STREEFKERK A (2010) Why do birds fly in flocks and formations?. LIMOSA 83 (4): 168-175.
In this literature review I seek answers to the questions of
how and why birds fly in flocks. Flying flocks can be divided
in two types: clusters and line formations. Clusters have a
three-dimensional structure and tend to be rather disorganised.
Birds are mainly thought to fly in clusters to prevent
predation through increased predator detection, a dilution
effect, physical protection by neighbours and a confusing effect
on the predator. People have always wondered how
birds avoid collisions in such rapidly moving clusters.
Historically it was believed that such movements were coordinated
by telepathy or electromagnetic communication.
The development of high-speed filming enabled scientists
to discover that manoeuvres are initiated by one individual
bird and followed by the rest of the flock. There is no single
initiator but this role is taken over by different individuals
from moment to moment. The most likely explanation for
movement in a cluster is a self-organization mechanism
where each bird applies a set of simple behavioural rules in
response to local information from neighbouring birds.
There are different kinds of formations, ranging from an
acute V-formation to a U-shaped bow. Although the total energy
saving is the same for different formations, it differs for
individual birds in the group. In an acute formation, the
leader gains little or no energy savings, while in a U-formation
the energy savings are more equal. A reason for attaining
a specific formation may be related to kin-selection or
reciprocity, although no empirical evidence for this hypothesis
has been found yet. Two hypotheses exist for the occurrence
of structured formations like the well known V-shape.
The first stresses the aerodynamic advantage of birds conserving
energy by taking advantage of the vortices created
by the wings of the birds in front. The second theory suggests
that by flying in formations at a fixed angle to each
other, birds can optimally visually communicate and orientate
to avoid collisions. These explanations are not mutually
exclusive and various studies have given support to both of
them. Small birds never engage in formation flight, probably
because their small wingspan and high wingbeat frequency
make it more difficult to gain energetic advantages from
wake vortices created by birds flying in front.
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