Breeding males can be recognised by their booming call, which can be heard from three miles away.
The birds had come under threat when their habitat - damp, reedy fenland - was drained, damaged or neglected.
But intensive work to create new places for the birds to live has helped bring them back to places like Norfolk.
Graham Madge, of the RSPB, said "There's no reason why the population shouldn't grow and grow now."
Bitterns were once very common, and even considered a bit of a luxury meat.