Feather plucking is like humans biting our nails. Some birds do it out of boredom, some because they're nervous. Of course, it could be something deeper than that, so a trip to a good avian vet may be in order. Since I have close to 40 finches, one that appears plucked is taken out of the flight and put in a "hospital cage" and given a product called "feather-up". 9 times out of ten, this does the trick. Since I have a colony of birds, I watch for a while to try and piece together which bird is picking on him or if its just self mutilation.