Much of a parrot's wild language is intended to give information to the flock or family. Vocal behaviors can develop any time before or after weaning. Screaming must be dealt with patiently, compassionately and sympathetically because it does have a purpose. If your bird is screaming excessively, then something's wrong. It's a good idea to seek immediate professional help because parrot volume can be easily turned down if annoying new vocalizations are addressed quickly.
For more information, please read the story Is Your Bird Talking or Squawking