It’s more efficient. Every time a bird flaps its wings, it generates a vortex of air at each wingtip that spirals behind the bird, widening as it gets farther away. A bird following behind can position itself at the edge of this vortex, at the point where the air is circling upwards. This updraught provides lift, which means that the bird needs less energy to stay aloft. For large birds like geese, the effect can reduce drag by up to 65 per cent. They take it in turns to fly at the front, so that each one benefits from the vortex effect.
Answered by Luis Villazon
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