Moulting in birds:
Moulting in birds is a comparatively slow process, as a bird never sheds all its feathers at once; it must keep enough of its feathers to regulate its body temperature, repeal moisture, and fly. A moulting bird should never have any bald spots. If a pet bird has any bald spots, the bird should be brought to an avian veterinarian to search for possible causes for the baldness, which may include giardia, mites, or feather-plucking.
The process of moulting
1. The bird begins to shed some old feathers
2. Pin feathers grow in to replace the old feathers
3. As the pin feathers become full feathers, other feathers are shed.
This is a cyclical process that happens in many phases. In general, a moult begins at a bird's head, progresses down the body to its wings and torso, and finishes with the tail feathers.