Because Pelt No Longer Considers Them Part Of Itself - Tumblr Posts

If you know about how selkies/swan maidens pelts/feather cloaks work, pelts for bird people work in essentially the same way. Stealing the pelt would make bird person lose their free will and obey the thief. Stealing just a couple of feathers would make bird person attached to thief, but still somewhat aware that it is wrong. In certain bird people it can create painful dissonance in their head mind splitting between loving the thief and being in anguish over lost freedom