bulbul bird losing feathers

I have a bulbul bird which keeps losing its wing and tail feathers then growing them back. It happens so frequently the bird can no longer fly. I feed it bird pellets, small insects, fruit and egg and biscuit. Is there something nutritional that would be causing my birds feathers to fall out so frequently? Or perhaps it bumps its feathers easily jumping around its cage? I let the bird out of its cage a daily basis for several hours under my supervision.