Monty's body clock

I don't know how he does it but every night at 9 pm Monty starts squawking loudly. There is nothing that causes him to start - not TV theme music or bedtime or anything else except the fact it is exactly 9 pm. It isn't even a minute either side of 9 pm but on the dot.
One night he had been playing in the back of his cage and I thought he wasn't going to start but at 9 pm he stopped playing, went to the front of the cage and started squawking.

Jane