  Animals that travel together in groups display a variety of fascinating
motion patterns thought to be the result of delicate local interactions among
group members. Although the most informative way of investigating and
interpreting collective movement phenomena would be afforded by the collection
of high-resolution spatiotemporal data from moving individuals, such data are
scarce and are virtually non-existent for long-distance group motion within a
natural setting because of the associated technological difficulties. Here we
present results of experiments in which track logs of homing pigeons flying in
flocks of up to 10 individuals have been obtained by high-resolution
lightweight GPS devices and analyzed using a variety of correlation functions
inspired by approaches common in statistical physics. We find a well-defined
hierarchy among flock members from data concerning leading roles in pairwise
interactions, defined on the basis of characteristic delay times between birds'
directional choices. The average spatial position of a pigeon within the flock
strongly correlates with its place in the hierarchy, and birds respond more
quickly to conspecifics perceived primarily through the left eye - both results
revealing differential roles for birds that assume different positions with
respect to flock-mates. From an evolutionary perspective, our results suggest
that hierarchical organisation of group flight may be more efficient than an
egalitarian one, at least for those flock sizes that permit regular pairwise
interactions among group members, during which leader-follower relationships
are consistently manifested.
