Why do geese fly in a V formation?
Why, when geese fly south for winter, do they fly in a v formation?
As a bird's (or aeroplane's) wing passes through the air, in order to hold itself up it creates a high pressure area of air below it's wing and a low pressure area above.
The air rushes up around the end of the wing and swirls around, creating a vortex behind each wing tip. This wastes energy for the bird in front but if a second bird flys slighty behind and outside the first bird, the air will be moving upwards. So it will actually give a bit of extra lift, for free, to the second bird, reducing the energy needed for flight.
Air will try and cancel this out by leaking around the end of the wing. This wastes energy and reduces the amount of lift the wing makes (this is why modern aircraft have vertical foils at the end of their wings to try and reduce this).
As flying at the front is more tiring than flying further back, migrating birds actually take it in turns to go at the front, so no one bird gets too exhausted.