South Wales Police are investigating the deaths of three birds of prey, a peregrine falcon and two buzzards at Ruthin Quarry in the Vale of Glamorgan, which were found dead in the quarry on March 26.
A toxicology report confirmed that the birds were killed using a poisoned bait bird which was laced with a banned pesticide.
PC Mark Goulding, wildlife and environmental crime officer, said: “The killing of birds of prey is a serious wildlife offence. Raptor persecution is a National Wildlife Crime priority.
“The poisoned birds ingested bait laced with the banned agricultural pesticide which was deliberately set out.
“I would urge anyone with may have witnessed this crime or who has information about this incident to come forward.
Anyone with information on illegal use of pesticides against wildlife can call us on 101 quoting 1800106122 or Crimestoppers anonymously on 0800 555 111.”