If you are fed up, at daybreak, to hear incessant croaking under your windows, tell yourself that you may be attending… a referendum! Researchers have discovered that crows, the jackdaw, seek to reach a consensus before flying en masse from the treetops where they usually spend the night. To reach this conclusion, Alex Dibnah of the University of Exeter, UK, and his colleagues recorded and filmed six groups of birds nesting in Cornwall, across the English Channel. Some 130 hours of audio and 55 hours of video that allowed them to examine collective decision-making by communities of up to more than 1400 birds!
Jackdaws left their nocturnal roosts less than an hour before sunrise or shortly after, the article in the journal Current Biology on Monday explains. In almost two-thirds of the cases studied (21 out of 33 mornings), the majority or all of the individuals flew away within five seconds. The other times, the corvids took off in small groups, twenty minutes passing, at most, between the first convoy and the last. Two opposite eventualities that owe nothing to chance.
Maintain group cohesion…
The researchers found that, most often, the intensity of the caws increased during the hour before the departure of the main group of crows. Once past a certain threshold, the jackdaws left en masse, forming a swarm in the sky. Conversely, on days when this noise level was not reached, take-off took place in dribs and drabs, as if the birds had not reached a consensus.
To confirm a link between vocalizations and collective departures, the scientists used trickery. They have, in some cases, broadcast the sounds of jackdaw cries among the trees themselves! In doing so, they provoked hasty departures. When, on the other hand, they emitted recordings of the sound of the wind, the take-offs occurred at the expected time. It was therefore not the general rise in the sound level that decided the crows, but the call of the majority of their congeners.
“For the birds scattered on the trees, it is difficult to see the other members of the group, especially since they can be very numerous and it is dark, early in the morning. Voice cues are a useful way to convey information in these circumstances,” said study co-author Alex Thornton. But why this taste for “democracy”? To maintain group cohesion, answers the study: in order to reduce the risk of predation, to promote the search for food and to guarantee “better access” to potential partners.
….and overcoming differences
Interested in jackdaws for about ten years, the team from the University of Exeter quite naturally pushed their study of these birds further. “In principle, similar decision-making processes may well occur in other bird species. It would be very interesting to find out! Until now, research on the role of acoustic signals in the collective decisions of animals had only focused on bees and small groups of vertebrates, of less than fifty individuals, such as meerkats. Ants are also known to communicate in very sophisticated ways, but using pheromones instead.
“Bees and ants live in family groups where the interests of the members of the group are all aligned because they are all relatives and therefore share genes. In contrast, groups of jackdaws are made up of a wide range of individuals, from many different family groups. This means that individual preferences for when to leave the roost are likely to vary widely and there is a high potential for conflicts of interest,” says Alex Thornton. “Individuals will have different levels of hunger, which in turn will influence when they want to leave to fetch food. We were particularly interested in understanding how animals can overcome their differences and reach consensus in such circumstances. »
The team’s next work should focus on the consequences of human activities (such as light and noise pollution) on the ability of animals to make group decisions. Stars of these studies: always corvids, whose intelligence has been demonstrated by numerous studies in recent decades, but also other species of birds such as greenfinches, redwings or even starlings.