The number of protected birds of prey found dead in one small area has risen to 13 in the past week.

Conservationists have condemned the killings in a 2sq-mile area of Ross-shire , with some of the cases confirmed as poisonings.

RSPB Scotland said yesterday it was considering offering a substantial reward for information leading to the conviction of those responsible for what appears one of the worst episodes persecution. Nine red kites and four buzzards have been found dead to the south-east of Conon Bridge around Conon Brae, Balvail, Leanaig and Alcaig. The number of raptors poisoned throughout the whole of last year was six.

The red kite was hunted to extinction in Scotland over a century ago. But in a joint RSPB and Scottish Natural Heritage project they were reintroduced in four areas of Scotland, between 1989 and 1994, including the Black Isle; near where the dead birds have been found.

Duncan Orr-Ewing, head of species and land management at RSPB Scotland, said: "This is devastating news and confirms our worst fears. 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."