Why do birds go in a arrow?

The reason that many birds fly in a "v" formation or an "arrow" formation is for aerodynamics. The first bird "parts" the air and helps the ones behind it. It is like the front end of a car, how it "parts" the air for the rest of the car. If you had a car with a front that was a flat wall, it would move slower and therefore use more gas, be less efficient, and use more energy. It is the same idea with the birds. The front helps the birds in the back, and they rotate, and take turns in front, sort of like an Indian run because the birds get tired.

I hope that this explanation answers your question clearly.