Cleon
Legend
How's this?
Feather Garment (Su): All bird maidens possess an enchanted feather garment which allows them to assume bird form (see Bird Shape, above), low-light vision, damage reduction, and evasion. Should another creature steal this garment, a bird maiden is unable to attack them unless the thief attacks the bird maiden first (as per the affect of the sanctuary spell). Furthermore, the silkie has a -4 penalty to resist all Charisma-based skills and saves against enchantment spells performed by the garment-thief.
If a bird maiden's feather garment is destroyed, the bird maiden dies.
Looks a good start. I'm inclined to drop the penalty to Cha-skills and enchantments, as there's nothing like it in the original.
The mention of evasion needs tweaking as only more experienced maidens have it.
Finally, I'd like a bit more info in the "destroying the garment kills the maiden" bit.
How hard is it to destroy the garment?
Presumably it can't be damaged while the maiden wears it (like a symbiont) and only has a few hit points with no hardness when seperated. I'd suspect it's also rather flammable.
How quickly does the maiden die?
Is it immediate, at the end of the round, gradual (Con damage per round or a constantly increasing Fort save vs death?)
What happens if they're raised or ressurected?
My guess is a raise dead returns to life but without the Bird Maiden template. We could have high-powered magic (wish? true resurrection?) restore the garment and return them to life as Bird Maidens.
...which rather begs the question as to how they gain this template in the first place.