I've seen these elven-bound immigrations before, but i want to know is whether it is the site map (drawn to the wild places, forest retreats etc.) or the civ that prompts these events to happen.
However as a bit of trivia, if you open up the goblin civilisation (by modding full player functionality for dwarf & adventure mode, so you're playing the civ faction), though they have no events like this in legends & steal babies to create surplus population non goblin in origin they have a large variety of often large carnivore & vermin animalmen from a variety of places already within the concurrent society, alligator men are a fun one, coyote men, tick men etc. (though unless fly men are present, all other races generally are outscaled by the number of trolls & goblins)
> Dwarves on the other hand have plump helmet men quite consistently based off their adjacency to the underground, while goblins often have Ratmen who are also a underground race, both the goblins & dwarves have site z levels with underground access that let them accommodate these races.
> No need to be too specific since generally animalmen adventurers travel around a lot, but moreso about the specific places where they 'settle' & where they were brought up is important, and this is what the carnivorous & large predator selection in goblin (sometimes across humans too) animalmen adventurers comes from.
> This crow woman necromancer prior to being a crow was originally 'settled' in the dull hills as a historical figure (whether they were historical before or after they got struck with necromancery desire is unknown) and moved into the nearest settlement to fulfill their needs to contact a death god (which my modded goblins provided directly) and fufill the wish.
Beside from immigration, i would suggest this means that based on values or atleast subtle preferences based off existing tags, certain animalmen are more inclined to side with 1 civilisation in worldgen rather than another as a trigger based off the site location range (for immigrations to elven sites) & undefined values we are not aware of (though there's nothing else im aware of mentioned that would support that, that is my hypothesis)
Technically every animal person is a non historical figure in legends when it appears that (from my crow necromancer picture) every single living thing has the same (settled in '______') at world generation but it's not relevant to legends or the player so its left out for beaurocratic reasons.
Animal people could move out at literally any time if you gave them the inclination to through a personal wish implanted on them or a command to move out of the non-relevant historical figures & exist but otherwise they happily trundle about the landscape where they were born & will die. If somebody could DF therapist a animalperson actively immigrating to a elven site that'd be insightful to see if a desire like 'survive' is being acted on.
Edit - I've just had a bit of anomaly that's thrown my logic a bit off centre, Ratmen in my small world generated elven civilisation exclusively, which is confusing unless the ratmen have literally emigrated to them through the ground with no underground access (
) i honestly expected them to be the next site over, but maybe there's another reason like a cave involved unless its random placement.