This does actually mean killing the birds when they are in flight which can be tricky.
The most obvious place is the beach camp but there is a better area; the first camp you come across in the game is Coastal Forest. Just down from the camp there is ALWAYS a flock of crows on the ground. Go past them and to your right, climbing up the platforms until you find yourself directly above the crows. Using your SHOTGUN, fire into an area next to the birds; they will all take flight at once. Unload your shotgun into the flock; you'll kill 3-4 . Loot the dead ones and go back to the platforms. The flock will have already came back. Rinse and repeat until the trophy pops!.