Are they breeding? if yes then he's using them as nesting material. Give him other nesting material.
If no then maybe stress? or he's being aggressive. You'll have to watch them to catch the one doing the act. she might be doing it herself out of boredom.
If it is the male separate them into different cages but keep the cages close so they can interact!
When her feathers grow back introduce her again, if he starts plucking again you'll have to separate them for life.
My female zebra finch has a few bald patches - she originally started losing feathers around her back area, but is now lacking them on her neck, too.
She's just over a year old at the moment and she shares an indoor aviary with one male zebra finch who is slightly older than her.
For a moment, we thought that it may have been mites/lice, and we have been using anti parasite sprays for a while now but she hasn't been getting any better.
Why is she losing her feathers?
