I have modified the spawning system in the world in such a way that the mob is added to the normal spawning algorithm of minecraft, avoiding that it can be substituted for other mobs.
You can go to the randomBird.yml file and change the "chance" from 0.10 to 1.00
You can change the worlds where it should spawn.
Can be single world or a comma separated list.
world snowyworld, lavaworld, islandworld