Four golden eagles were found illegally poisoned in Scotland last year, the highest number since records began 18 years ago.

The poisoned carcasses are just the tip of the iceberg, according to the RSPB, and the continued persecution of the golden eagle is preventing it from recapturing old territories to the south and east.

Tests by the Scottish government also confirmed the poisoning of 13 buzzards, seven red kites, two peregrines and one white-tailed sea eagle in 2010. A further 20 illegal incidents related to the illegal shooting, trapping and nest destruction of birds of prey.

All of the 442 pairs of golden eagles in Britain nest in Scotland. While breeding pairs are surviving in their north-western strongholds, the victims of poisoning were all young adult birds attempting to establish new territories in eastern and south-west Scotland, according to Bob Elliot, head of investigations for RSPB Scotland.

"These are pioneering eagles which are trying to find territories and are ending up dead," said Elliot. "Human killing is suppressing the population so it's on a flatline. The birds of prey are top of the food chain and can slide into a decline very quickly."

Most of the poisoned birds were found in landscapes managed for grouse shooting. Three golden eagles were found poisoned on the Skibo Estate in Sutherland, where Madonna married Guy Ritchie in 2000.

Dean Barr, sporting manager on the estate, was this year fined £3,300 pounds for keeping Carbofuran, a banned pesticide. He had enough of the lethal substance to kill every bird of prey in Scotland six times over but denied using it to protect game on the estate from predators.

No one was charged or convicted in connection with the poisoning at Skibo and the Scottish government this year introduced tougher laws to make landowners liable for prosecution over the illegal killing of birds of prey by their employees. Some gamekeepers continue to use banned poisons to protect young grouse from predators, killing crows, birds of prey and even dogs in the process, according to Elliot.

The RSPB, which compiled the bird death figures, said it was likely that the recorded poisonings – mostly discovered by chance by walkers – were a fraction of the true number. Four rings from golden eagles were found in a building at Moy, near Inverness, in 2010 but their carcasses were never found.

Duncan Orr-Ewing of RSPB Scotland thanked the public for reporting suspected wildlife crimes to the police. "Such reports were pivotal in finding illegally killed birds," he said. "Everybody can play a part in supporting efforts to bear down on these crimes in our countryside, which are so detrimental to some of Scotland's tourism and other rural industries."