Most people would say that they do it to save energy, which would be right. As a bird flaps, a rotating vortex of air rolls off each of its wingtips. These vortices mean that the air immediately behind the bird gets constantly pushed downwards downwash , and the air behind it and off to the sides gets pushed upwards upwash. If another bird flies in either of these upwash zones, it gets free lift. It can save energy by mooching off the air flow created by its flock-mate.
This all makes sense, but it represents decades of largely theoretical work. Scientists calculated how air should flow around a flying bird based on what we know about planes, but almost no one had taken any actual measurements. Henri Weimerskirch changed that in , when he fitted pelicans with heart-rate monitors. He found that birds at the back of the V had slower heart rates than those in the front, and flapped less often.
It was an interesting study, which confirmed that birds benefit from flying in a V. First, he needed the right technology. His colleagues at the Royal Veterinary College, UK developed tiny data-loggers that are light enough to be carried by a flying bird and sensitive enough to record its position, speed and heading, several times a second. If you strap them to, say, a flock of geese, the birds would fly off into the distance taking some very expensive equipment with them.
Johannes Fritz had a solution. He works for an Austrian conservation organisation that is trying to save the northern bald ibis —a critically endangered species that makes vultures look handsome. The ibis went extinct in Central Europe in the 17th century, and Fritz is trying to reintroduce it into its old range.
His team have reared several youngsters and teach them to fly along their old migration routes by leading the way in a microlight aircraft. That gave Portugal plenty of chances to fit the birds with loggers, record every flap of their wings for long stretches, and retrieve the data a few hours later.
The recordings revealed that the bird fly exactly where the theoretical simulations predicted: around a metre behind the bird in front, and another metre off to the side.
Some ibises preferred to fly on the right of the V, or on the left. Some preferred the centre, and others the edges. But on the whole, the birds swapped around a lot and the flock had no constant leader. The air that has been moved by car is pushed above and to either side, resulting in areas of high pressure. Directly behind the moving car is an area of low pressure, where the car used to be. The car following behind, in this low-pressure area, faces less air resistance, resulting in a boost in performance and efficiency.
For the practicalities of flight, the low-pressure area directly behind the leader is actually not an optimal place to fly. Flying requires an area of high pressure beneath the wing and low pressure above the wing that results in lift.
Air flows faster over a convex surface than it does over a flat or concave surface. Because the air moves faster over the top of the wing than the bottom, there is a higher pressure beneath the wing than there is above it. Instead of the low-pressure areas directly behind the leader, used by Nascar, birds utilize the high-pressure areas that occur from their aerodynamic shapes. The shape of a bird naturally focuses air out from the body and along with the wings to either side.
At the wingtip, the high-pressure air beneath spirals into the low-pressure area above the wing, generating a high-pressure vortex. By catching these high-pressure areas beneath their wings, the birds following are able to utilize this additional high pressure to augment their lift, and that of those, again behind them.
As described, the ideal location for the gain in lift is just behind, above, and to either side of the leader, but directly behind, is not optimal. This naturally leads to the classic V shape formations or echelons that many migratory birds adopt when crossing our skies around the equinoxes.
The V and J echelons are also useful for members of the flock to keep an eye on one another for safety and support. On trans-continental journeys, support from the flock is crucial. Birds will adopt this type of formation in a crosswind.
The strength of the wind determines how wide the formation flies. In high wind, the long side will usually be longer and fly straighter behind the leader, while the short side will fly wider with fewer members, sometimes as little as 2 or 3 birds.
This is due to the pressure areas being affected by the strength and direction of the prevailing wind. The reason the long side is into the prevailing wind is due to the way wingtip vortices work. At the wingtip, the low and high-pressure areas, respectively above and below the wing, come together, creating a vortex. The direction that the vortex spirals is up, over the wingtip, and in towards the body.
A crosswind coming from the right will keep the vortex from the right-wing in a tight spiral. This helps to maintain the integrity of the vortex for longer, so more birds can use it. The vortex from the left-wing, however, is spiraling into the wind. As such, a short distance behind the leader, its rotation begins to slow down, undermining the integrity of the vortex, and with it, the added efficiency. Birds will seldom fly directly into the wind , but even more rarely, fly with a direct crosswind, depending on the wind direction and speed, and of course, their destination.
Birds will usually find an angle that takes advantage of the lift generated by a slight headwind. The leaders of formations change from time to time, but the causes, frequency and characteristics of these changes have not yet been determined.
Sustained observation from the ground of flocks covering great distances in the air is very difficult. There are plenty of intuitive predictions about leader choice that quickly come to mind relative to the age, experience, sex, condition and social status of the leaders, but researchers have not figured out how to overcome the prohibitive logistic issues to test them.
Some scientists have trained birds to fly in formation with small aircraft; perhaps their experiences will yield opportunities to test these ideas. Answer originally posted Feb.
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