Why do some birds fly in a V-formation?
When birds gather into groups, they fly in a variety of different formations and patterns.
Some birds fly in a formation that looks like the letter "V". Several species of geese, pelicans, and ibis are just a few of the more commonly known. This is their normal flying behavior, but can be more evident during migration due to the large numbers headed in the same direction.
Many of the other common birds fly in swarms or flocks without any obvious pattern. There's also a type of flock, exhibited breathtakingly by starlings, which is called a murmuration. It's a group of literally thousands of birds flying in magnificent and unique patterns.
I took the picture below over a lake in Rhode Island, United States, recently. The geese had just taken off and were flying in some side-by-side V-shaped groups. As they got farther away they gathered into smaller, tighter, individual V-shaped groups. Of course I've seen this behavior before, but on that particular evening, as I watched these geese leave the area, I began to ponder it more deeply.
Why do some birds fly in a V-formation? Is it just a random pattern or is there a reason for it?
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They fly on the wing currents produced by the birds in front of them. It's similar to drafting someone in front of you while cycling or racing. The leader is breaking trail, and the followers are benefiting by exerting less energy. They end up in a V-formation as birds join up one after the other and find a wing to fly on. After a while, the leader gets tired and falls back onto the wing of another, giving a different bird the opportunity to spear-head through the wind.
Here's a National Geographical article that details the specifics:Birds That Fly in a V Formation Use An Amazing Trick.
From the article:
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)
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I have heard that air-pressure explanation too. There might be some truth to it, but it sounds rather complicated to be a sound explanation.
In my opinion, the real reason birds fly in a V-formation is because the "leader" is either front-right or front-left in their field of vision. This is due to the location of their eyes on their heads: they are on the sides, rather than in front like we humans. A bird sees most easily what is either left or right of its head, not what is directly in front of its head. Example, watch a robin cock its head to the right or to the left when it spies a worm in the grass. The "lead" bird is in front, and the bird(s) behind it are looking at the leader and it is either on its right or left side and somewhat to the front. This is a simpler explanation than the air pressure differentials. Occam's razor. Apply this concept to a flock of birds and you'll get a natural V-formation, even multiple V-formations within the same flock.
Humans might be displaying a cognitive bias when they expect the birds to fly in a straight line.
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