Once a bird fledges, therefore, it leaves its parasites behind.

因此，一旦鸟长出羽毛，鸡蚤就离开了。

But they do not (as, for example, lice do) live on their hosts continually. Once a bird fledges, therefore, it leaves its parasites behind.

但是它们不会一直寄生在它们的宿主身上(虱子才会)，当鸟儿羽翼丰满时，鸡蚤就会离开。