A starling in a flock of several thousand is not watching the flock. It keeps track of six or seven of its nearest neighbours, however close or far apart they happen to be, and that is all; a team in Italy worked this out in 2008 by reconstructing whole flocks in three dimensions. Counting heads instead of measuring distance is what holds the thing together, because when a hawk punches a hole and the birds spread out, each one still has its six or seven to follow. So the shape everyone stops on the pavement to watch is one that no bird inside it can see.