迁徙是一个复杂的问题，生物学家对此有不同的定义，定义的方法部分取决于他们研究的动物种类。
Migration is a complex issue, and biologists define it differently, depending in part on what sorts of animals they study.
很多种鸟常常混群共同迁徙，但是当他们停下来觅食休息的时候，你就会发现不同的种类有的在地面上，有的在中层高度，有的则在树顶处。
Mixed flocks of several species often travel together, but when they are feeding or resting you will spot different birds on the ground, at mid-level heights and at the tree tops.
企鹅有迁徙的习性，迁徙路程可长达几千英里，因此专家尚不清楚这些企鹅的出处、年龄及种类。
At present experts cannot say where the penguins were from, their age or their species. It is not unusual for some penguin species to migrate thousands of miles.
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