The wild bird Pokémon of Kalos have no clear pattern however there is a method I mentioned in the comment above to find them more easily. They will reside in a final area that shows up on the Pokédex.
As for the other legendaries they don’t need to be found 11 times(or even twice):
Mewtwo can be found sitting in the Unknown Dungeon (Requires surf) in the Pokémon Village.
Zygarde can be found in terminus cave. Lowest floor.