A murmuration looks like it should have a conductor. Tens of thousands of starlings pour across the sky as a single shape that folds, splits, and snaps back together, and the natural assumption is that something is directing it. Nothing is. There is no lead bird, no signal, and no bird anywhere in the cloud with a picture of what the whole flock is doing.
That absence is what makes the collision question serious. Thousands of bodies at flight speed share one patch of air and turn constantly, and intuition says this should end in carnage. The actual mechanism was pinned down in a short run of papers between 2008 and 2014, and it is much smaller and much stranger than a leader.
The seven-neighbour rule
In 2008, Ballerini and colleagues published a paper in PNAS whose title gives the game away: interaction in these flocks depends on topological, not metric, distance. In plain terms, a starling does not watch every bird within some fixed radius. It tracks a fixed number of nearest neighbours, six to seven of them, and ignores the rest of the flock entirely.
That count holds regardless of how dense the flock is, and the density part is the clever bit. A distance-based rule fails as the flock deforms: spread out and a bird loses its references, compress and it drowns in them. A count-based rule survives both. However hard the flock stretches or packs, each bird's workload stays exactly the same: your nearest six or seven, and nothing else.
How seven neighbours steer ten thousand birds
A seven-bird watchlist explains a clump. It does not obviously explain one coherent shape made of tens of thousands of birds. That step was measured in 2010, when Cavagna and colleagues, in PNAS again, found scale-free correlations in starling flocks: velocity correlations span the entire flock, so birds on opposite edges are still statistically linked. Alignment copied neighbour to neighbour, seven at a time, compounds into global order with no central control anywhere in the system.
The collision answer proper arrived in 2014, in Nature Physics. Attanasi and colleagues showed that when one bird turns, the turn propagates through the flock as a wave travelling at tens of metres per second, which is faster than any individual starling flies. The information outruns the animals carrying it: by the time the far side of the flock needs to bend, the instruction has already arrived.
The choreographer is a falcon
None of this evolved for the audience on the ground. Predation pressure drives the display, and the predator in question is the peregrine. A murmuration is an anti-falcon measure that happens to look sublime from below, and it scales: flocks can exceed tens of thousands of birds.
The detail I keep returning to is how unglamorous each bird's job is. No starling improvises, and no starling ever sees the pattern it belongs to. It watches six or seven others, continuously, and matches them. The shape people stop their cars to film is a side effect of ten thousand animals each doing something too small to admire on its own.
One disclosure, since it is the whole reason this page exists: I build Curio, an iOS app that serves five short curiosity stories a day, so I am professionally biased toward thinking facts like the seven-neighbour rule deserve two minutes of your attention. If that sounds right, it's Curio on the App Store. Either way, the starlings work for free.