Larger flocks tend to have wobbly shapes (Image: Amir Cohen/Reuters)

Flocks of birds can get only so big before falling apart. A new study shows that flocks of different sizes behave differently, and could help explain how animals like birds and fish coordinate their movements.

Inspired to study the starlings flying around their city, Andrea Cavagna of the Sapienza University in Rome and colleagues had previously shown that in large flocks, changes in distance between members propagate rather like sound waves. Now the team has found that changes in flying direction also spread like a wave, but dampen more quickly – as if moving through mud rather than air.

They examined 3D datasets of flocks of starlings flying and turning, and found that while sound-like waves spread best in bigger flocks, the waves transmitting information about direction spread best in smaller flocks.


Cavagna and colleagues found their models showed up a range of medium-sized flocks that were too small for traditional density waves to spread, and too large for direction changes to spread, making any sort of coordination between members impossible. “Collective behaviour requires that the system is able to propagate information successfully across the group,” says Cavagna.

Such “silent flocks” don’t exist in nature, yet could provide insight into why flocks behave the way they do, he says. For example large flocks of around 10,000 birds tend to have softer outlines: one part may change direction, but the rest of the flock does not, leading to a wobbling appearance. Flocks of 1000 birds or less behave more rigidly as direction changes quickly spread across the entire flock.

Cavagna says the theory may also apply to animals other than birds. The exact size limit of a flock, school or herd depends on the specifics of the animals involved, such as how strongly individuals align with their neighbours, and how fast each can respond to the others’ changes in behaviour.

“I think it is interesting because it identifies purely physical mechanisms for the propagation of information across the flock,” says Cristina Marchetti of Syracuse University in New York. “More importantly, it imposes constraints on such a propagation, which imply constraints on the size of the flock.”

Journal reference: Physical Review Letters, 10.1103/PhysRevLett.114.218101