I have a one-year-old cockatiel who constantly seems to be plucking out his feathers.  Everything else seems normal; he is active and enjoys his diet of seed and fruit.  What are some possible causes for his behavior?

You raise a very good question about your cockatiel.  Your bird is lucky to have an owner like you who pays attention to its behavior and health!   What you have described as behavior may actually be a symptom.  The first thing that you need to do is to find an avian veterinarian and have them examine your bird.  There are a number of things that could be going on to cause this problem, and a good physical exam and laboratory tests including blood work, fecal exams and cultures will be needed to diagnose your bird. 

Something as simple as removing the bird from exposure to cigarette smoke or cooking fumes may be the answer.  Other problems could include diseases such as infection, malformation of the feathers, parasites, thyroid deficiency, allergies and neuroses.  This problem could also be associated with poor husbandry.  Frequent baths and exposure to sunlight are very important for your bird.  

The diet you are feeding your bird may not be nutritionally complete.  You make no mention of vitamin or calcium supplements, which are essential to the proper growth of feathers.  We strongly recommend the use of pellets designed for birds to supplement a diet of seed and fruit.  Remember, seed diets are not a balanced diet for your bird!

Finally, this may indeed be a behavioral problem that needs to be modified, either by a change in the bird's environment, or with the help of medication.