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 upwash 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 long-distance migration, orientation and communication remain necessary, but there is also much to be gained for each bird in the flock by optimizing its position to conserve energy.
Fluid dynamics and energy wave configuration calculations have been used to test predictions of where birds should position themselves in relation to others to conserve the most energy as they travel through the air. Analyses of flock formations using photography have measured bird positions and found them to almost always be located such that they gain some energetic advantage. The animals are not very often in the expected optimal location, however, indicating that other factors also influence position in the formation.
Knowledge of birds¿ visual axes, "blind spots" and field of vision have allowed researchers to pinpoint the best locations for birds within a flock to maintain optimal visual positioning. Actual positions of the animals are usually positively related to these predictions but are, again, not always optimal. Studies have categorized the positions of birds and found that some individuals take positions that are most closely predicted to satisfy the energy conservation hypothesis; others are in better visual contact positions; and still others are not apparently responding to either benefit or are in a position that should gain some advantage from both benefits.
The leaders of formations change from time to time, but the causes, frequency and characteristics of these changes have not yet been determined. Sustained observation from the ground of flocks covering great distances in the air is very difficult. There are plenty of intuitive predictions about leader choice that quickly come to mind relative to the age, experience, sex, condition and social status of the leaders, but researchers have not figured out how to overcome the prohibitive logistic issues to test them. Some scientists have trained birds to fly in formation with small aircraft; perhaps their experiences will yield opportunities to test these ideas.
Answer originally posted Feb. 23, 2004.