Squirrels listen to the surrounding birds to know when they're safe

James Gorman, NYTimes, reports, Squirrels Relax When They Hear Birds Relaxing, Sept 6, 2019.

Researchers at Oberlin College reported Wednesday in the scientific journal PLoS One that squirrels pay attention not only to alarm calls, as many animals do, but also to the background chatter of birds, and that they relax a bit when the birds sound relaxed.

Of course they do. They’re squirrels. They pay attention to everything. I’d be willing to bet that they can tell the difference between an irate bird lover banging on a window in pajamas (no problem) and one with her snow boots on (better get ready to skitter away). [...]

The researchers hypothesized that the squirrels were paying attention, and over the course of a couple of years Dr. Tarvin and two undergraduates, Marie Lilly and Emma Lucore, designed and conducted an experiment to test that idea. They played a recorded screech of a red-tailed hawk, which put the squirrels on alert and caused them to act vigilant.

Then they played recordings of desultory bird chatter, or of background noise without birds. [...] Once she found a squirrel, she set up the sound system and played the different recordings: the hawk followed by bird chatter, and the hawk followed by birdless background noise. [...] The results were that when the squirrels heard the relaxed birds, they, too, relaxed. And they did so more quickly than when they heard background noise without bird chatter.

The research paper:  Lilly MV, Lucore EC, Tarvin KA (2019) Eavesdropping grey squirrels infer safety from bird chatter. PLoS ONE 14(9): e0221279. https://doi.org/10.1371/journal.pone.0221279.

Abstract: When multiple species are vulnerable to a common set of predators, it is advantageous for individuals to recognize information about the environment provided by other species. Eastern gray squirrels (Sciurus carolinensis) and other small mammals have been shown to exploit heterospecific alarm calls as indicators of danger. However, many species–especially birds—emit non-alarm auditory cues such as contact calls when perceived predator threat is low, and such public information may serve as cues of safety to eavesdroppers. We tested the hypothesis that eavesdropping gray squirrels respond to “bird chatter” (contact calls emitted by multiple individuals when not under threat of predation) as a measure of safety. We compared vigilance behavior of free-ranging squirrels in the presence of playbacks of bird chatter vs non-masking ambient background noise lacking chatter after priming them with a playback recording of a red-tailed hawk (Buteo jamaicensis) call. Squirrels responded to the hawk call playbacks by significantly increasing the proportion of time they spent engaged in vigilance behaviors and the number of times they looked up during otherwise non-vigilance behaviors, indicating that they perceived elevated predation threat prior to the playbacks of chatter or ambient noise. Following the hawk playback, squirrels exposed to the chatter treatment engaged in significantly lower levels of vigilance behavior (i.e., standing, freezing, fleeing, looking up) and the decay in vigilance behaviors was more rapid than in squirrels exposed to the ambient noise treatment, suggesting squirrels use information contained in bird chatter as a cue of safety. These findings suggest that eastern gray squirrels eavesdrop on non-alarm auditory cues as indicators of safety and adjust their vigilance level in accordance with the vigilance level of other species that share the same predators.