I know normally you don't see a bird's ears because they are covered with feathers. However, for about the last week or so I have been able to see my tiel's right ear. Is that okay? Could it just be a molting issue?
Unless there is another bird that could be plucking him it is a molting issue. Birds cannot pluck their own heads. If you don't see improvement in a month, take him in to see if there is something else going on.