Starling flocks turn as one because each bird only tracks its seven nearest neighbours

Physicists analysing huge starling murmurations found each bird adjusts its flight based on roughly its six or seven closest neighbours, regardless of how densely packed the flock is. That simple local rule lets a ripple of turning propagate through thousands of birds almost instantly, with no leader and no central signal.

— Andrea Cavagna et al., Scale-Free Correlations in Starling Flocks — PNAS, 2010