A) Assuming the poults have been taken by a person, not an animal, you are the victim of theft because while in any sort of pen, gamebirds are regarded in law as the property of their owner. This means that any other person taking them dishonestly and without permission commits the crime of theft and you should report the loss to the police. If the suspect is caught, he should be prosecuted through the criminal court system and, on conviction, could be fined or imprisoned.
If the poults are killed or taken outside the pen, they are regarded in law as being wild and they cease to be the property of any person at this stage, though the exclusive right of killing and/or taking them remains with the owner or occupier of the shooting rights. Any person killing and taking them away from outside the pen would be committing various offences of poaching and trespass.