Cockatiel bird- stops singing

Question
Hello, please help.  I have a 9 year old male cockatiel who recently stop singing this year after singing so beautiful for the last 8 years.  What could have caused this? He still says "Pretty Bird" and squeals for food and treats. Thanks for your help.

Answer
There could be several reasons.
One is that the bird is not interested in a mate anymore so it stops singing.
Two is that he is thinking that he is alone in the cockatiel world so there is need of singing, because no cockatiel female would possible answer.
Three is that the bird is thinking that he is only male around so he does need to defend his area anymore.

What would help is playing sounds of other cockatiels in another or the same room as the cockatiel is in.