The etiology could be of genetic,/ physiological / nutritional deficiencies/menace from predators during nights/cannibolism, etc. Remedial treatment depends on the specific etiological causes .
From your explanation of detail condition, it is almost surely concluded that it is genetical, as mentioned by Prof. Loknath. Treatments would be unnecessary then. However, the ragged feather could be plucked if smooth appearance is desirable, though this is also unnecessary.