It is quite remarkable to witness hundreds, even thousands, of jackdaws simultaneously taking flight. Around sunrise on winter mornings, the birds—found across Europe, North Africa and western Asia—leave their roost all at once, creating large, noisy masses in the sky.
In a study published last month in Current Biology, researchers concluded that the birds use vocalizations to “vote” on when to depart. Their distinctive calls sound something like “tchaw, tchaw” or “tchack, tchack.”
“When a bird calls, it’s casting a vote or signaling it wants to leave,” Alex Thornton, an ecologist at the University of Exeter, U.K., told BBC News’ Georgina Rannard in May. When the calling reaches a “sufficient level,” they take off, Alex Dibnah, a researcher also at the University of Exeter who led the study, said in a statement.
“They all leave together, which is a really striking sight. The sky just suddenly fills with black birds. It’s like a black snowstorm,” Thornton told New Scientist’s Matthew Sparkes. “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.”
Jackdaws are members of the Corvidae (corvid) family, which includes crows, ravens, jays and magpies, and practice complex behaviors like food sharing, which is rare even in primates.
The researchers collected sound and video data from six jackdaw roosts in Cornwall, U.K., across two winters. They found that the mass departures happened between 45 minutes before sunrise and 15 minutes after. The birds seemed to agree on leaving as the collective noise reached its highest intensity, since most birds left when “voting” was at its loudest. All birds left within an average of five seconds of each other, according to the study. The scientists were able to push mass departure times forward more than six minutes by playing recordings of calls, but other noise recordings, like the sound of a whipping wind, had no effect on the time the birds took off, showing that the birds responded specifically to the sound of their birdcall.
Sometimes, the birds failed to reach a consensus and departed in smaller groups, per the statement.
“After roosting in a large group at night, each jackdaw will have a slightly different preference about when they want to leave, based on factors like their size and hunger,” said Dibnah in the statement. “However, it’s useful to reach a consensus. Leaving the roost together has various benefits, including safety from predators and access to information such as where to find food.”
Because jackdaws rely on sound to communicate with each other, researchers say human activities may hinder these interactions.
“Our findings provide further evidence that vocalizations are really fundamental in allowing some species to reach group decisions – so we need to investigate what happens when we as humans create noise pollution that might influence how information spreads through these social groups,” Thornton said in the statement. “The next stage of our research will look into this.”