The iconic V formation of geese in flight has been found be aerodynamically the exact way to minimise drag and optimise effort in long flights. But a detailed study of birds in flight, by James R Usherwood, Marinos Stavrou1, John C Lowe1, Kyle Roskilly and Alan M Wilson at the Royal Veterinary College, London, shows 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 because of other gains in community living.
The classic geese
Just like pushing through a crowd in a 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 keeping it from falling to the ground. The angle to generate this lift doesn’t come easy but it causes a ‘drag’, 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 ‘lift’, 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.
A study a few years ago monitored the heartbeat of pelicans in flight and showed that its 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 mutual benefit and it has been shown that the effort saving can be as much as 70 per cent.
As the leader birds would naturally tire faster than the followers, geese and other birds that fly long distances rotate the leader position, so that the group as a whole is able to cover the largest distance before a break for 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 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 flights 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 pin-pointing 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 has yielded data in sufficient quantity. 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 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 ‘effective body weight’, which then needs more ‘lift’ to maintain flight.
The observed higher flap frequency, which is mechanically less efficient, is required to provide greater control, essential for flying in close proximity with, particularly directly behind neighbours. This is a substantial additional cost of flying in a close cluster.
Conclusion: 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 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 feeding.
Their priorities may include mutual observation, collective guidance and navigation, enhanced security, as a result of greater numbers of individuals, fitness display, and assessment of group numbers.