Marvellous murmurations

The following is an extract from Animal Behaviour: A Very Short Introduction by Tristram D. Wyatt and explains in detail how mumurations occur.

Shortly before sunset, especially in winter from October to February, flocks of tens of thousands of European starlings (Sturnus vulgaris) fly in aerobatic displays called murmurations. The flocks swirl and morph, transforming from, for example, a teardrop shape into a vortex, and then into a long rope. The spontaneous synchronised flock turns as if of one mind. An early 20^th century British bird watcher and author Edmund Selous was mystified how such big flocks could be so beautifully coordinated.

We now know that this kind of collective animal behaviour can emerge from relatively simple individual behaviours by each animal. This can produce complex, emergent behaviours larger than the parts.

The collective motion of flocks of birds (and other animals such as fish) can be modelled as a flock of self-propelled agents. The coordinated formations do not require ‘leaders.’ Each agent (animal) bases its own movement decisions on the positions, orientations, motion (or change in motion) of a few near neighbours. Using this local information, each agent follows simple rules to keep close but not too close, and align with and match the velocity of neighbours. While each agent reacts only with its neighbours, these interact in turn with their own neighbours, so a change in movement by an agent on one edge of the flock can ripple across it. The resulting simulated ‘flocks’ behave convincingly like real ones, changing shape, dividing and reforming.

As murmurations of up to a million starlings form in the skies above Rome, featured in the BBC’s Planet Earth II, it’s proved to be a good place to test the theories. Andrea Cavagna and colleagues used stereoscopic photography and digital image processing to reconstruct the positions, tracks, and velocities of individual birds. The results have supported the theoretical idea that local interactions between birds can explain the flock’s movement, but surprisingly the real shape of the flocks is more like a sheet rather than the bulbous shapes we see them as from the ground. Another unexpected finding is that the flocks are densest towards the edges, and that flocks can differ more than two-fold in their density. This has lead researchers to propose that the birds respond to the nearest 6 or 7 neighbours, rather than only neighbours within a certain radius as models had often assumed before.

The speed and orientation of the starlings are highly correlated across the flock, no matter how big the flock, which accounts for the flock’s synchronized movements. When birds in one part of the flock are disturbed (by seeing a predator such as a hawk, for example), their individual response in change of direction and/or speed transmits as a wave across the flock, amplifying the change of direction by just a few individuals initially. Transmission of information in this way creates an effective sensory range for the flock far greater than the perception of any individual.

Featured image credit: Flock of Birds, Pigeons, Starlings by YvonneH. Public domain via Pixabay.