You have not said how old he is, maybe he has reached maturity and is calling for a female or wants out.
If he is older then i would get him checked by the vet. Maybe he misses the other one.

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Grandma to Jinja Dog, Mum to Tilly the Cavalier, Tattycat, Moomy, Choo, Sonic, Niven Mc Twisp and Lawna the rabbits and 1 guinea,17 budgies, 4 Cockatiels, 8 Goldfish.