Sixteen birds - 12 red kites and four buzzards - have been found dead in the area of Conon Bridge, Ross-shire, in recent weeks.
At least some of the birds had been poisoned, and police are carrying out an investigation.
Nature conservation charity RSPB Scotland put up a £5,000 reward for information leading to a conviction and the sum has now been doubled by an anonymous donor said to be "deeply concerned" about the situation.
Duncan Orr-Ewing, head of species and land management for RSPB Scotland, said: "This appalling incident highlights the very real threat illegal poisoning poses to fantastic species like red kites.
"The vulnerable Black Isle population in particular has been repeatedly hit by deaths due to illegal poison use. The Chilterns population in southern England is nearly 10 times bigger, yet both projects started at the same time in 1989 with the same number of birds released into the wild. That is a shocking indictment on behaviour of some in this part of rural Scotland.
"An anonymous donor deeply concerned at the illegal killing of the red kites has come forward to increase our reward for information leading to a prosecution to £10,000. We urge anyone with information to contact Police Scotland as soon as possible."
All of the dead birds have been discovered in a two square mile area to the south east of Conon Bridge around Conon Brae, Balvail, Leanaig and Alcaig.
Police are also working with the Scottish SPCA and experts from the Scottish Agricultural College on the investigation.