Lissaman, Shollenberger and formation flight in birds.

For centuries, mankind has been fascinated by V-formation flight in birds. Back in AD 79, Pliny the Elder noted that flocks of geese flew ‘like fast galleys, cleaving the air more easily than if they drove at it with a straight front’, and since then, numerous ideas have been proposed to explain the function of the V-shaped flocks that are a such a familiar sight. Many of these explanations often implied that there was an energetic benefit to be had from V-formation flight and that each bird within the flock was profiting from travelling as a group; essentially, flockmates were helping each other out. In 1970, Peter Lissaman and Carl Shollenberger published a paper in Science that was the first to detail the precise aerodynamic interactions that were likely to be taking place within a flock that could produce an energetic benefit. The authors made specific predictions about where each individual bird should position itself within the V for maximal energetic benefit. Subsequently, these predictions have persisted as the gold standard in comparison with all subsequent work, and as such, the paper was a pivotal point in the study of formation flight. The premise for the paper was founded in the basic principle that an object flying in a fluid produces lift by creating downward momentum within its span. When a wing is generating lift, the air on the upper side of the wing has lower pressure relative to the bottom side, and air flows from below the wing and out around the wingtips. At the wingtips, vortices – circular patterns of rotating air around the wingtip – are generated (Fig. 1), with a wingtip vortex trailing from the tip of each wing; this results in a vortex trailing from the righthand wing and a vortex trailing from the left-hand wing. These vortices generate upwash, creating a favourable airflow for other birds flying abreast that they could take advantage of if they flew in the optimal position to capture the upwash. The lift provided by the upwash causes a reduction in the lift power that trailing individuals must produce, and thus can bring about an energetic saving. Between these two regions of upwash, however, there is a large region of downwash – created as a result of air being pushed down as the bird moves forward – that most birds want to avoid.