A research team at London's Royal Veterinary College has discovered the aerodynamic reasoning for this pattern through extensive data collection during flight. Their work was published in the most recent edition of Nature.
The research team studied a flock of 14 northern bald ibises. Specialized GPS devices provided the researchers with detailed information on the bird's movements during flight-compiling location, speed, frequency of flapping of their wings, and direction-during a flight of 43 minutes in a nearly 45 degree angle V formation (relative to the bird flying at the point of the formation). Over 180,000 wing flaps worth of information was gathered and analyzed. What the research team discovered was that the spacing of the birds and the timing of the wing flaps were essential to maximize beneficial air currents, and make for more efficient flight.
This makes sense on a fundamental level-anyone who has driven their car behind an 18-wheel truck on a windy day can attest to the powerful aerodynamic forces. The backwash from the aerodynamics of the 18-wheeler is buffeting your car, and the effect is very dependent on your position. Race-car drivers have also understood this for years, positioning themselves precisely behind fellow racers to "draft" off of them and maximize the aerodynamic effect-and of course pilots must consider interfering wind patterns as they fly. On a different scale, the birds are essentially doing the same thing.
The birds phase their wing beats to create a coherent airflow path throughout the flock, gaining lift from the rising air (upwash) and bypassing the drag associated with sinking air (downwash). Their position is equally important to utilizing upwash and avoiding downwash. This is why the adjustments are always in unison-there's a cascading effect on lift relative to the lead bird, and to get the full benefits of the lifting air, it's important that the movements be as uniform as possible.
Without understanding the complex aerodynamics behind their movements and synchronization, the birds automatically adjust their flapping pattern and relative positions to take advantage of positive air currents in a way that would make the Blue Angels proud. This implies a level of sophistication that many scientists did not expect birds to possess.
There probably aren't any new aerodynamic principles to be uncovered here, but the coordination effort of the birds is worth further study. Even though the scales are significantly different, by connecting aerodynamic data with the flight pattern of birds, this type of research could lead to new developments in flight feedback systems, or new fuel efficiency protocols for flying machines.