Why do geese fly in a V formation?

Breaking down a few articles, here are the reasons scientists think geese fly in a V formation.

1) Field of vision. From first to last each has the same view (as opposed to when flying in a single row, like straight drafting, or in a group, like a bicycle peloton or swarm of birds).

2) Aerodynamics. Geese can achieve a greater distance of about 70% when flying in groups than each flying solo, using the same amount of energy. In this formation, the bird in the lead position will experience a greater air resistance and will have to work a little harder, however, when the lead bird is tired it falls out of position and goes behind into one of the V position lines. Another bird from behind will then quickly move forward into the leading position thus, maintaining the V formation. The same happens for those birds in trailing positions especially those at both extreme ends of the V formation, since they tire more quickly than those in the middle. This cycle continues as they travel to their destination.

Also, heart rate seems to be lowered by anywhere from 12-20%.

3) Social aspect. If a bird in the formation falls ill or is shot, two other birds will accompany it on the descent, aiding and protecting the injured bird until it either recovers or dies. The two helpful geese will then rejoin the formation.

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Here's some more in-depth information regarding the aerodynamics of the situation and wingtip vortices.These vortices are generally undesirable because they create a downwash that increases the induced drag on a wing in flight. However, this downwash is also accompanied by an upwash that can be beneficial to a second wing flying behind and slightly above the first.

A bird flying in one of these upwash regions essentially gains free lift so that it can fly at a lower angle of attack. As angle of attack is reduced, the induced drag is also lowered so that the bird does not need to flap its wings as hard or as often to generate the thrust needed for forward flight. Flapping the wings less often means that the bird's muscles do not work as hard and its heart rate drops. As a result, the bird does not tire as quickly and is able to fly farther. Researchers explored this theory by monitoring pelican heartbeats during flight. Examination of the data showed that the heart rates of pelicans flying in formation were much lower than that of a bird flying alone. Other studies have estimated that a flock of 25 birds in formation can fly as much as 70% further than a solo bird using the same amount of energy.

The majority of the benefit goes to the birds further aft, but the front bird does still gain some reduction in drag. The presence of the two birds flanking the leader helps to dissipate the downwash off the lead bird's wingtips and reduces the induced drag this bird experiences. These two flanking birds also benefit from a similar reduction in drag if outboard birds flank them as well. In other words, the birds in the middle of each of the lines forming the V are in the best position. These birds benefit from the upwash off the lead birds as well as off the trailing birds. This additional bonus means that birds in the middle experience less drag than either the lead bird or the bird at the end of each line.

ref:
http://kottke.org/09/10/the-flight-patterns-of-geese
http://www.associatedcontent.com/article/348057/why_do_geese_migrate_in_a_v_shape.html
http://en.wikipedia.org/wiki/Swarm#Birds
http://www.myuniversalfacts.com/2005/11/why-do-geese-fly-in-v-formation.html
http://news.bbc.co.uk/2/hi/science/nature/1608251.stm
http://www.aerospaceweb.org/question/nature/q0237.shtml