Why do geese fly in a V-formation? To aid communication and to fly more efficiently.
Geese don’t always migrate, but when they do, they can fly for thousands of kilometers and Canada geese can fly 2,400 km in 24 hours! If they find a good tailwind, they can reach speeds of 110 km/h. You may have seen them flying and they always fly in a V-shape. However, they don’t fly in a horizontal V. Every goose flies slightly above and to the side of the goose in front. There are several reasons for this. The first reason is for communication. All of the geese fly as a group and their tiered position lets any goose in the formation see the front goose. This helps them synchronize and move as one unit. They also honk while they are flying to ensure that they all stay in formation.
All birds fly by flapping their wings. They have very strong breast muscles, and they can produce a lot of force when they flap. On the down flap, they angle the wings so that the air flows faster over the wing than under it, producing low pressure above the wing and providing lift. At the same time, they push the air backwards, generating thrust and propelling themselves forward. When they flap their wings up, they fold them to produce as little resistance as possible. All birds have to fight against air resistance, which increases the effort they need to fly. Heavier birds need to go faster than smaller birds to ensure that they get enough lift to fly, which means they encounter more air resistance. Large geese can weigh over 6.3 kg, so they need a lot of speed to be able to stay in the air.
Heavier birds need more lift, and they need to flap their wings more, which requires energy. To get around this, a lot of heavy birds, such as eagles, glide on air currents. Geese can’t use this method because they need to travel a long distance forwards. They do very occasionally glide, but they generally flap constantly. That requires an enormous amount of energy, so they have evolved a way to make it easier. And that is the second reason why they fly in a V-formation.
When birds flap their wings down, they produce a vortex of air. Directly behind the bird this vortex heads down, called downwash. Just to either side of the wing this vortex heads up, called upwash. If the geese flew directly behind each other, they would be pushed down by the downwash and would have to flap harder to generate lift. However, flying slightly to the side of the bird in front, they are pushed upward by the upwash. Each bird going towards the back of the V requires less effort than the bird in front. This has been tested by using heart rate monitors on pelicans, birds that also fly in a V-shape, and the birds at the back of the V had slower heart rates than the birds at the front. All of this means that the bird at the very front of the V has to make the most effort to fly because there is no upwash to benefit from. The geese take turns being the bird at the front, so they all get a chance to benefit from the upwash. This method of flying theoretically increases their efficiency by 71% and the birds at the back can flap more slowly. I say “theoretically” because the birds need to keep 0.16 meters above and to the side of the bird in front of them to get the most out of the upwash and it is almost impossible to keep this exact distance, so the actual efficiency is much lower. Still, it is enough to warrant this style of flying.
So, if this method of flying is so efficient, why don’t other birds fly in a V-formation? It only works with birds that have large wings and can flap slowly. Smaller birds that have to flap a lot create far too much turbulence with their wings and it is impossible to keep in a formation.
As an interesting extra, one type of goose called the bar-headed goose migrate across the Himalayas and regularly fly at heights of 5,000 to 6,000 meters. To get to that height requires a long and sustained climb, probably the longest sustained altitude gain of any bird. At this height the air is very thin, and they have to flap even harder to produce the same amount of life. They also have to produce this extra flapping while coping with the very low oxygen content of the air at that height. Birds generally have more efficient lungs than mammals and larger hearts, so can survive hypoxia, but the bar-headed goose has other adaptations. They can increase their metabolism, control their breathing, breathe more deeply, have more efficient gas exchange in their lungs, and have 25% larger lungs than other geese. And this is what I learned today.
Photo by David Dibert: https://www.pexels.com/photo/a-flock-of-geese-flying-7177008/
Sources
https://www.loc.gov/everyday-mysteries/zoology/item/why-do-geese-fly-in-a-v/
https://www.quora.com/Why-dont-all-birds-fly-in-the-V-formation-Isnt-it-more-efficient
https://en.wikipedia.org/wiki/Goose
https://www.nationalgeographic.com/animals/article/do-canada-geese-still-fly-south-for-winter