Bird
Chatter Acts as Signal for Safety
For
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