In-air measurements of northern bald ibises flying in a V formation show that the birds conform to predictions for saving energy by regulating their relative body position and synchronizing their flapping motion. See Letter p.399
The elegant V formations of migrating birds provide a picturesque harbinger of summer's end, but why do the birds fly in such a precise formation? There are rumours that Allied bomber pilots during the Second World War noticed that their fuel economy increased when their squadrons flew in a V formation. Although these apocryphal tales have not been confirmed, the energy-saving benefits of formation flying have been reported in both civil1 and military2 aviation. For example, by maintaining one wing tip in the wake of a forward plane, a fighter jet can reduce its energy consumption by up to 18% (ref. 2). However, exploiting the benefits of formation flight is more challenging for birds than for fixed-wing aircraft — birds not only need to adjust their position relative to each other, but also must synchronize their wingbeat patterns3. On page 399 of this issue, Portugal et al.4 show that at least one bird species exhibits the requisite synchronization of body position and flapping motion to reduce energetic cost during migratory flight.
The principle by which formation flight saves energy derives from the way wings disturb the air as they move1,5. To create lift, wings accelerate airflow over their top surface compared with their bottom surface. Thus, relative to the still air through which they move, wings create a net circular flow of air that is directed rearward over the top surface and forward under the bottom surface. The greater the circulation a wing creates, the higher the lift it produces. At each wing tip, however, the circulation around the wing rolls up into a tip vortex, which extends backward like a tube, creating a horseshoe-shaped structure — the wake — that extends as the animal moves forward (Fig. 1a).
Owing to the laws of conservation, the circulation around the wings is equal in strength to the circulation around each of the two tip vortices. Air flows down through the middle of the wake, and the steady change in momentum as this downwash region elongates is equal to the lift created by the wings. Any animal flying behind another should avoid the downwash region, which would literally push them earthward. Just outside the downwash zone, however, there is a small region of upwash, created by the circular flow of air in the tip vortices. By careful placement of its own wing tip, a trailing bird can exploit the upwash generated by the tip vortex of a leading bird, thereby generating lift more efficiently and reducing its flight cost (Fig. 1b).
For an aeroplane pilot, keeping one wing precisely within the small upwash region of a leading plane's tip vortex is tricky enough, but for a bird the problem is further complicated by the flapping wings of its neighbours, which create tip vortices that undulate up and down. A bird that is following another bird must carefully adjust its own flapping motion, not in perfect temporal synchrony with the leader, but rather at a precise phase lag that tracks the tip vortex as it oscillates. When flying most efficiently, all the birds in a formation should flap with a precise metachrony (a wave-like synchrony), such that the flapping phase changes systematically from the leader to each bird down the line. Several theoretical studies3,5,6,7,8 have predicted how birds flying in formation could optimize energy savings by tuning their spacing and wing motion, and geese flying in a V formation have been observed to align their body positions in a way that might save energy7. But until now, no experimental data have shown that birds are capable of the precise adjustment of flapping phase that is necessary to track undulating tip vortices.
Northern bald ibises (Geronticus eremita) often fly in a V formation when they migrate. To examine their behaviour during formation flight, Portugal and co-workers mounted custom-built data loggers on 14 ibises that accurately measured the body position and flapping dynamics of each bird. The authors found that trailing birds flew so as to keep their inner wing in the upwash zone of the bird in front of them, just as predicted by theory3. This requires not only correct regulation of body position, but also proper adjustment of the flapping phase, so that each bird's wing tip follows the undulating tip vortex of the individual in front of it. Because the birds occasionally shifted position within the formation, situations occurred in which trailing birds briefly flew directly behind a leading bird. In these situations, the trailing birds tended to flap their wings in strict antiphase with the leading bird, thereby minimizing the negative interactions with the downwash region of the wake. This change in behaviour indicates that the ibises actively adjust their flapping pattern under different conditions.
Although these findings are qualitatively consistent with theoretical predictions, many challenging questions remain. For example, how much energy do the birds actually save? The best existing evidence that V formations save a significant amount of energy is that pelicans have a lower heart rate and show reduced flapping frequency when flying in formation compared with when flying solo9. Accurate measurements of metabolic rate will be crucial for a more precise understanding of the underlying aerodynamics of formation flight and for greater insight into the ecology of bird migration. Do ibises and other birds instinctively flap in an efficient manner when flying in formation, or do they learn to adjust their body position and wing motion because it 'feels' easier? And if the strategy is so useful, why do many species of small migrating birds not fly in a V formation? Might the benefits of formation flying decrease with body size, or is the requisite control of body position and wing motion more difficult for smaller, faster-flapping birds? Although our understanding of V formations has improved, there is still much to ponder when looking skyward on late summer days.
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Biology Direct (2015)