Seven tiny falcon chicks which will survive only 48 hours without specialist treatment have been stolen.
The chicks were destined for sale in Dubai or the UK
They are two to three weeks old, worth about £15,000 in total and were being hand-reared at the Welsh Hawking Centre in Barry, Vale of Glamorgan.
They are last known to have been fed at 2200 BST on Monday, and were found to be missing on Tuesday morning.
Centre owner Ceri Griffiths said the chicks needed four meals a day and would not survive unless correctly fed.
Their theft follows that of a tawny owl from the nearby Barry Swan Rescue Centre at the weekend, but it is not known if the two are connected.
The chicks were ringed by Defra who were informed of the theft
Police are also investigating the disappearance from the Welsh Hawking Centre, which is home to about 400 birds in total.
"It's a welfare issue," said Mr Griffiths.
"They are fed specialist food. If you buy meat from the butchers for them they will end up with rickets because there's no calcium in it.
The birds will become noisier as they get hungry and were destined for sale in Dubai or the UK, although some may have been retained at the centre for display purposes.
The chicks were taken from a special rearing room at the hawking centre some time on Monday night.
They were the only seven birds in the room who were not being treated under heat lamps, said Mr Griffiths.
Dai the owl
All the chicks were still without feathers and unable to fly and looked like white balls of fluff.
They have also been ringed with the Department for Food, Environment and Rural Affairs (Defra), which has been informed of the theft.
Mr Griffiths said he had "no idea" if the birds had been stolen to order, but they would be very difficult to dispose of because documentation and tests - including one for bird flu - would be needed.
It was the second bird theft in a day in Barry.
On Sunday a tame tawny owl called Dai was taken from the Barry Swan Rescue Centre a couple of miles away.
Anne Bird, who runs the sanctuary, said a cash reward was on offer.
"He was brought to us in 2000 after someone had bought it for their child after the Harry Potter film came out," she said. "And he has been here since."