(单词翻译:单击)
The passenger pigeon was once the most abundant bird in North America—maybe even the world.
候鸽曾是北美数量最多的鸟类—或许甚至是世界上最多的鸟类。
In the first half of the 19th century, estimates put the number between 3 and 5 billion,
在十九世纪上半叶，其数量估计在三到五十亿之间，
which would have been at least one-third of the total bird population of North America today.
这个数值已占到了如今北美鸟类总数量的至少三分之一。
But while flocks could block out the sky in 1833, by 1900 there were no wild birds to be found.
虽然鸟群在1833年能布满整个天空，但到了1900年，已经找不到这种鸟类的踪迹了。
And by 1914 there were none alive even in zoos.
到1914年，连动物园里都见不着活的候鸽了。
So what happened to make the population go from billions to zero in less than a century?
所以到底发生了什么让它们的数量在不到一个世纪的时期里从数十亿降至零呢？
To understand how passenger pigeons died, we need to understand how they lived.
为了弄清候鸽为何消亡，我们需要明白它们如何生存。
Because… it was kind of our fault.
因为...我们也有过错。
Passenger pigeons were native to eastern North America and migrated between the regions around the Great Lakes in the north and the Gulf of Mexico in the south.
候鸽原产于北美东部地区，在北部五大湖地区和南部墨西哥湾地区之间迁移。
They traveled in huge flocks with hundreds of thousands of pals.
它们成群结伴的飞行，一群鸟中有数百只同伴。
When roosting or nesting, it wasn't uncommon for so many birds to perch on a tree that the branches would snap under their combined weight.
当栖息或筑巢的时候，众多只鸟栖息在一棵树上，树枝在它们的重压下断掉是很普遍的事情。
Sometimes birds would even perch on top of each other just for a place to sleep. Real cozy.
有时鸟儿们甚至栖息于彼此头顶只为了有个睡觉的地方，真的是非常舒适了。
This strength in numbers was their main defense against predators like wolves, foxes, weasels, and hawks.
数量上的优势是它们抵御捕食者，比如狼、狐狸、鼬鼠和老鹰的主要防御。
The local carnivores could eat themselves sick and still not make a dent in the flock.
当地的食肉动物即使吃到吐了，仍没有削弱鸟群分毫。
Ecologists call this predator satiation, and it's a pretty common defense tactic in both plants and animals.
生态学家称这种捕食者厌足，这在植物和动物界是一种相当常见的防守战术。
It's why groups of oak trees produce almost no acorns for two years, then on the third year it's an acorn party.
正是因为如此，橡树在两年时间中不生产任何橡子，然后第三年则是它们的橡子盛宴。
It's also why cicadas reproduce periodically, why salmon clog streams during their breeding season, and why caribou travel in massive herds.
也是因为如此，蝉会定期繁殖、鲑鱼在繁育季节会阻塞溪流、北美驯鹿会大群的迁徙。
Now, passenger pigeons worked together to raise their young and to find food like nuts, seeds, and berries.
现在候鸽共同养育后代，寻找食物，如坚果、种子和浆果。
So when humans started cutting down forests to plant crops, pigeons went after those grains too.
所以当人类开始砍伐树木种植庄家的时候，鸽子也为了谷物而来。
And, as you might guess, that made them pretty unpopular with farmers.
不难猜到，农民们非常讨厌这些鸽子。
But, really, everything changed when we started hunting the pigeons as a commercial food source…because they were apparently super tasty.
但是当我们开始捕食鸽子，将它们作为一种商品食物来源时，一切都开始改变了...因为它们超级美味。
The expansion of train networks made it possible for hunters to kill and ship the birds nationwide. And it wasn't hard.
列车网络的扩张让猎人能够捕杀鸽子并将它们送往全国各地。这并不困难。
Because they traveled in dense, low-flying flocks, hunters could basically just swing a stick and hit a couple birds.
因为它们大群的飞行，飞得很低，猎人们基本上挥舞着长杆就能打下几只鸽子。
To satisfy the demand, hunters started using baited traps and controlled fires to kill more birds in one go.
为了满足需求，猎人们开始使用诱饵陷阱和控火的方法一下子捕获更多的鸽子。
They even burned sulfur below pigeon nests to suffocate them, collecting the bodies as they fell from the trees.
他们甚至在鸽子的鸟巢下燃烧硫磺使这些鸽子窒息而亡，然后收集从树上掉下来的鸽子尸体。
The pigeons were apparently so tasty that no one questioned whether eating birds killed by fire and smoke was a good idea or not.
鸽子显然太过美味了，以至于没人质疑吃这些被大火烧死和被烟熏死的鸟是好是坏。
Even as late as the 1870s, people thought there were so many pigeons that it wasn't possible to hunt them too much.
甚至是到了1870年代后期，人们还认为这种鸽子数量非常多，猎人是捕不尽的。
But once their flock numbers dipped too far, the situation got dire.
但是一旦它们的数量急剧下降，情况就会走向极端。
By 1900, the only surviving birds lived in zoos across the midwest.
1900年，仅存的几只鸽子生活在美国中西部的动物园里。
And on September 1st, 1914, the last captive passenger pigeon, named Martha after Martha Washington, died at the age of 29.
1914年9月1日，最后一只被捕获的候鸽，29岁的玛莎（以 玛莎·华盛顿命名）死亡。
Looking back, scientists have tried to figure out why the passenger pigeon couldn't survive in smaller flocks —
回顾往事，科学家们试图探寻候鸽无法小群体存活的原因—
like, maybe they couldn't defend themselves or breed successfully.
