Squirrels eavesdrop on the casual chitchat of birds to figure out when it's safe enough to be out in the open and foraging for food.
Researchers have found that a squirrel becomes incredibly vigilant when it hears the shriek of a red-tailed hawk, but it will relax and resume its food-seeking behavior more quickly if the predator's call is immediately followed by the easygoing tweets of unconcerned birds.
The findings, described in the journal PLOS One, add to a growing body of research that animals take advantage of all available "public information" when trying to assess threats in their environment.
"Lots of animals listen in on the alarm calls of other species," says Keith Tarvin, a behavioral ecologist at Oberlin College in Ohio. "This has been found in a variety of squirrels — ground squirrels, tree squirrels. It's been found in monkeys. It's been found even in lizards."
While other researchers have focused on how animals tune into other species for warnings of danger, his team focused on whether animals also watch their neighbors for signs of safety. "I wanted to know if they might be paying attention to other information, as well," says Emma Lucore, one of Tarvin's former students.
Her idea for a study of how squirrels respond to lighthearted bird tweets got picked up and expanded by fellow former student Marie Lilly, who did the fieldwork one cold January in Oberlin.
"It definitely did look very ridiculous," says Lilly, who explains that she set up audio equipment in a couple of repurposed cat litter buckets. "And I put those on my bicycle, and basically rode around town looking for squirrels."
Once she found a suitable eastern gray squirrel, she'd set up her equipment and hide behind a bush to perform the experiment.
First, the squirrel would hear the recorded cry of a hawk. Then, the squirrel would hear either several minutes of casual bird talk that had been recorded around a bird feeder, or several minutes of silence that was recorded around the same feeder at night.
Using a special app created by a computer science friend, Lilly logged the amount of time that the squirrel spent either freezing, foraging, fleeing, resting, or standing. She also recorded whenever the squirrel looked up at the sky.
"In the field, I couldn't tell if their vigilance level was going down or not because I was just recording their raw behavior," says Lilly.
But once the data had been collected and analyzed, it was obvious that hearing pleasant, relaxed bird chatter made the squirrels resume foraging more quickly.
"When squirrels are hearing chatter coming from other birds, that chatter conveys a message or a cue that apparently these birds feel pretty safe," says Tarvin. "And the squirrels apparently interpret that to mean that the environment is relatively safe."
Daniel Blumstein, a behavioral ecologist at the University of California, Los Angeles, says this is a great study with convincing results.
"Most of us have been thinking about the risky side of things, not the safety side of things," Blumstein says. "Yet both sorts of public information are out there for the taking if you know what to clue in on."
He says researchers have long focused on how individual species respond to other individual species. But increasingly, people are thinking about more complex communication networks in nature, which Blumstein describes as being sort of like "Facebook for birds."
He's recently been doing some modeling work that suggests cues of safety from other species "can be really important in certain situations for allowing animals to better estimate the likelihood that they're going to be able to engage in things without being killed."
MARY LOUISE KELLY, HOST:
Birds constantly tweet and chirp. They are talking to each other. What they might not realize is they are surrounded by eavesdroppers - squirrels. NPR's Nell Greenfieldboyce reports.
NELL GREENFIELDBOYCE, BYLINE: A squirrel would make a tasty snack for a predator like a hawk, so squirrels have to watch out.
KEITH TARVIN: They're vulnerable critters, and I think that they tend to be on a pretty high level of alert most of the time.
GREENFIELDBOYCE: Keith Tarvin is a behavioral ecologist at Oberlin College in Ohio. He says it's been known that squirrels pay attention to the alarm calls birds make when they spot a predator, and that's not so unusual.
TARVIN: Lots of animals listen in on the alarm calls of other species. This has been found in a variety of squirrels - ground squirrels, tree squirrels. It's been found in monkeys, even in lizards.
GREENFIELDBOYCE: He wondered if the squirrels also listen for other kinds of bird calls, ones that might mean safety. So he and a couple of students came up with an experiment. Marie Lilly did the fieldwork one cold January. She says she loaded some audio equipment into two repurposed cat litter boxes.
MARIE LILLY: And I put those on my bicycle and basically rode around town looking for squirrels.
GREENFIELDBOYCE: When she found one, she'd first play a recording of a red-tailed hawk.
(SOUNDBITE OF RED-TAILED HAWK SCREECHING)
GREENFIELDBOYCE: This definitely got the squirrel's attention. Then she'd play one of two recordings - either three minutes of bird-free outdoor sounds or three minutes of happy, peaceful bird chatter.
(SOUNDBITE OF BIRDS CHATTERING)
GREENFIELDBOYCE: She watched the squirrels and logged how much time they spent freezing, foraging, relaxing or looking up at the sky. The results appear in the journal PLOS ONE. Tarvin says squirrels clearly relaxed and went back to foraging much more quickly if they heard birds chattering.
TARVIN: That chatter conveys a cue that apparently these birds feel pretty safe.
GREENFIELDBOYCE: This study convinced Dan Blumstein - he's a behavioral ecologist at the University of California, Los Angeles. He says this is a new way of thinking about what animals can learn from other species. Most of the research has focused on alarm calls.
DAN BLUMSTEIN: Most of us have been thinking about the risky side of things, not the safety side of things, yet both sorts of public information are out there for the taking if you know what to clue in on.
GREENFIELDBOYCE: He says nature seems to have communication networks, almost like Facebook, that let animals get vital information from their neighbors.
Nell Greenfieldboyce, NPR News.
(SOUNDBITE OF MATT ULERY'S "RESILIN") Transcript provided by NPR, Copyright NPR.