There have been several theories as to why some birds fly in a V formation.
One of the explanations has been that it helped with lift, as birds generally change position, taking turns being the lead bird. Now scientists at the Royal Veterinary College in England have attached data loggers to the back of migrating ibis. The data indicates the ibis time their wingbeats and change their position to give them an aerodynamic advantage.
The ibis species was the endangered Bald Ibis, a species a friend of mine studied in South Africa many years ago. I was able to visit him during his study and had the opportunity to spend the night in a hut with cow dung floor. Not as bad as it sounds, and the ibis were nesting on a cliff along side a beautiful waterfall.
Here is more info on the study. http://www.bbc.co.uk/news/science-environment-25736049