they must be in position before the thaw starts. 企鹅们必须赶在解冻期结束前做好就位的准备

21. Instead of waiting for the summer melt, the new arrivals hasten south over the frozen sea.
    还没等到夏季解冻期的来临 这些鸟儿就急急忙忙从冰封的海洋赶到南极
22. They have come here to breed,
    它们来这里繁衍
23. but polar summers are so short,
    但是极地的夏天是非常短暂的
24. they must be in position before the thaw starts.
    企鹅们必须赶在解冻期结束前做好就位的准备
25. As the sea ice retreats,
    当海冰的面积缩小的时候
26. life can journey farther south.
    动物便能长驱直入来到南极的腹地
27. Antarctic waters are so rich that visitors come from far and wide to harvest them.
    南极富饶的海水吸引着千里之外的客人来觅食

来自《行星地球 第一季 第六集 Planet Earth S01E06 冰雪世界》第 24 行