Look upwards on a clear day and maybe you’ll spot a strange mark across the sky, a big wobbly V made up of small dots. Listen hard enough, and those dots may be squawking or honking. Geese and other birds fly in a set formation, but why is this, and why that shape?
Migrating birds take to the sky in the autumn and spring to find warmer places to live out their winters or summers. Many do so alone, like the grebe, or in large flocks like swallows or puffins.
The common factor among all of these types of birds is that they have very long wings.
There are also those birds who will fly in a smaller group and in a specific V formation shape, like ibises, ducks, pelicans, storks and geese. The common factor among all of these types of birds is that they have very long wings. This dictates the actual style of flying they will then use to get where they’re going.
The military have long understood the fuel-saving benefits of flying in a staggered formation. The physics of flight meant that fuel consumption could be reduced by flying in wake of the air speeding off the back of the forward aircraft. It was supposed that birds who fly in a V formation function in more or less the same way, to save energy somehow. However, this was always just a theory, and had never actually been studied, until 2001, when a team fronted by Henri Weimerskirch, a researcher specialising in seabirds, fitted heart rate monitors to eight pelicans and monitored them as they flapped about over the Senegal River. The resultant data showed that when the birds aligned in a V formation, their heart rates decreased by 10%, using considerably less energy and getting more mileage. The data also showed that the birds beat their wings less frequently and could glide for longer periods when they flew as a group. So much for proving the energy conservation theory. But still the question remained, why always a V? Why not, for instance, the staggered one as in the above image?
All birds (except for a handful of flightless ones) use their strong breast muscles to flap their wings, which are shaped in such a way as to make the air flowing over the top of them speed up, as it has to travel further over the top of the wing compared to the shorter distance taken by the air flowing along the flatter underside of the wing. This causes the difference in air pressure above and below the wings that then gives them the lift to fly. Some birds need to use gravity to get that lift started, by jumping from a perch or launching off a cliff. Others need to get the lift by running across an expanse of land or the tensile surface of water first, flapping as they go, generating the movement of air required for takeoff. Planes have fixed wings, specifically designed in such a way as to leave the air behind them more or less stable so as not to disrupt the flight of aircraft behind them. They also have engines to provide the thrust needed.
Birds do of course flap, a lot. The air coming off a flapping wing is far more unstable than an F-16. No one really wanted to think how that might affect the birds behind the leader because we had no real-life experience of it. Until, that is, one wonderful pioneering project that took place just a few years ago.
The loggers recorded each bird’s specific position, heading and speed, as well as every wing flap.
Northern bald ibises are critically endangered, and in 2014 a team in Austria wanted to reintroduce them to Europe, so they set about hand-rearing 14 chicks. By being at the birds’ sides from birth, the ibises imprinted onto their human foster parents, following them everywhere for everything. Using specially designed data loggers developed by the Royal Veterinary College, UK, the team flew across Europe in microlights following the ibis’ ancestral migratory routes, and the trained birds followed them. This time, instead of heart rates, the loggers recorded each bird’s specific position, heading and speed, as well as every wing flap. This data showed that not only did the birds often change their position in the V, they also adjusted their wing flap timing depending on where they were in the group.
The team were now able to see how the birds utilised the unstable air coming off the flapping wing tips either side of the birds’ body. Air passing over a bird wing produces something called downwash and upwash. Downwash is the majority of air produced from passing over and under the broader surface of the wing. Upwash, however, is the result of the spiralling air called a vortex that is produced at the tips of the wings. This vortex provides the upwash: air that drops down off the wingtip but very quickly flows upwards again, for a brief but vital time.
Birds with large wingspans, like geese and cranes, and these ibises, produce an exceptionally strong upwash. It is this upwash that provides the bird additional lift for their flapping, thus conserving energy. To benefit from this regular but momentary rush, birds need to catch this surge where they need it the most, under the broad part of their wings, so they will position themselves to be just a little behind and, crucially, off to the side of the leader. Subsequent birds in the flock follow suit, so after the leader bird you get this triangle or V shape, as the downwash generated by any birds in front of you is no good. If the leader bird changed position a little placing the following bird in the downwash, the bird would change their flapping so that they were doing the opposite of what the bird in front was doing: rather than tracing the same path, it flew almost perfectly out of phase. They would then just move in line with the upwash again.
Quite how they could tell where the upwash would be is still not known, but it is assumed that they felt it in their flight feathers. Birds were seen to take turns at being in the front, allowing their companions a little free-loading in the back. This meant that each bird had the best chance of surviving the long trip, therefore the colony had more chance of successfully breeding and saving the species as a whole.
A final interesting thought that stemmed from this project meant that the theory that birds learn from their elders was thrown out – these birds were all raised from chicks with no adult ibis in sight, just humans and microlights. This meant that they were not only capable of learning from each other, but were also self-taught. One mystery solved; another one appears.