候鸟,是随季节变更作定时迁徙的鸟类。它们通常在一个地方产卵、育雏,到另一个地方越冬,有时只是从山的南坡到北坡,而后返回;有时则要远涉重洋,例如金鸥,每年秋季从北冰洋迁往南半球的阿根廷越冬,第二年春天再飞回原地繁殖后代,往返2万公里。按照达尔文的“自然选择”法则,鸟类在进化过程中,为 Migratory birds are birds that migrate regularly with seasonal changes. They usually lay eggs, brood in one place, go to winter in another place, sometimes only from the southern slope of the mountain to the north slope, and then return; sometimes they have to go to distant oceans, such as the golden gull, which moves from the Arctic Ocean to the southern hemisphere every autumn in Argentina. After the winter, the second spring will fly back to the original breeding grounds and return to 20,000 kilometers. According to Darwin’s “natural selection” rule, birds evolved