Provide a larger cage or place on a perch out of the cage. Giardia, an intestinal parasite, seems to be an important factor in cockatiel feather picking. Your veterinarian will perform a trichrome giardia test and, if positive, treat your cockatiel. Skin parasites are not common in pet cockatiels. Do not use the mite sprays or mite discs available commercially.
Lice infestations are rare in pet birds.
Separate them for a while. Hormonal imbalances or feather cysts are occasionally associated with feather picking. As you can see, the diagnosis and treatment of feather picking can be difficult. Have patience and work with your veterinarian. A medical workout must be done to rule out the many causes of feather picking.