Have you ever noticed a large group of birds flying high in the sky? If you have, then you must have noticed that many groups fly in a shape that basically resembles the English alphabet letter “V”
Here is the standard explanation of V formation.
When bird in the front flaps, a rotating vortex of airolls off each of its wingtips. These vortices mean that the air immediately behind the bird gets constantly pushed downwards (downwash), and the air behind it and off to the sides gets pushed upwards (upwash). If another bird flies in either of these upwash zones, it gets free lift. It can save energy by mooching off the air flow created by its flock-mate.
Every time bird in the lead flaps it's wings the flock will also follow some how the fellow mates sense the free lift and flaps it's wings.
Henri Weimerskirch in 2001,He found that birds at the back of the V had slower heart rates than those in the front, and flapped less often.
Most often position of the first bird or leader bird changes frequently because it will get tired because it breaks the air first,so they change place often.
We can see in military operations they also used V formation.It saves fuel by 10 to 20%.Reference:Graham war wick
Name of the birds in image:Indain Black Cormorant.
Image Shot on Coolpad note.