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  • Think of a flock of birds and you probably imagine that classic V shape — a leader

  • with sets of trailing birds on either side.

  • But not all flocks fly this way. Starlings, for example, travel in large, three-dimensional

  • clusters that seem to move like a wave.

  • So why do some species fly in Vs and others in clumps? It turns out to have a lot to do

  • with the individual birds themselves.

  • Some, like geese heading south for the winter, are making long treks. The V formation helps

  • them stay in visual contact, avoid collisions, and conserve energy.

  • It's the structure of their wings that lets them take advantage of the V.

  • As the wing flaps, each wing tip creates a vortex that spirals up from the bottom of

  • the wing and over the top. This vortex trails off behind each bird as it moves forward and

  • is encountered by the next one in line.

  • The trailing bird positions itself to catch just the upwash of that vortex, or upward

  • force, and that requires being behind and just to the side of the leading bird.

  • Lots of birds behind and to the side of one another creates the V shape.

  • Studies have estimated that birds flying this way can save around 15% of their energy.

  • So, why don't all birds fly this way?

  • We talked to Professor Erick Greene from the University of Montana Bird Ecology Lab and

  • he explained that this has to do with the size of the bird.

  • You may have noticed that birds that fly in a V — like geese, pelicans, swans, and ibises

  • are typically larger creatures with a long wingspan.

  • These species move their wings only a few degrees up and down with each flap. This motion

  • creates vortices that lie pretty neatly behind the bird.

  • Small birds, on the other hand, tend to flap their wings all the way up and down. The vortices

  • created by these motions are all over the placenot consistent enough for their

  • flock mates to actually use.

  • And the small birds that do flap their wings like larger ones just don't generate a big

  • enough vortex because of their size.

  • For small birds, flying in groups sometimes even uses more energy, not less. But these

  • species have another need that's even more important: protection.

  • In 1971, evolutionary biologist William David Hamilton proposed a theory called the selfish

  • herd.

  • It suggests that the risk to an individual is reduced if that animal places another between

  • itself and a possible predator. Repeat this across enough individuals and you end up with

  • a herdor in the case, a flock.

  • Other theories offer similar explanations, but, whether you're talking schools of fish

  • or swarms of insects, it's clear that this is a pretty common survival strategy.

  • So, the next time you see a group of birds flying by, you'll know it might be to save

  • energy. Or it could just be to stay alive.

  • Thanks for asking. If you can't get enough of SciShow in your eyeballs, it's also available

  • for your earsin the form of our podcast, SciShow Tangents!

  • Join me and our other hosts as we do stuff like try to stump one another with weird trivia

  • and write science poems. Download it wherever you download podcasts!

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B1 中級

為什麼所有的鳥兒都不是V字形飛行? (Why Don't All Birds Fly in V Shapes?)

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    林宜悉 發佈於 2021 年 01 月 14 日