when a bird suddenly changes in personality and becomes aggressive or reclusive, it’s imperative to have a vet evaluation to rule out a variety of possibilities in the way of illness. Remember, a bird that’s weak or injured is at risk for predation. Even though your bird is perfectly safe, their instincts don’t know this and will perhaps overcompensate by becoming defensive.
Once a full blood panel, crop and vent swab/cultures are done on top of a good physical exam and illness or injury is ruled out, approaching it as a behavior issue is your next move.
At this age it's a good idea to have these chemistries done anyway, just as a precaution.
If your bird has been a seed eater and millet addict, as most cockatiels are, you may be seeing early symptoms of liver, kidney and/or thyroid disease. These are extremely common in seed eaters, along with tumors - so this is just another reason to get the full evaluation.
Another option is to change the inside of his cage around about once a month. Move the perches, the dishes, change out the toys, make it different. We do this with all of our birds to prevent the females from wanting to egg lay, curb aggression in both the males and females and to prevent all birds from ending up cage bound (not wanting to come out).
It could be a pattern he's now established. A habit. To determine this, change his routine by about 1/2 hour and don't do the usual things. If you go to his cage and open the door and walk away, hesitate, stand at the cage for a while, talk to him or sing and whistle for a minute before opening the door. Even change the color or consistency of the towel - whatever makes it different.
This started about a month ago, right at a vulnerable time with moult. During moulting season a bird may become a bit more aggressive and self-protective as well. They lose some important feathers at this time and their flight (if they were fully flighted and in a natural environment) would be slightly impaired.
During a moult they don’t travel as far from their immediate home territory as they might otherwise (in the wild). This instinct remains and can effect their behavior in their human home. Even in a bird that is clipped and perhaps has never been fully flighted.
Find an avian vet near you http://aav.org/vet-lookup and
http://veccs.org/hospital_directory.php
Another very productive search site is
http://www.vetwebdirectory.net/index.php?option=com_mtzcssearch&template=2
These days, with birds growing fast in popularity as in home companions, many DVM’s are quite experienced and able to see and treat many birds. If you have a pet store that sells birds or know of any bird breeders – ask them who they use for their bird care.
To be sure the vet is a good one, make sure you’re there for the exam. This should include hands on, feeling the chest area, peering into the mouth with a well placed flashlight and lifting the tail feathers to examine the vent.
This exam should also include any one or more of the following: Blood tests, gram stains/cultures, x-rays, even oral/crop/tracheal swabs and so on.
If the examining vet doesn’t perform a hands on exam, or worse, leaves your bird in their cage or carrier, leave immediately. This is not the vet for you or your bird.
Certified Avian Specialist
Cert. Avian Specialist; Int. Assoc.Animal Behavior Consult; Pet Ind. Joint Advisory Council; author