What formation do geese and other migratory birds adopt for long flights?

Scientists have determined that the V-shaped formation that geese use when migrating serves two important purposes: First, it conserves their energy. Each bird flies slightly above the bird in front of them, resulting in a reduction of wind resistance. The birds take turns being in the front, falling back when they get tired. In this way, the geese can fly for a long time before they must stop for rest. The second benefit to the V-formation is that it is easy to keep track of every bird in the group. Flying in formation may assist with communication and coordination within the group. Fighter pilots often use this formation for the same reason.

Source: Library of Congress