Rapidly changing shapes of bird flocks inspire us and fill us with wonder: How do they do that? How do so many birds move in three dimensions in such dynamic and tightly-packed aggregations without crashing into each other?

Hypotheses abound. One hypothesis, proposed in the 1930s by British writer and ornithologist, Edmund Selous, went so far as to suggest that bird flocks coordinate their movements through telepathy.

“Surprisingly enough, in an era in which splitting the atom into its tiniest parts has become commonplace for science, obtaining empirical data on large groups of animals moving in three dimensions is still a very difficult task”, write statistical physicists Andrea Cavagna and Irene Giardina, in their paper, “The Seventh Starling” (doi:10.1111/j.1740–9713.2008.00288.x). Dr Cavagna and Dr Giardina study the physics of flocking and other collective animal behaviour at the Centre for Statistical Mechanics and Complexity in Italy.

Palazzo Massimo alle Terme. (Credit: RyanFreisling/Public Domain.)

After perching atop the Palazzo Massimo at sundown for three winters to film the aerial ballet of flocking starlings, Dr Cavagna and Dr Giardina and their collaborators have learned that these flocks have several basic rules — traffic rules, if you will — to avoid collisions.

“The clearest structural feature is that a bird’s nearest neighbours are typically found at the bird’s sides, rather than ahead or behind the bird, so that the probability that a bird’s nearest neighbour is approximately ahead or behind is very low”, the authors write.

Perhaps the reason is anatomical. Since the bird’s eyes are on the sides of its head, it sees sideways better. Or maybe the birds are keeping a safe distance between themselves and those in front to avoid rear-end collisions — similar to humans driving cars at speed on crowded motorways.

The research team also found that any particular starling’s spatial orientation and velocity correlates with the orientation and velocity of its six closest neighbours, regardless of flock size. But why not five or seven or …? Dr Cavagna and Dr Giardina think that seven may serve as the cognitive limit for starlings: they simply cannot track the movements of a larger number of neighbours. (Perhaps not coincidentally, this limitation appears to be shared with humans too.)

Nevertheless, starling flocks are ultimately democratic. If any one bird turns or changes speed, so will all the others. Such changes radiate outwards in a wave from the individual to affect the flock. Statistically speaking, every individual bird is interconnected within the same dynamic web of interactions.

My favourite living poet, Mary Oliver, has a few ideas about starling flocks:

…they are acrobats

in the freezing wind.

And now, in the theater of air,

they swing over buildings, dipping and rising;

they float like one stippled star

that opens,

becomes for a moment fragmented, then closes again;

and you watch

and you try

but you simply can’t imagine how they do it

with no articulated instruction, no pause,

only the silent confirmation

that they are this notable thing, this wheel of many parts, that can rise and spin

over and over again,

full of gorgeous life. ~ excerpted from “Starlings in Winter” by Mary Oliver; Owls and Other Fantasies: Poems and Essays (Beacon Press, 2003).

If you wish to experience a murmuration of starlings for yourself, November is the best month to do so.

Additional reading (includes scary mathematical formulae and lots of other details):

Cavagna, A., & Giardina, I. (2008). The seventh starling. Significance, 5 (2), 62–66 doi:10.1111/j.1740–9713.2008.00288.x

Cavagna, A., Cimarelli, A., Giardina, I., Parisi, G., Santagati, R., Stefanini, F., & Viale, M. (2010). Scale-free correlations in starling flocks. Proceedings of the National Academy of Sciences, 107 (26), 11865–11870 doi:10.1073/pnas.1005766107

Hemelrijk, C., & Hildenbrandt, H. (2011). Some Causes of the Variable Shape of Flocks of Birds. PLoS ONE, 6 (8) doi:10.1371/journal.pone.0022479