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One of the most remarkable sights over Europe in the spring is the migration of large groups of birds from the south. Each group is composed of individuals of different strengths and speeds yet, thanks to an aerodynamic trick, they manage to move together as a group. The trick is to fly in a tight V-shaped formation in which the strongest and fastest birds stay at the front. These birds take the brunt of the air resistance, which slows them down. The weaker birds fly behind the leaders, taking advantage of the low-pressure region behind the stronger birds. This allows the weaker birds to move faster and with less effort. The overall result is that the group moves as a whole without dispersing, held together by air pressure at a uniform velocity.
Izo Abram (Mon,) studied this question.