曾有一场贫困农民向城市的大规模迁移。

There was a mass ___migration of poor farmers into the cities.

蝴蝶在它们的年度迁徙期间来到墨西哥。

The butterflies come to Mexico during their annual ___migration.

同时，也存在从农村地区向海岸的显著人口迁移。

There is also significant ___migration from rural areas to the coast.