Energy conservation and visual assurance.
Why do geese fly in a V? Because it would be too hard to fly in an S! Just kidding. 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 authors of a 2001 Nature article stated that pelicans that fly alone beat their wings more frequently and have higher heart rates than those that fly in formation. It follows that birds that fly in formation glide more often and reduce energy expenditure (Weimerskirch, 2001).
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 the communication and coordination within the group. Fighter pilots often use this formation for the same reason.