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")
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