The capacity of birds to fly in astonishingly tight formations, turning with apparently one mind to avoid predators is among the wonders of nature. A study of jackdaws has shown how this order emerges from chaotic behavior when birds are sparse, providing guidance for the makers of aerial drones.
Like schools of fish, the collective movements of birds are remarkable because they can shift so swiftly even with all the choices three-dimensional movement allows. For this to happen either individuals need to take their cues with barely believable speed from others, or each needs to be following the same secret algorithm.
Dr Alex Thornton of the University of Exeter said in a statement: “Traditionally, it is thought that flocking… occurs when each individual follows an identical set of rules to every other in the group." The assumption has been these rules need to be consistent for a species, or how else would they know when to apply the same one as their peers?
Thornton and co-authors tracked wild jackdaws using high-speed cameras and reconstructed the movements of every bird. In Nature Communications they show the system depends on the circumstance.
"When flying to their winter roosts, jackdaws follow what we call 'topological rules' – meaning that they respond to the movements of a fixed number of neighbors, and groups remain ordered regardless of how many birds are flying together,” Thornton said. The same has been found with starlings’ murmurations, a sight so beautiful videos attract millions of views.
However, jackdaws also have a different sort of flocking, where they band together to drive predators from their nests. "Jackdaws issue an alarm call to draw other birds to the mob, and at first these groups are completely disordered,” Thornton said. “Then, when the density of birds reaches a certain threshold, it suddenly flips into an ordered, cohesive state where the birds are aligned with their neighbors and move together in an organized way.”
These defensive mobs follow what are known as “metric rules” where each bird pays attention to those around it, and takes its lead from those within a certain radius. Metric rules have previously recorded among chimney swifts seeking roost sites. Thornton compared the shift to a phase transition, such as when a disordered liquid turns to a regularly structured crystal. However, instead of loss of energy driving the change, the shift is caused by bird numbers.
The paper notes there have been hints of fish adjusting their schooling behavior depending on circumstances, but this is the first time we have seen clear evidence of animals adopting different flocking rules depending on circumstance. The need to be able to identify the appropriate behavior for the conditions may have contributed to the development of corvids’ famous intelligence.