On cold, dark winter mornings, little black crows known as jackdaws can be heard calling loudly to each other from their winter roosts in the UK before simultaneously taking off just as the sun rises. Now researchers who have studied their daily activities in unprecedented detail report evidence that these groups of hundreds of individuals rely on a “democratic” decision-making process to coordinate with each other and all fly away at the same time. The results are reported in the journal Current biology May 23.
“Like humans, large groups of animals can use decision-making processes to overcome their individual differences and reach a kind of ‘democratic’ consensus,” says Alex Thornton of the University of Exeter.
Previous studies have investigated consensus decision-making in groups of animals that are relatively small or made up of family members, he explained. What struck them about the jackdaw roosts was their size. Not only are their groups large, but they also include individuals of different ages, sexes, family groups, and colonies, all spread across the treetops.
It is unlikely that all these different people would naturally prefer to take off at precisely the same time. And yet, staying together has benefits, such as reducing the risk of predation and giving individuals information from their peers about where to find food. The researchers wanted to explore how birds decide when it’s time to leave.
To take a closer look, Thornton and colleagues, including first author Alex Dibnah, recorded hours and hours of audio and video from six different jackdaw roosts in Cornwall, UK, over the months of ‘winter. The size of the roosts varied from 160 to nearly 1,500 individuals. They quantified the loudness of bird calls before and just after taking off in flight.
Evidence shows that the time of departure is closely related to the intensity of calls in the roost. While some mornings the birds left in small groups for about 20 minutes, most days they left in droves, with hundreds of birds taking off within about 4 seconds of each other.
Most mornings, they found that the intensity of the calls increased during the hour before the largest group left. It was occasionally delayed by rain or heavy cloud cover. They concluded that changes in call intensity could serve as a reliable source of information, allowing birds to synchronize their daily takeoffs.
To confirm cause and effect, the researchers played the sounds of conspecifics calling them to see if they could get the birds to take off sooner than they would have otherwise. And they found they could. Adding calls to the mix, the birds took flight on average about 6.5 minutes earlier.
“Through their calls, jackdaws appear to effectively signal their willingness to leave, providing large groups with a means to reach consensus to make cohesive, collective departures from the roost,” the researchers write.
They say observational data indicates consensus is reached when the intensity of calls reaches a point that prompts them to act. Playback experiments help show a causal link between call intensity and takeoff. Together, they offer new insight into how animals make decisions about mass movement in nature.
It is also worth noting that, on the rare occasions when the intensity of the calls did not develop sufficiently, the birds apparently failed to reach a consensus. As a result, they took off “bit by bit” instead of all at once.
In future studies, the researchers hope to learn more about how human activities can affect these dynamics.
“As human impacts on wildlife increase, we are very interested in understanding if and how human disturbances – for example, light and noise pollution – can affect the abilities of groups of animals to communicate and reach decisions. consensual,” Thornton said.
This research was supported by the Swedish Research Council, the Human Frontier Science Program, the Whitten Lectureship in Marine Biology, the Royal Society Dorothy Hodgkin Research Fellowship and a Severo Ochoa Postdoctoral Fellowship through the Spanish Program for Centers of Excellency.
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