Flocks move with remarkable synchrony and order such as this can have radically different origins. It can be the result of a top-down centralised control mechanism - i.e. the presence of one or more leaders. Or, it can arise as bottom-up self-organisation.

How the group behaves under strong predatory pressure provides crucial information about its organisational structure. In a group where all individuals follow a leader, without interacting with each other, there is strong order, as everyone will move in the same direction, yet information transfer is poor. Unless the falcon is detected by the leader, the group will not respond to it.

Instead, flocks are self-organised. When one bird detects a falcon attack it will turn to avoid it. This turn is noticed by its neighbours who also turn. They influence their neighbours, and so on.

[Scale-free correlations in starling flocks - A. Cavagna, G. Parisi et al 2010, PNAS]

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