Science

Demo-crow-cy: Jackdaws leave the roost by voting

Hundreds or even thousands of jackdaws are known to launch into the winter skies at once from the treetops. Now, ecologists know that this synchronized exodus is timed with a sophisticated behavior called “consensus decision-making,” where the group’s majority chooses the birds’ actions.

Jackdaws, Corvus monedula, pitch into the sky in huge numbers come sunrise and then split into smaller groups to feed throughout the day. To figure out how such large numbers of these birds decide when to take off, researchers in the United Kingdom recorded hundreds of hours of their bird calls in Cornwall over several months. They measured when the first jackdaws began their calls, how loud the birds were, and how quickly the swell of calls rose, and then compared these sounds to footage of those birds’ departures. The team found that the jackdaw ensemble left together once the chorus of calls reached a critical mass—the more rapidly the chorus swelled, the earlier the birds left. The findings were published on Monday in Current Biology.

“They all leave together, which is a really striking sight. The sky just suddenly fills with black birds. It’s like a black snowstorm,” Alex Thornton, an ecologist at the University of Exeter, UK, told New Scientist. Every call is a jackdaw casting its vote to leave. “At first you just get a few calls, then more and more birds join in and it builds and it builds, and the steeper that increase, the earlier they leave,” Thornton added. On rare occasions, when the intensity of the chorus doesn’t build enough and the jackdaws don’t agree, the birds instead launch off in “dribs and drabs.”

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The team also found that once jackdaws reached consensus they departed almost immediately, with hundreds of individuals taking off in less than five seconds of each other. When scientists played past recordings of calls back to the jackdaws, interfering with the group’s natural crescendo, they were able to push forward the birds’ launching by more than six minutes. The birds did not, however, change their launch times in response to other noise—which means the group is specifically listening for the calls of its members.

The gregarious birds prefer to leave as a group, although “each jackdaw will have a slightly different preference about when they want to leave, based on factors like their size and hunger,” Alex Dibnah, a University of Exeter graduate student and lead author of the study, said in a statement. “Leaving the roost together has various benefits, including safety from predators and access to information such as where to find food.”

This research shows the critical role vocalizations play in group decision-making for this species. One next step for this research, according to the authors, is to figure out how human-created noise pollution might affect this process and the spread of information throughout these communities. 

“Imagine a big roost near a town or busy road,” Thornton said to the BBC. “If the birds can’t hear each other and can’t form a consensus to leave together, it could have big impacts on their population.”



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