Male Cockatiel getting really aggressive

[notThtDeep]

I have two cockatiels. My male cockatiel, 11 years old, has suddenly gotten really aggressive. Whenever try to change water or food in the cage, he comes to bite my hand. He also comes aggressively to the edge of the cage even when I pass by it. This behavior is completely new and never happened before. Aggression is not just limited to me. He also often chases the female cockatiel around the cage, trying to bite her. He does get normal when I get him out of the cage though. But, I cannot keep him out of the cage 24x7.
What can I do to stop this behavior, especially towards the female?

 

[Shezbug]

I personally would remove the female and give her somewhere safe and stress free to live (like her own cage) - it can not be fun nor comfortable for her being chased about by an aggressive male who seems to be getting possessive of the cage and food dishes.
I would rearrange the cage, reposition the cage, add extra food dishes (for your own ease of changing them out) and work on treating/praising for calm suitable behaviours from him when you need to change things in there. Target training can be a great help for birds who need to be redirected to an area away from a certain area. If it is a smaller cage I would invest in a larger one with feeder doors.

 

[Mizzely]

Sudden behavior changes can also be a sign of a medical issue. Usually for birds it's just hormonal, but worth a vet visit if he hasn't been in a while just to be sure