Cockatiels may bite themselves for several reasons, not necessarily because of a bruise:
Over-preening: Stress, boredom, or skin irritation can cause excessive grooming.
Malnutrition: Vitamin or mineral deficiencies may trigger abnormal behavior.
Mites or Parasites: Infestations cause itching and discomfort.
Skin Conditions: Dermatitis, allergies, or infections can lead to self-biting.
Stress or Anxiety: Environmental changes or lack of attention may cause stress-induced biting.
Medical Issues: Underlying health problems or pain may be responsible.
Solutions include creating a calm environment, providing toys, ensuring a balanced diet, checking for parasites, and consulting an avian veterinarian if the behavior persists.