It is not always for economy that birds fly in formation., says S.Ananthanarayanan.

The iconic V formation of geese in flight has been found be aerodynamically the exact way to minimize drag and optimize effort during long flights. A detailed study of birds in flight, by James R. Usherwood, Marinos Stavrou, John C. Lowe1, Kyle Roskilly and Alan M. Wilson at the Royal Veterinary College, London, published in the journal, Nature, showed that flying in groups is often costlier, in terms of effort, than flying alone, and the reason that birds choose to fly in groups may be other gains of community living.

The classic geese

Just like pushing through a crowd in single file is easier than going three abreast, there is an advantage in flying behind another flying object. Any flying object is able to stay airborne because its forward motion generates ‘lift’ that is equal to its weight, and thus keeps it from falling to the ground. The angle of the object to generate this lift, however, doesn’t come free but it causes ‘drag’. This is the force which the object must overcome by effort, like burning fuel in the engine, in the case of an aircraft or flapping of wings, in the case of a bird. But if one is flying behind a leader, the drag is partially overcome by the leader’s effort. There is also some advantage in getting ‘lift’ from the vortices, or the upward movement of air caused by the leader. The followers can thus fly at a shallower angle and there is an overall economy of effort. The best saving, in fact, is not directly behind the leader but slightly to the side, which gives rise to the ‘V’ formation of geese in flight.

There was an early study which monitored the heartbeat of pelicans in flight and showed that the heart rate was much slower when flying in the V formation. When a large number of birds are involved, the group can position itself to further tweak the mutual benefit and it has been shown that the effort saving can be as much as 70%. As the leader birds would naturally tire faster than the followers, geese and other birds that fly long distances in formation have evolved to rotate the leader position, so that the group as a whole is able to cover the largest distance before needing to rest.

Flying in a cluster

The group at the Royal Veterinary College noted that the V formation, however, was limited to large birds on long, migratory flights, the more common flight mode being in groups of birds flying close together. Did such flight also yield aerodynamic advantage? The London group used state of the art monitoring devices to study the internals of a flock of pigeons in flight and found that it was the contrary that was true!

The study was of 18 pigeons during seven bouts of voluntary, straight and circling flight, around their home loft, over a period of more than 9 pigeon-hours of flight, 400 pigeon-km, and over 243,000 flaps. Back mounted GPS (Global Positioning System) devices enabled pinpointing the position of each bird at every instant of flight and gyroscopic sensors recorded the acceleration and centrifugal forces the birds experienced. Wingbeat motions were monitored with a 300 Hz sensor and the study took into account the local wind conditions with the help of an anemometer mounted on a nearby rooftop. The arrangement yielded data in sufficient quantity and with the quality to allow, for detailed, mathematical and statistical analysis of how the effort expended, as measured by the flap frequency and body motions, was related to airspeed, induced, climbing and accelerating power, and proximity to other pigeons. The data was sufficient to separately examine the effect of each factor, to be assessed for its cost, in terms of effort, and the economy, if any, of different modes of flight

The result of study showed that in the case of pigeons flying locally around their roosting spot, there are a number of effects that do not arise in solitary flight. For instance, turning to the left or right, while flying in a group, calls for tilted or ‘banked’ turns, like an aircraft, which increases the effective body weight, which then needs more lift to maintain flight. The observed higher flap frequency, which is mechanically less efficient, is needed to provide greater control, essential for flying in close proximity, and particularly directly behind other birds in flight. This is a substantial additional cost of flying in a close cluster. The reasons of economy, which are celebrated in the case of geese and pelicans, are obviously not the motivators of group behaviour in the case of smaller birds which stay together over short distances.

Other benefits

At the same time, it is seen that even the classic long-distance geese do not always stick to the mathematically ideal flying formation, for economy of effort alone. The V formation itself, is not only for energy efficiency, it also provides the possibility of each bird being able to see the largest number of other birds, so that the group stays together. In the case of smaller birds on shorter flights, the benefit of energy conservation is also not a major factor. Even if energy saving is a goal, it may not be paramount, unlike on long flights, across stretches of water, for instance, where it is important that the group be capable of reaching the next place for resting and possibly feeding. The priorities may include mutual observation, collective guidance and naviga¬tion, enhanced security as a result of greater numbers of individuals or of eyes, fitness display, and assessment of group numbers. The coordinated bursts of flying in groups, by pigeons, may be for testing and maintaining fitness and their ability to move fast and accurately, which is important for security. It is evident that there is more than one reason for the way behaviour and flight, in birds, has evolved in the animal kingdom.

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