Squirrels Eavesdrop on Bird Chatter as Sign of Safety

Gray squirrel.

Source: Charles J. Sharp, via Wikimedia Commons. Distributed under a CC BY-SA 4.0 license.

Eastern gray squirrels (Sciurus carolinensis) listen in to birds’ conversations for cues about nearby predator risk, according to a study by Oberlin College scientists published in the journal PLOS ONE.

Squirrels and other animals have been shown to exploit the alarm calls of other species as signals of danger. While alarm calls are informative about predation risk, it occurred to biologist Keith Tarvin that a lack of alarm calls from a group of animals that would likely give alarm calls if they did detect a predator could also provide information to a listener.

For instance, many songbirds produce alarm calls in response to potential danger, such as a nearby bird of prey, but they also emit contact calls to each other when there is no imminent threat. This “chatter” from multiple bird species could therefore be a useful cue to other animals to relax.

“If several birds are foraging within an area, they represent several pairs of eyes and ears that are all tuned in to predator detection,” reports Tarvin. “It seemed to us that such groups would produce reliable information about predation risk that would be freely available to any other animal that was able to recognize and interpret it.”

Scaring Squirrels

Tarvin and his colleagues tested whether eavesdropping gray squirrels respond to bird chatter as a measure of safety. First, they played back a recording of the call of a red-tailed hawk (Buteo jamaicensis), then they followed it with a playback of either bird chatter or ambient background noise lacking chatter.

“I did have quite a few people ask what in the world I was doing as I biked around Oberlin looking for squirrels,” says Marie Lilly, who conducted the field work. “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.”

Gray squirrel.

Source: Rhododendrites, via Wikimedia Commons. Distributed under a CC BY-SA 4.0 license.

The researchers found that the squirrels showed an increase in vigilance behaviors, such as freezing, looking up, or fleeing, after they heard the hawk call. However, squirrels that heard the bird chatter afterward performed fewer vigilance behaviors and returned to normal levels of watchfulness more quickly than squirrels that were not played bird chatter after the hawk call.

This suggests that the squirrels are able to eavesdrop on the casual chatter of many bird species as an indicator of safety, allowing them to quickly get back to normal behaviors like foraging rather than remaining on high alert after a threat has passed.

Inter-Species Eavesdropping

Many animals pay attention to alarm calls produced by other species, but we know less about responses to non-alarm cues of safety. Tarvin says that scientists have studied attendance to auditory non-alarm cues of safety in at least six species of birds, mammals, and even a frog. In most of those cases, the eavesdropping species attended to specific “all-clear” or “sentinel” calls emitted by the calling species, but in some cases, they seemed to attend to more general sounds.

However, in all those instances, the eavesdropping and calling species had tight ecological relationships, meaning they consistently move around in co-foraging groups, or in the case of the frogs, they occupy the same limited space when calling for mates.

“In each of those cases, the eavesdroppers probably 'know' the calling species fairly well, at least in the sense that they have myriad opportunities to learn the sounds the caller makes and evaluate their reliability,” reports Tarvin. “Although the squirrels in our study probably encounter the chattering species periodically, they don’t move around with them in co-foraging groups or otherwise share tight ecological relationships with them.”

This study suggests that “public information networks” may be more widespread in ecological communities than previously recognized. Tarvin says such information networks may increase the success of the species that occupy those communities by reducing predation risk and reducing the degree of vigilance required to be safe, thereby allowing individuals to allocate more time to foraging, finding mates, and rearing offspring.