Have you ever looked toward the sky on a fall day and witnessed a group of migrating birds? If so, you probably noted the V-shaped formation of the birds or the birds flying in a ball-like formation. Why do birds fly this way? Many theories have been developed to explain the formation patterns of different types of birds.
One theory is that birds fly in certain formations to take advantage of the laws of nature. The birds know that flying in a V-shaped pattern will save energy. Like the lead cyclist in a race who decreases wind force for the cyclists who follow, the lead bird cuts wind force for the birds that follow. This decrease in wind force means that the birds use up to 70 percent less energy during their flight. When the lead bird becomes tired, a more rested bird takes over that position.
But saving energy is important for more than one reason. Sometimes food is short during migration flights. Keeping energy enables the birds to fly longer distances between meals.
When food is sighted, the birds guide one another in a different way. When a bird identifies a familiar feeding area, it might turn around in order to signal the group to change direction. Then, this bird becomes the new leader. It helps guarantee that other birds will know exactly where it is going. Then the whole group makes a change in direction, gently streaming from the sky down to the ground. This formation is like an arrow pointing to the location of food.
Scientists have also studied the birds that sometimes fly in a ball-like formation. Researchers believe that the birds come together if a predator（天敌） is spotted. The predator may then become impatient waiting for a single bird to fly away from the group. The birds will often dip and dive as a group, frustrating even the most persistent enemy. Scientists report that this is a very effective method of defense against an attack.
The birds care for their fellow fliers through teamwork. As transportation expert Henry Ford once said, “If everyone is moving forward together, then success takes care of itself.” When it comes to teamwork, these feathered fliers are a soaring success!
According to the passage, we can learn that birds ___________.

A．move faster than cyclists

B．prefer to fly in a V formation

C．are smarter than other animals

D．play different roles in a formation

When food is sighted, ____________.

A．the group follows the discoverer

B．the lead bird decides what to do

C．the discoverer leaves the group

D．the whole group forms a stream

To protect themselves from attack, birds will _____________ .

A．break into groups

B．come close together

C．change directions

D．fly up and down

Which is the best title for the passage?

A．A Bird’s-Eye View of Teamwork.

B．Success Takes Care of Itself.

C．The Strongest Will Survive.

D．A Science Behind Flying.