If you’ve ever looked up to see a skein of geese or a plump of ducks flying overhead, then you’re probably aware that many birds that fly in flocks do so in V-formation.
For centuries, scientists have pondered the question of why birds fly in V-formation. Piny the Elder, the Roman author, philosopher, and naturalist, noted in AD 79 that flocks of geese ‘flew like fast galleys, cleaving the air more easily than if they drove at it with a straight front’.
However, despite being a readily observed phenomena, studying birds that fly in V-formation presents several challenges, and even today there is still not a single consensus in the scientific community that explains why they do it.
In 1914, Carl Wieselsberger, a German engineer and passionate pilot, was the first to suggest that birds flying in a V-formation expend less energy. He noticed that birds alternate being at the apex of the V; when one gets tired, another takes its place. From his knowledge of the principles of aerodynamics he proposed that the air flowing over a bird’s wingtips rises upwards with the bird behind taking advantage of the upwash to reduce drag and therefore using less energy flapping their wings.
Just over 40 years later, a group of German scientists suggested that flying in formation meant that the birds didn’t experience turbulence from the bird in front. However, if you look closely, you’ll notice that birds in formation don’t fly directly behind each other. They actually fly in echelons at a slight diagonal and are slightly staggered, which means this theory doesn’t hold out very well.
In 1967, William John Hamilton III, a graduate of Cornell University, and research scientist in ecology and animal behaviour, proposed that birds flying in V-formation have a better field of view and can communicate visually with neighbouring birds. However, this doesn’t explain why only some birds that fly in flocks fly in formation.
It wasn’t until 1970 that two scientists, Dr. Peter Lissaman and Carl Shollenberger at the California Institute of Technology, first calculated the exact interactions that take place between birds flying in V-formation.
Using their knowledge of the laws of aerodynamics, their theory was based on the principle that an object flying in a fluid, in this case, air, produces lift by creating a downward momentum. When a bird generates lift in its wing by flapping it, the air on the upper side has lower pressure than the air on the underside. This causes air to flow from underneath the wing and out around the wingtips, where circular patterns of air called vortices are formed which generate an upwash.
A bird flying behind this bird positioning itself to take advantage of the upwash, while also avoiding the downwash created as the first bird moves forward, means it doesn’t need to use as much energy to generate lift in its own wings.
According to Lissaman and Shollenberger , birds flying in a single line in echelon formation would also make energy savings due to the same principle, but it is only when they fly in V-formation that they share the energy savings between them for optimum efficiency.
They calculated that for a V-formation of 25 individuals, the energy required for lift is reduced by a factor of 2.9 which equates to an increase in flight range of about 70% when compared to solo flight.
The results of their experiments were published in a paper Formation flight of birds in Science, the peer-reviewed journal of the American Association for the Advancement of Science.
But the idea that birds fly in V-formation to save energy was still only a theory. It wasn’t until the turn of the century and the invention of micro loggers that could be attached to birds was any empirical evidence found.
In 2001, a team of researchers from the French National Centre for Scientific Research, led by ecologist Dr. Henri Weimerskirch, taped tiny heart rate monitors to the backs of eight great white pelicans. Pelicans, like ducks and geese, usually fly in V-formation following a leader, and these birds had been trained to follow a light aircraft and motorboat for a film Le Peuple migrateur or Winged Migration, a documentary about winter bird migration.
This gave Dr. Weimerskirch and his team a unique opportunity to observe and study them as they flew over the Senegal River. The data from the monitors showed that the pelicans’ heart rates, a good measure of energy expenditure, decreased by about 10 percentage points when flying in formation. The scientists also found that the birds glided more often leading them to conclude that they were saving energy not by taking advantage of the bird in front’s upwash to flap with less effort as Lissaman and Shollenberger had prediected, but because they were able to flap less often.
Scientists now has a pretty good understanding of why some birds fly in v-formation, but they still weren’t clear how they did it.
In 2014, Steven Portugal, a biomechanist at the Royal Veterinary College in London, and his colleagues developed a tiny GPS tracking device that was light enough to be attached to a flying bird and could record its position, speed, and direction of travel several times a second.
However, there was one problem; the loggers weren’t able to emit any of the data, so they needed to be attached to a flock of birds that wouldn’t fly off with them.
To solve this, Portugal teamed up with an Austrian conservation group, Waldrappteam, who were working on saving the northern bald ibis (Geronticus eremita) from extinction and reintroducing them to Europe.
14 juvenile birds that had been raised in captivity were being taught to fly along their old migration routes from Austria to Italy by following microlight plane. With several stops along the way, the team from the RVS had plenty of opportunities to fit the birds with the trackers, record their flight, and retrieve the data a few hours later.
As predicted by aerodynamic simulations, the data showed that the ibises flew about a metre behind the bird in front and about a metre out to the side. Some birds preferred to fly on the left of the formation and some on the right, while some preferred the centre and others preferred the edges. But the ibises swapped around a lot and there wasn’t an individual that stood out as a leader. Overall, they spent about a third of the time following another bird and about the same amount of time leading a formation.
The data also showed that they flapped their wings at exactly the right moment to minimise the effects of the downwash from the wings of the bird in front, essential tracking the air throughout their flap cycle.
If they moved somewhere else in the flock and got caught directly behind a bird in front, they were quickly able to change their wingbeats and flap the opposite way and out of phase, again to minimise the downwash.
