Some birds can have an allergic-type reaction to some components of seeds or the seeds may be contaminated with mycotoxins. Peanuts are often a culprit. They may be contaminated with mycotoxins and other bacterial or fungal organisms. Remove all kernel peanuts from the diet. Remove all nuts in the shell from the diet.
Mycotoxins again are of concern.
I don't recommend that birds be fed kernel peanuts or dry seeds or nuts in the shell. Offering Planter's type nuts from the supermarket may be safer if you choose not to remove nuts from your bird's diet. Avian Medicine pages 536 and 1043 . For information on fungal organisms, a listing of URLs is included at the end of this section. If the plucking is of recent onset, the seeds may be contaminated.