Is it the whole crown, or just little bits of individual feathers. If its the whole crown:
they can dispose of their feather when distressed/hurt/sick or whatever. Fortunately, if they get better, they can regrow their feathers. Unfortunately, when my feather dusters lost its feathers, they both died.
keep params stable and at good levels, dont overfeed, and keep an eye on it. When things start to eat it, it is probably dead, or when the worm has fully left the tube.