They fly on the wing currents produced by the birds in front of them. It's similar to drafting someone in front of you while cycling or racing. The leader is breaking trail, and the followers are benefiting by exerting less energy. They end up in a V-formation as birds join up one after the other and find a wing to fly on. After a while, the leader gets tired and falls back onto the wing of another, giving a different bird the opportunity to spear-head through the wind.

Here's a National Geographical article that details the specifics: [Birds That Fly in a V Formation Use An Amazing Trick](http://phenomena.nationalgeographic.com/2014/01/15/birds-that-fly-in-a-v-formation-use-an-amazing-trick/).

From the article: As a bird flaps, a rotating vortex of air rolls off each of its wingtips. These vortices mean that the air immediately behind the bird gets constantly pushed downwards (downwash), and the air behind it and off to the sides gets pushed upwards (upwash)