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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