Hundreds of jackdaws roosting in trees coordinate when to take off together by squawking until the noise is loud enough to signal a consensus
A crescendo of squawks from hundreds of roosting jackdaws may help the flock reach a “democratic consensus” about when to take off all at once.
“They all leave together, which is a really striking sight. The sky just suddenly fills with black birds. It’s like a black snowstorm,” says Alex Thornton at the University of Exeter, UK.
Jackdaws (Corvus monedula), a close relative of crows, roost overnight in huge numbers and then split up into smaller groups during the day to feed in different areas.
Thornton and his colleagues recorded hundreds of hours of video of six jackdaw roosts in Cornwall, UK, over a period of several months. They quantified the intensity of the birds’ calls before, during and after they took flight, then compared it with the footage of their departures.
The researchers found that the calls grew in intensity before take-off, and decided to test whether there was a causal link. By playing back recordings of these intense calls, they found they were able to hasten the birds’ departure by several minutes, while recordings of other noises had no effect.
Thornton says this is evidence that the birds are effectively casting their vote to signify they are ready to leave, and that when these calls reach a certain threshold, it is taken as a sign for the entire group to depart en masse.
“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,” he says.
They also found that the final intensity of the calls just before taking flight correlated with how cohesive the departure was. Most of the time, the birds took off as one early in the morning, with hundreds getting into the air within around 4 seconds of each other.
On rare occasions when the intensity of calling didn’t build up enough, the jackdaws seemed to fail to reach a consensus, and instead took off in “dribs and drabs”, says Thornton.
The researchers think sticking together has advantages, such as reducing the risk of predation and pointing each other to food sources.
The roosts studied varied in size from 160 birds to almost 1500. Thornton says that some of these roosts still exist on the same spot where they were recorded in the Domesday Book in 1086.
Journal reference: Current Biology, DOI: 10.1016/j.cub.2022.04.032
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