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Saturday, September 21, 2019

Eavesdropping Squirrels

Bird Chatter Acts as Signal for Safety

snacks nom nom nom GIF by CheezburgerFor the small mammals foraging on the forest floor, reading your environment is essential to survival. 

When a hawk calls, a gray squirrel (Sciurus carolinensis) will stand tall and scan the trees. 

Now researchers in the U.S. have found the squirrels are also eavesdropping on the conversations of birds, who are unwittingly letting them know when they should flee, or continue searching for seeds.

“We knew that squirrels eavesdropped on the alarm calls of some bird species, but we were excited to find that they also eavesdrop on non-alarm sounds that indicate the birds feel relatively safe,” say the study authors from Oberlin College.

The researchers played their local squirrels a recording of a red-tailed hawk call. In the three minutes following, some squirrels were presented with a recording of bird chatter, while others listened to only ambient background noise.

An app was designed and customized for the project, enabling the scientists to record squirrels movements with timestamps, providing them with a timeline of observed behaviors — before, during, and after the hawk call.

They found squirrels exposed to the chatter of song birds exhibited fewer signs of vigilance in the ‘post-hawk period,’ compared with those who listened to the relative quiet of distant traffic and river sounds.

Although gray squirrels share a habitat with song birds, it’s not an intimate friendship, and they’re not known to follow a specific bird species while foraging.

“This is a big difference between the kind of eavesdropping that we report on in our paper and the kind that other researchers have reported on from other species that attend to cues of safety,” says Professor Keith Tarvin, one of the authors of the study.

“In those other cases, the eavesdroppers and the ‘callers’ tend to move around together, which provides the eavesdroppers with many opportunities to learn the sounds made by the callers and learn to associate those calls with relative safety.”

So how did they learn to listen in to these so-called community informants? The researchers have hypothesized that it could be a trick of evolution.

“I think that the squirrels are able to reduce the costs of being vigilant if they can exploit the information produced by the birds,” says Professor Tarvin.

“If squirrels can ‘interpret’ the information about safety, then perhaps they won’t have to allocate quite as much time or energy toward being self-vigilant… That would allow them to re-allocate that time and energy toward gathering more food per hour, seeking mates, caring for offspring, preening and other self-maintenance behaviors.”

While the study doesn’t directly address the issue, the authors do point out the potential for noise pollution to disrupt species that rely on the sounds of their ecosystem to get by.

“If bird chatter were masked by anthropogenic noise, this publicly available safety cue could be lost to the network of eavesdroppers.”

The findings are published in the journal PLoS ONE.
M.V. Lilly et al. 2019. Eavesdropping grey squirrels infer safety from bird chatter. PLoS ONE 14 (9): e0221279; doi: 10.1371/journal.pone.0221279