比如，或许它们无法保护自己或是成功繁育。
In 2014, a group of scientists published a paper suggesting that passenger pigeon populations likely fluctuated even before modern humans arrived.
2014年，一组科学家发表了一篇论文暗示候鸽数量或许在现代人类到来之前就是浮动的。
In fact, they may have been in a natural decline that they had recovered from in past cycles.
事实上，它们只是从过去的周期中恢复，它们的数量是在遵循自然性的下降。
But then humans came in, messed around, and ended up being the last nail in the coffin.
但是人类闯了进来，混乱了它们的周期，最终成为了钉死棺材的最后一颗钉子。
These scientists used ecological models that generally say that bigger populations mean larger effective population size —
这些科学家所利用的生态模型通常表明更大的种群意味着更大的有效群体大小—
which is a measure of how many individuals are making babies and contributing to the total gene pool.
这是多数个体如何繁育下一代，如何为基因库做贡献的测量方法。
And the neutral theory of molecular evolution suggests
分子演化的中性理论表明
that the effective population size should be linked to DNA variations called neutral mutations, which don't really affect survival.
有效群体大小应与被称为中性突变的DNA变异有关联，这并不影响存活。
Basically, more breeding adults means more neutral mutations randomly cropping up..
从根本上说，更多的繁殖成年意味着或有更多的中性突变随机产生...
So, theoretically, large populations should result in high genetic diversity from bird to bird.
所以从理论上来讲，大种群的结果应该是鸟类之间更多的基因多样性。
But — according to their DNA analyses — genetic variation between passenger pigeons was low, despite how many there were.
但是，根据他们的DNA分析来看，候鸽之间的遗传变异很低，虽然它们数量很多。
Based on that fact, the researchers estimated that the pigeon population dipped down in cycles,
基于事实，研究人员对周期中下跌的鸽子种群进行预估，
which could explain why the billions of birds seen in the 1800s seemed like not-so-distant relatives.
这可以解释为什么1800年代数十亿只鸽子看上去像是它们的近亲。
This idea was seemingly supported by pollen records from the last 20,000-odd years suggesting that
这种想法表面上得到了最后两万余年花粉记录的支持，其表明
acorn production varied enough that huge bird populations couldn't always survive.
橡子生产十分多变从而导致大量鸟类种群无法生存。
However, a 2017 study suggested that the lack of genetic diversity came from rapid evolutionary adaptation instead.
但是一项2017年的研究表明缺乏基因多样性源于快速的进化适应。
These scientists pointed out that there are plenty of populations whose size doesn't correlate with genetic diversity.
这些科学家指出有大量的种群，它们的规模与基因多样性并无关联。
And this might have to do with how new mutations can spread.
这或许和新型变异如何延伸有关。
When organisms make sperm and eggs, there's a mix-and-match process where DNA gets swapped around called recombination.
生物体在制造精子和卵子时，会出现一种混合搭配过程—重组，DNA在此过程中进行交换。
This makes it so every offspring has a unique set of genes.
这使得每个后代都有着其独特的基因组。
But recombination involves swapping big chunks of DNA.
但是基因重组中还涉及交换较大的DNA块。
So as an advantageous gene spreads like wildfire, whole stretches of DNA spread.
所以当一个有利基因像野火一样伸展时，DNA整端都在伸展。
That could explain why there doesn't seem to be a lot of genetic diversity in some big populations, like passenger pigeons
这可以解释为什么一些大型种群，比如候鸽中似乎没有很多基因多样性
despite what the neutral theory suggests.
虽然中性理论如此。
So these researchers think that the pigeon population was always big,
所以这些研究人员认为鸽子种群总是很大，
and that their huge drop-off in their numbers probably wasn't normal… so it was largely our fault.
并且它们数量上的急剧下降或许并不正常，所以大部分是人类的错。
It's a gloomy thought that we caused the passenger pigeon extinction, but it did inspire the first wide-scale laws aimed at species conservation.
或许是人类导致了候鸽的灭绝，但是这也确实引发了首个针对物种保护的大规模法律。
In the 1890s, there was almost no regulation on hunting or killing any kind of wildlife.
1890年代，几乎没有关于捕获或猎杀任何野生动物的规定。
But in 1900, this looming extinction inspired a federal law prohibiting interstate shipping of illegally hunted game.
但是在1900年，这种隐约的灭绝授意了一项禁止州之间非法捕获运输的联邦法律。
13 years later, the Weeks-McLean Act added a ban on hunting migratory birds in the spring, when many species are breeding.
13年后，威克斯一麦克林法案中添加了一项法律规定—在多数物种繁育的春季里禁止捕获候鸟。
It also banned imports of wild bird feathers used for fashion.
法律还规定禁止进口用于时尚的野生鸟类羽毛。
Today, we have many such laws protecting wildlife and we keep a much closer eye on endangered species —
今天，我们有很多类似法律保护野生动物，我们更加密切的关注这些濒临灭绝的物种—
so we can hopefully prevent more stories like the passenger pigeon.
希望能够预防类似候鸽这种事件的再次发生。
Thanks for watching this episode of SciShow, which is produced by Complexly —
感谢收看本期《科学秀》，本期视频由Complexly出品—
a group of people who believe the more we understand about the world, the better we are at being humans.
Complexly中有很多人相信，我们对这个世界了解的越多，我们就能更好地尽到人类的本分。
Even if humans aren't always so great.
虽然人类并非总是如此伟大。
If you want to learn more about all kinds of animals that we still share the planet with, check out Animal Wonders at youtube.com/animalwondersmontana!
如果你想了解更多与人类共享地球的动物，登录youtube.com/animalwondersmontana，观看《动物奇迹》栏目！
