Why don’t all birds fly in the V formation?
The V-shaped formation allows birds to conserve energy and avoid collisions with other flock members.
By flapping their wings they create vortices of air beneath them, and nearby birds position themselves in such a way as to take advantage of the upwash of that vortex.
This requires the birds to stand a little further back and to the side than those in front of them, thus forming a V. This technique saves up to 15–20% energy.
It is therefore natural to ask why this technique is not exploited by all birds.
The answer lies in the size of the birds. Larger ones, such as pelicans and swans, have a larger wingspan and need to move their wings up and down only a few degrees to fly. This creates a fairly neat vortex that other birds can take advantage of.
Smaller birds move their wings much more frequently and in a more chaotic way. The eddies created are not consistent enough to produce the necessary upwash.
And even those that move their wings more slowly do not seem to generate a useful vortex due to their small size alone.
Flying in flocks, for smaller birds, is instead a strategy used as a protection measure (Selfish herd theory — Wikipedia).