When birds fly in formation, why don't they fly even on both sides?

Answer 1

The sides of the V do not have to be the same length for efficient flying, a few birds more or less makes no difference. They could use one side only but as the birds are social animals they like to be able to see each other so they use both sides of the V.

Answer 2

Birds, particularly the migratory ones, couldn't fly evenly on both sides because it would be detrimental for them especially during long flights.

Flying in a V formation (sometimes called a skein) aids the birds in covering long distances very efficiently by transferring much of the shedding of drag-inducing vortices to the birds at each tip of the formation, rather than allowing shedding from each bird individually. Energy efficiency can be as much as doubled in this manner. The birds flying at the tips are rotated in a timely cyclical fashion so as to spread out flight fatigue equally among the flock members. The formation also facilitates communication and allows the birds to maintain visual contact with each other.

Answer 3

Someone has to lead them to where they are going that knows the way.