它们往返数千公里,付出减轻三分之一的体重,甚至生命的代价,只为遵守几百万年来一直延续的一个约定。归来,无论往南或向北,不会有罗盘和坐标。能为它们导航的,只有基因里它们自己才懂的密码。 They travel thousands of kilometers and pay a third of their weight, or even their life, for just a deal that has continued for millions of years. Back, no matter south or north, there will be no compass and coordinates. The only ones in the gene that they can understand are the ones that can navigate for them.