Squirrels are what biologist Keith Tarvin calls "public information exploiters". Clodagh Kilcoyne/Reuters

The skittish rodents are always listening for cues that tell them if they’re safe or not—including to the sounds of their avian friends.

The next time you’re at a park and you see a squirrel standing up and shaking its tail, look up. You might just catch a raptor flying by.

The Eastern grey squirrel is a cautious creature, with its ears constantly perked and monitoring for potential threat—be it the screech of a hawk flying above, the rapid pattering of a eager dog, or the rumbling of a oncoming car.

But that’s not all they’re listening for.

It turns out that when the songbirds sing, the squirrels are also listening. Or, more accurately, they’re eavesdropping on the casual “bird chatter” to gauge their own safety from potential predators nearby, according to a new study in the journal PLOS One.

Squirrels are what Keith Tarvin, a biologist at Oberlin College and Conservatory in Ohio who led the study, calls “public information exploiters,” meaning they often take cues from other prey animals nearby. They’re not the only ones that do this. Early animal behavior studies have shown that birds, mammals, and even fish and lizards can recognize the alarm signals of other species that share similar geographic locations and predators. Within the bird family, a nuthatch may tune into the high-pitched call of a chick-a-dee, which might also be paying attention to the panicked tweet of a tufted titmice.

Squirrels and chipmunks, meanwhile, speak bird too. The biologist Erick Greene, at the University of Montana, told NPR in 2015 that not only do squirrels recognize the seet and mob calls of birds (the former warns others of a predator, the latter draws a mob of birds to chase a predator away), but they can also mimic them to near perfection.

“Those alarm calls become public information out there for the taking,” says Tarvin. But with so much noise out there, he says responding to every warning sound—even ones that turn out to be false alarms—can become costly for the squirrels as it cuts into time used to feed or reproduce. So he and his team wanted to know if squirrels also listen for cues that signal to them their surrounding is safe and predator-free. This is where the bird chatter comes in.

Tarvin and his team began their experiment by playing each squirrel in both the experimental and control groups recordings of hawks, which heightened their vigilance behavior—things like freezing, looking up, and fleeing. After a few minutes, they played ambient noise for squirrels in the control group. The other group was played recordings of “chatter” from birds feeding in his backyard. They then observed each squirrel for three minutes, tracking how their vigilance levels changed over time.

“We counted the number of times it would move its head really quick, [as if] scanning the environment,” he says, “or freeze, which turns out to be most common response.” Those in the group that was played bird chatter indeed froze and looked up less frequently and were overall quicker to return to their normal state of vigilance. To Tarvin’s team, that indicated that these cues of safety can be just as important to squirrels as warning calls are, especially when it comes to survival.

The field is relatively new and there are a lot of questions biologists like Tarvin want answered. For example, he wants to know if squirrels tune into a specific kind of chatter. Or if urban noise might affect how prey animals rely on alarm calls from other species. He suspects it does: “If those information exploiters tend to rely on information producers, and you mask them [with noise],” he told CityLab, “they may allocate more of their own time toward self-vigilance, which leaves less time to forage and mate.”

Meanwhile, as squirrels are eavesdropping on birds, Tarvin suggests city dwellers tune in on the furry rodents. “Next time, pay attention to the squirrels,” he says.

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