题目内容：

    For a bird, to fly hundreds or thousands of miles each spring and fall is a difficult and dangerous journey, one that not all birds can survive. 1. There is more than one single reason for different birds to migrate(迁徙), but it all comes down to survival.

Birds are migrating for a meal.

For all birds, one of the main driving forces behind migration is food. If all birds were to stay in the same places year-round, food there would become not enough. 2. But as the food supplies decrease in the fall, they return to the southern areas.

3.

Over millions of years, birds have developed different migration patterns, times and destinations. This helps birds take advantage of a wide variety of suitable conditions to raise their young, which increases the survival chances of the young birds.

Birds are migrating because of climate changes.

Changes in climate can affect migration. Many birds leave the Arctic for example, when temperatures begin to fall. They need warmer habitats. Similarly, the hottest regions can be an unpleasant environment for raising young birds. 4.

Birds are migrating because of predators.

Habitats that have rich food sources also attract a greater number of predators who may kill birds. 5. Many birds even migrate to specialized habitats that are nearly inaccessible to predators, such as coastal cliffs or rocky offshore islands.

A.So why do birds migrate?

B.Birds are migrating for their young.

C.So how do young birds migrate?

D.Birds are migrating because of diseases.

E.It is beneficial to lay eggs further north in cooler areas.

F.Birds that migrate to different habitats can avoid being eaten by predators.

G.As food sources are rich in the north each spring, millions of birds migrate from south to those areas.