By following some simple rules, the patterns and behaviours seen in flocks of starlings can be described. Firstly, there is an element of short range repulsion which ensures collision avoidance. Secondly, attraction to the surrounding birds keeps the flock cohesive. The third rule is that each bird mimics the direction and speed of the birds around it.

By comparing these rules to the data, scientists found that each bird interacts with its closest 6 or 7 neighbours. This is far fewer than the number of birds it can see. The starlings have optimised how much information to take in. If the bird takes information from too many neighbours the data is too noisy. Its decision making is ill-informed. If the information is taken from too few neighbours, the information is too short-ranged. Interestingly they found that this 6 or 7 number persists, regardless of the density of the flock. In fact, even sparse flocks keep their cohesion and because their visual range is so far any stragglers can quickly rejoin the flock.

[Interaction ruling animal collective behavior depends on topological rather than metric distance: Evidence from a field study - M. Ballerini, G Parisi et al 2008, PNAS]

Read more about my work on starling murmurations in my latest photo essay.