阅读下面的对话，掌握大意，然后从各题的三个选项中选择能填入相应空白处的最佳的一项。

Nancy: Look—Mum. There are many big birds in the sky.

Mum: They're wild1. They are changing homes.

Nancy: Why do they change homes?

Mum: 2winter is coming. They cannot find food in the3in winter. So they have to fly south.

Nancy: Will they come back?

Mum: Yes. In spring, it is4in the north. They will fly back.

Nancy: Wow! They move around so much. They have busy5.