Why do birds fly in a V formation?

The V is aerodynamics. A flapping wing leaves behind a rotating wake, with downwash directly behind and upwash off the wingtips, so a bird positioned slightly behind and to the side of another can ride that rising air and generate lift for less effort. Research using heart rate monitors and, in one notable study, data loggers on ibises trained to migrate behind an aircraft, found birds precisely positioning themselves in the upwash and even timing their wingbeats to match the favorable phase of the leader's wake, which is a genuinely sophisticated bit of flight control.

The savings are substantial, with estimates that formation flight can meaningfully reduce energy expenditure over long migrations, which is the difference between arriving and not. The lead position gets no such benefit, which is why birds rotate through it rather than having a permanent leader, sharing the cost across the flock. The staggered arrangement also gives every bird a clear line of sight to the others and to the direction of travel, aiding coordination and collision avoidance. Not every flocking species uses a V, it appears mainly in larger birds with slower wingbeats, geese, swans, pelicans, cranes, ibises, where the wake is large and stable enough to exploit, while small birds flocking in dense clouds like starlings are doing something entirely different, largely predator avoidance.