Feather plucking is one of those frustrating problems that occur from time to time in the birdroom. There are several causes raging from an inherited behaviorism to boredom.
Most often it is the hen but can be the cock. If feather plucking occurs in a nest and it is the cock then remove him and leave the hen to do the work of bringing up the chicks.
However, prevention is better than cure and as feather plucking appears to be more prevalent in smaller clutches of chicks it is worthwhile transferring chicks from other nests, so that the small-numbered nests are brought up to four or five chicks per nest. Do this by removing one chick from, say, two or three larger nests. This method works because the hen is too busy feeding a full nest box to allow boredom to kick in.
Other methods of relieving boredom include leaving a radio on all day. Budgerigars definitely like the noise, as when it switches off at night all will go quiet, and the activity will diminish.
Another thing that can be done to help alleviate feather plucking is to put a piece of millet spray into the nest box; this will often distract the hen away from her chicks as a relief from boredom.