Unfortunately, theft of birds does happen. Luckily, it is relatively rare, but if you have a good, well known show strain then take extra precautions.
Prey: We're talking theft of poultry here. Normally it will either be good show birds or birds that have been complained about by the neighbours before.
Control: CCTV (perhaps in the form of camcorder or webcam)
If and when you have proof, a trip to the police station.
Prevention: Padlock on the coop
Don't advertise their presence to all and sundry
Keep the neighbours happy about their presence. No roos in the city, and a box of eggs can go a long way.
How do I know it was a human that took my birds? Several birds gone, with no clues. Padlocks cut etc.