题目内容：

    Migration(迁徙), is a long journey carried out by some birds to other areas where they hope to find more food and a warmer place to spend winter. Hundreds or thousands of miles' journey is actually difficult and dangerous. So why do birds migrate?1.However, it all comes down to survival.

Migrating for a Meal

For all birds, one of the main driving forces behind migration is food shortage. If all birds were to stay in the same warm regions all the year round, food would become lacking and producing their young would be less successful. As food resources are rich in the north each spring, millions of birds migrate there for the food supplies.2.

Migrating for Family

Over thousands of years, birds have evolved different migration patterns, timing and destinations to fly around the world to produce their young. Birds take advantage of a wide variety of suitable conditions to raise their young.3.

Migrating for Climate

Birds have evolved different types of feathers to survive different climates.4.So many birds leave the Arctic producing grounds, for example, when temperatures begin to fall and they need warmer habitats. Similarly, the hottest regions can be a bad environment for raising little chicks, and they will choose cooler areas to lay eggs.

Migrating for Predators(掠食动物)

Habitats that have rich food resources all the year round also attract a greater number of predators that can threaten their nests.5.It gives their young a better chance of reaching maturity. Many birds even migrate to specialized habitats that are nearly inaccessible to predators.

A.Birds always tend to fly to warmer places.

B.And changes in those climates can affect migration.

C.This increases the surviving chances of little chicks.

D.Birds have adapted themselves to their own surroundings.

E.There is more than one reasons for different birds to migrate

F.As the food supplies decrease in fall, they return to warm regions.

G.So birds migrate to different habitats to avoid being eaten by predators.