A U of M site says: In flight, flocks form large V's or diagonally straight lines. This is because each bird doesn't fly directly behind the others, but off to an angle. This minimizes drag on each individual bird, allowing them to take advantage of the slipstream created by the bird in front of them.
http://animaldiversity.ummz.umich.edu/site/accounts/information/Branta_canadensis.html
I honestly don't know whether straight line or V-formation makes for better aerodynamics, but I do know that if bicyclists tried to ride in a vee they'd be very limited in numbers, because roadways aren't as wide as the sky!