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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.
想象一群鸟，你可能会想到经典的V型——领导者的两侧各有几只尾随的鸟。
But not all flocks fly this way. Starlings, for example, travel in large, three-dimensional clusters that seem to move like a wave.
但不是所有的鸟群都是这样飞的。例如，椋鸟以大型三维集群的形式飞行，它们的移动就像波浪一样。
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.
为什么有些物种会以V形飞行，而有些却成群结队飞行呢？结果证明这和鸟类个体本身有很大关系。
Some, like geese heading south for the winter, are making long treks.
有些，比如冬天南下的鹅，正在长途跋涉。
The V formation helps them stay in visual contact, avoid collisions, and conserve energy.
V字队形可以帮助它们保持视觉接触，避免碰撞并保存能量。
It's the structure of their wings that lets them take advantage of the V.
正是它们翅膀的结构让它们能够利用V字形。
As the wing flaps, each wing tip creates a vortex that spirals up from the bottom of the wing and over the top.
当翅膀扇动时，每个翼尖都会产生一个漩涡，从翅膀底部盘旋而上，越过顶部。
This vortex trails off behind each bird as it moves forward and is encountered by the next one in line.
当每只鸟向前移动时，这个漩涡就会在它们的身后消失，并被排成一行的下一只鸟遇到。
The trailing bird positions itself to catch just the upwash of that vortex, or upward force,
后边的鸟会调整自己的位置来捕捉涡流的上升流，或者向上的力量，
and that requires being behind and just to the side of the leading bird.
这就需要落在领头鸟的后面。
Lots of birds behind and to the side of one another creates the V shape.
许多鸟在彼此的后面和旁边创造了V字形。
Studies have estimated that birds flying this way can save around 15% of their energy. So, why don't all birds fly this way?
研究认为以这种方式飞行，鸟类可以节省约15%的精力。那么为什么不是所有的鸟都以V形飞行？
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.
我们和来自蒙大拿大学鸟类生态学实验室的Erick Greene教授进行交谈，他解释称这和鸟的大小有关。
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.
大家可能已经注意到了，飞成V字形的鸟类—比如鹅、鹈鹕、天鹅和朱鹭—通常体型较大，翼展较长。
These species move their wings only a few degrees up and down with each flap.
这些鸟每次拍动翅膀时，只上下移动几度。
This motion creates vortices that lie pretty neatly behind the bird.
这种运动产生的漩涡就在鸟儿身后。
Small birds, on the other hand, tend to flap their wings all the way up and down.
另一方面，小型鸟类则会上下拍打翅膀。
The vortices created by these motions are all over the place — not consistent enough for their flock mates to actually use.
这些运动产生的漩涡到处都是——由于一致性不足，所以它们的同伴无法使用。
And the small birds that do flap their wings like larger ones just don't generate a big enough vortex because of their size.
而那些像大鸟一样扇动翅膀的小鸟，因为体型的原因，不会产生足够大的涡流。
For small birds, flying in groups sometimes even uses more energy, not less.
对小鸟而言，组团飞行有时甚至会消耗更多的能量。
But these species have another need that's even more important: protection.
但这些鸟类有另一种更重要的需求：保护。
In 1971, evolutionary biologist William David Hamilton proposed a theory called the selfish herd.
1971年，进化生物学家威廉·戴维·汉密尔顿提出了一种被称为“自私牧群”的理论。
It suggests that the risk to an individual is reduced if that animal places another between itself and a possible predator.
该理论认为如果动物将另一只动物置于自己和潜在捕食者之间，那么它遭遇风险的可能性就会减少。
Repeat this across enough individuals and you end up with a herd — or in the case, a flock.
在足够多的个体身上重复这一过程，你就得到了一群——或者在这种情况下，一群。
Other theories offer similar explanations, but, whether you're talking schools of fish or swarms of insects,
其他理论提供了相似的解释，但不论是一群鱼还是成群的昆虫
it's clear that this is a pretty common survival strategy.
很明显，这是一个很常见的生存策略。
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.
所以下次你看到其群鸟飞过，你就知道这可能是为了节省精力。或者只是为了存活。
Thanks for asking. If you can't get enough of SciShow in your eyeballs,
感谢提问。如果你还没有看够，
it's also available for your ears — in the form of our podcast, SciShow Tangents!
你也可以收听我们的播客《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.
加入我们，和我们一起用一些古怪的琐事来互相刁难，写一些科学诗歌。
Download it wherever you download podcasts!
下载播客，下载我们的节目！
