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Snowy owls make their summer nests north of the Arctic Circle on high ground.
雪鸮把它们夏季的巢穴建在北极圈北部的高地上。
Their plumage changes little with the seasons and their nests are easy to spot against the exposed tundra soil.
它们的羽毛几乎不随季节而变化，而它们的巢穴在苔原上裸露的泥土映衬下极易被识别。
This female has ten eggs to protect. Her plumage is darker and more mottled than the males.
这只雌性有十只卵需要保护，它的毛色比雄性略深斑点也略多些。
Some tundra animals do change with the seasons. This arctic hare's upper body will darken for the few short weeks of summer.
有些动物的确会随季节而改变，这种北极野兔的上半身毛色会在夏季短暂的几周里变得稍暗。
In places, the growing season lasts just six weeks, so the plants here have had to adapt.
在有些地方，生长季仅仅会持续六周时间，所以这里的植物必须适应。
This explosion of life attracts a massive gathering of migration birds.
这种暴发式的生态繁荣引发了候鸟的大量聚集。
In spring, hundreds of millions of birds make the annual trip to the arctic tundra.
舂天里，数以亿计的鸟类像往年一样踏上了前往北极苔原的旅程。
But one species, the arctic tern, flies to northern Europe all the way from the Antarctic. No animal on earth migrates this far.
而有一种鸟类，北极燕鸥，却从遥远的南极洲一路飞抵北欧，地球上没有任何动物能迁徙这么远。
Its annual trip of seventy thousand kilometers means it will fly nearly two million kilometers in its lifetime.
它们一年一度的旅程长达七万公里，这意味着每只鸟一生中的飞行旅程长达将近两百万公里。
And the reason that terns fly so far to these wetlands lies just below the surface.
燕鸥不远万里飞来这片湿地的原因就隐藏在这水面之下。
As the pools warm from the extra sunlight, insects are hatching.
水潭随着太阳光照的增强而变暖，昆虫因此得以孵化。
Right across the tundra, billions of mosquitoes are hatching.
整个苔原上，几十亿只蚊子开始孵化。
It's what the birds have been waiting for.
这正是鸟儿们所期待己久的。
But even a billion birds can't cope with this many mosquitoes.
然而即使这数以亿计的鸟类也无法对付如此多的蚊子。
The insects form vast swarms and head off in search of the meal.
昆虫大量汇集成群，飞到各处寻找食物。
In Alaska, the emergence of the mosquitoes coincides with the arrival of the porcupine herd.
在阿拉斯加，大量蚊子孵化时恰逢格兰特驯鹿刚刚抵达。
They finally completed their epic migration to take advantage of the lush vegetation.
它们好不容易完成了史诗般的旅程，正打算享用丰美的青草。
But the presence of the mosquitoes makes the tundra this time of the year one of the most inhospitable places on the planet and the herd must keep moving in search of respite.
但是蚊子的出现却把此时的苔原变成了地球上最不适宜居住的地方，而驯鹿群也不得不继续前行寻找栖息之地。
Once this massive reindeer herd is in motion, not even the hundreds of fast flowing melt water rivers can stand in their way.
一旦这一大群的驯鹿行动起来，那几百条河流中湍急的融冰之水也无法阻挡它们的脚步。
The reindeer can swim amazingly fast, up to ten kilometers an hour.
驯鹿游水的速度惊人，可以高达每小时十公里。
For many of the calves, this will be the first time they cross the river so big.
对很多小鹿来说，这将是它们首次跨越这么宽的河流。
The herd may have shaken off the mosquitoes, but crossing the river comes at a price. And some gets separated.
驯鹿群也许可以藉此摆脱蚊虫的纠缠，但过河也会付出代价，而有些驯鹿掉队了。
