay hears a chickadee’s frantic alarm in the face of a little pygmy owl and says, in effect, “I’m not worried”, Greene said.
Conversely, does the same jay hear a half-hearted chickadee alarm and suddenly perk up, understanding that this means a threat now lurks nearby for a bigger bird?
Here is where the stuffed animals come in. The researchers are using predators of different sizes – the owl, Cooper’s hawks, sharp-shinned hawks, goshawks – to elicit responses. Back at the lab, Greene pointed to the alarm call on the spectrogram in response to the pygmy owl.
“All of these notes are acoustically very different, and they might have different meanings,” Greene said. “Sound humans hear simply as ‘chick-a-dee’ actually could contain information differentiating between a Cooper’s hawk and a pygmy owl. We know birds hear this as if it’s slowed down,” he said.