Why do geese fly in a V?

Energy conservation and visual assurance.

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 him, 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. Fighter pilots often use this formation for the same reason.