Nine red kites and four buzzards were found dead recently within the two square miles to the south east of Conon Bridge around Conon Brae, Balvail, Leanaig and Alcaig.
It has been confirmed that some were poisoned.
Duncan Orr-Ewing, Head of Species and Land Management for RSPB Scotland said it was a shocking indictment on behaviour of some in this part of rural Scotland.
He added: "We are offering a £5,000 reward for information that leads to a successful conviction. We urge anyone with information relating to this incident to contact Police Scotland so the perpetrators can be identified and brought to justice."