候鸟要迁徙。 ( Migratory birds are to migrate. )

候鸟迁徙要飞行很长的距离 ( Migratory birds migrate long distances )

随着全球气候变暖，候鸟的习性也发生了变化，有些已经不再按时迁徙。 ( With the global warming, the habits of migratory birds have also changed, and some are no longer migrating on time. )