The sight of migrating swans, geese, cranes, cormor-ants, pelicans, flamingos, or other large birds is a spectacular sight that gives rise to interesting questions. Why such regular formations?

A vortex is formed in the wake of each wingtip, creating downflow behind the wing and uplift outside the wake, as indicated at the tip of the right wing of the right-hand bird. A trailing bird can take energetic advantage of this uplift by flying at a suitably lateral position relative to the bird ahead. Theory suggests that the optimal wingtip overlap for the trailing bird is about one tenth of the wingspan b. A distance of about 0.78b separates the centres of the two trailing vortices from a bird or aircraft

Various reasons have been suggested for such formations, including reciprocal altruism, kin selection, and communication, in addition to the energy-saving benefits. Regardless of the social interaction reasons, the energy benefit is clear:

But when it gets down to it, a formation of geese in migration is a beautiful sight, that just about anyone can recognize and appreciate.

• Andersson M, Wallander J. Kin selection and reciprocity in flight formation? Behavioral Ecology 2004 Vol. 15 No. 1: 158-162. ISI
• Weimerskirch H, Martin J, Clerquin Y, Alexandre P, Jiraskova S. Energy saving in flight formation. Nature 2001 413:697-698. Pubmed

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