题目内容：

    More than 5, 000 species of birds manage annual round-trip migrations（迁徙）. These journeys can be thousands of miles, with many birds often returning to the exact same nesting and wintering place from year to year.

Migration is very important in the life cycle of birds, and without this annual journey many birds would not be able to raise their young. Birds migrate to find the richest and most abundant food that will provide adequate energy to raise young birds. If no birds migrated, competition for adequate food during breeding（繁殖）seasons would be fierce and many birds would starve.

Of course, not all birds migrate. Some birds can take advantage of different food sources（来源）as seasons change, allowing them to stay in one place all year round. Other birds are better adapted to cold climates with thicker fat reserves and better feathers, and they can survive long cold seasons while they forage for winter food. For more than half the world's birds, however, migration is necessary to stay alive.

When the timing is right for their migrating needs, birds will begin their journey. Food, weather, temperature and illness or injury are several minor aspects that may affect migration by a day or two, but most bird species follow precise migration "schedule". While migration is at its finest during spring and fall, birds migrate all throughout the year. Migration is actually an ongoing process and there are always birds at some stage of their journeys. The distance the birds must fly, the length of time it takes to mate and the amount of young birds all affect when any one species is migrating.

Migratory birds have several changes before the journey. Among them is hyperphagia, the process of migration-related weight gain. As daylight changes and migration times near, a bird's hormone levels will change and they will build a greater fat supply. Besides, old, ragged feathers create more wind drag and air resistance, which requires a bird to use more energy in flight, so many birds replace old feathers with new ones.

1.Without migration, many birds would          .

A.stop reproducing during breeding seasons

B.abandon young birds for lack of food

C.be unable to find places for nesting

D.be in danger of starvation

2.What does the underlined phrase "forage for" mean in Paragraph 3?

A.Run out of. B.Cut down on.

C.Search for. D.Wait for.

3.What can we learn about birds' migration?

A.Most birds begin their journey at the arrival of spring.

B.Warm temperature usually advances it by a day or two.

C.There is almost little migration of birds in winter.

D.Birds' migration is affected by various aspects.

4.How will migratory birds benefit from the process of hyperphagia?

A.They will tend to use less energy in flight.

B.They will store enough energy for travelling.

C.They will replace old feathers with new ones.

D.They will better sense the changes of daylight.