  Bird flocking is a striking example of collective animal behaviour. A vivid
illustration of this phenomenon is provided by the aerial display of vast
flocks of starlings gathering at dusk over the roost and swirling with
extraordinary spatial coherence. Both the evolutionary justification and the
mechanistic laws of flocking are poorly understood, arguably because of a lack
of data on large flocks. Here, we report a quantitative study of aerial
display. We measured the individual three-dimensional positions in compact
flocks of up to 2700 birds. We investigated the main features of the flock as a
whole - shape, movement, density and structure - and discuss these as emergent
attributes of the grouping phenomenon. We find that flocks are relatively thin,
with variable sizes, but constant proportions. They tend to slide parallel to
the ground and, during turns, their orientation changes with respect to the
direction of motion. Individual birds keep a minimum distance from each other
that is comparable to their wingspan. The density within the aggregations is
non-homogeneous, as birds are packed more tightly at the border compared to the
centre of the flock. These results constitute the first set of large-scale data
on three-dimensional animal aggregations. Current models and theories of
collective animal behaviour can now be tested against these results.
