At least once a year -- sometimes more -- every bird sheds its old feathers and replaces them with new ones via a process called molting. When and how a bird molts can vary according to species, season, and age, but molting typically begins at the head and finishes at the tail. The older feathers loosen in their follicles and are pushed out by the new feathers underneath, and it can take anywhere from several weeks to a few months before all the feathers are new.
It's important to take special care of your bird while it molts by not letting the temperature of its room drop too much and keeping it in a quiet place with its cage partially covered. If your bird maintains a well-balanced diet, most of the nutritional demands of molting should be taken care of, but it's still a good idea to give it some extra proteins, vitamins, and minerals during this stage. Mist your bird daily to prevent the new feathers from becoming dry and hard, and remember that the new feathers heighten the bird's sensitivity, which can make it peevish and moody.