[单项选择]The linear flight formations of migratory birds are called echelons. The V and the J structures are typical and are the most readily recognized flock echelons, but other variations also occur. Studies of several species have shown that a true V-shaped echelon is, in fact, less common than a J formation is.
There are two well-supported and complementary explanations for why birds fly in formation. One is to conserve energy by taking advantage of the upward vortex fields created by the wings of the birds in front. The other is to facilitate orientation and communication among the birds. These explanations are not mutually exclusive, and both have been backed by a variety of studies. The relative importance of each undoubtedly shifts as various factors, such as the season of the year or the purpose of individual flights, change. During local feeding flights, for example, energy conservation is probably much less important than careful orientation and collision avoidance are. During l
A. conserve energy.
B. help orientation.
C. facilitate communication.
D. All Above.