I guess it's fair to leave it out and leave the interpretation to the DM. But what I was suggesting was just the second paragraph of the symbiont rules and the default for magic items.
Very good, then. I'd go for about 20 hp, maybe 10 hp if you want them fragile.
It's just a few feathers, but it is magical. And I wouldn't want it to be too easy to destroy with, say, just a match. How about 8 hp?
Let's say 8 hp, hardness 2. Does that work for you?
That'd be OK, although giving it hardness feels a bit odd:
Feather Garment (Su): All bird maidens possess an enchanted feather garment which grants them all the other special qualities of this template (bird shape, low-light vision, damage reduction, and possibly evasion). Should another creature steal this garment, the garment-thief gains a +4 bonus when using Charisma-based skills or attacks with Will saving throws against the bird maiden.
If the feathered garment is destroyed (8 hp, hardness 2, takes double damage from fire), the bird maiden automatically dies at the end of that round. Spells that restore souls to their bodies, such as raise dead, reincarnate, and resurrection, don't work on a bird maiden whose feather garment has been destroyed. It takes a different magical effect, such as limited wish, wish, miracle, or true resurrection, to restore it to life, which also recreates the maiden's feather garment.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.