When squirrels hear the shrieks of a red-tailed hawk, they shift into danger mode, alternately freezing in place, searching the skies or fleeing. But new research suggests the rodents aren’t attuned solely to avian alarms. As a trio of scientists from Ohio’s Oberlin College reports in the journal PLoS One, eastern grays rely on the cadences of everyday bird calls to sense whether threats have passed.
As Katherine J. Wu of NOVA Next reports, the researchers found that squirrels wary of predators resume their normal activities more quickly after hearing nearby birds’ casual chatter. Distinct from “all clear” alerts, these exchanges essentially act as background noise, signaling a return to normalcy for animals in the vicinity.
“There’s a lot of information in alarm calls,” explains Oberlin behavioral ecologist Keith Tarvin. “It dawned on us that cues of safety might be equally informative.”
To gauge eastern grays’ sensitivity to bird conversations, Tarvin and then-undergraduates Marie Lilly and Emma Lucore designed an experiment centered on Oberlin’s squirrel population. Per the New York Times’ James Gorman, Lilly spent several winter days biking around the city in search of squirrels. Upon spotting suitable subjects, she set up a special sound system, played a series of recordings and waited to see the rodents’ reaction.
“I did have quite a few people ask what in the world I was doing as I biked around Oberlin looking for squirrels,” Lilly says to Psychology Today’s Mary Bates. “I had two giant, repurposed cat litter buckets on my bicycle handlebars filled with sound equipment and binoculars and after setting up, I would crouch a few meters away so that my presence was not associated with the recording being played.”
Some of the 54 squirrels observed for the study heard red-tailed hawk cries succeeded by multi-species songbird calls. Others were treated to a playlist featuring hawk sounds and largely silent ambient noise.
As Gorman writes, Lilly used a customized app to track each squirrel’s actions in the three minutes following the simulated soundtrack. She classified freezing, fleeing and standing as vigilant states and foraging, preening and resting as more relaxed states.
In an interview with NPR’s Nell Greenfieldboyce, Tarvin explains that subsequent data analysis confirmed the team’s suspicions: “When squirrels are hearing chatter coming from other birds, that chatter conveys a message or a cue that apparently these birds feel pretty safe. And the squirrels apparently interpret that to mean that the environment is relatively safe.”
NOVA Next's Wu notes that the timing of such events is probably more varied in nature, where squirrels aren’t always in hearing range of birds enjoying a friendly catch-up. Still, Tarvin says, this chatter amounts to “free, public information” available to any animal in earshot; by offering up a chorus of calls, birds send out an unintentional signal to those cognitively equipped to interpret their message.
“This [study] highlights how interconnected ecosystems are,” Amanda Robin, a squirrel expert at the University of California, Los Angeles, who was not involved in the new research, tells Wu. “If you removed one species [like a bird], you might be changing the entire life of another species and not know.”
Moving forward, Nicola Davis reports for the Guardian, the researchers hope to examine whether squirrels keep an ear out for particular species and if the rodents rely mainly on bird chatter or general foraging and jostling sounds. The team also warns that rising levels of human-made noise could interfere with squirrels’ listening abilities.
Jakob Bro-Jorgensen, an evolutionary ecologist at the University of Liverpool who was not involved in the study, says the research highlights how animals can garner information from seemingly irrelevant cues.
Speaking with Davis, he concludes, “It makes you wonder how the more and more pervasive impact of human activities on natural soundscapes may compromise survival of wildlife in ways we haven’t thought of.”