Police in Perthshire are investigating the poisoning of two red kites.
Police said the poisoning was "reckless and irresponsible"
Toxicology tests on the bodies of the dead birds showed both had been killed with the same banned pesticide.
The first was found at Spittal of Glenshee in January through its tracking device, and may have been dead for some time.
The second, which was identified from wing tags as having hatched from a nest near Perth, was found by a walker at Little Glenshee in June.
The species is one of the rarest in the UK and is being reintroduced in Scotland.
Tayside Police have appealed for help from the public to trace those responsible for the deaths.
Wildlife officer Alan Stewart said: "Just when I thought we were making some progress in reducing the deliberate poisoning of birds of prey in Tayside this has set us back years.
"Setting out poisoned baits, whether to protect sheep, game or racing pigeon interests, is completely illegal and incredibly reckless and irresponsible.
"Baits are indiscriminate and likely to kill any wildlife from crows and ravens to golden eagles and red kites.
"There are many instances where dogs have been poisoned and there is a definite risk to humans who may pick up either the bait or its victim without being aware that it may have been laced with deadly pesticide."
Mr Stewart added, "Wildlife poisoning cases are some of the most difficult to investigate.
"Sometimes the evidence points to a clear suspect but on many occasions there are a choice of suspects and sometimes even a choice of interests to take into account.
"It is logical that in the vastness of the countryside the victims of poisoning that happen to be found and reported to the police must only be a small proportion of those that are killed."
Bert Burnett, of the Scottish Gamekeepers' Association said that he was disappointed that poisoned birds kept turning up and hoped that those responsible were caught and punished.
Bob Elliott, head of Investigations at RSPB Scotland added: "It is a national disgrace that the killing of rare birds such as the red kite still occurs in Scotland's countryside.
"These illegal poison baits set for foxes or crows are indiscriminate and kill protected birds and other animals.
"Red kites, as scavengers, are particularly sensitive to illegal poisoning.
"Numerous incidents of illegal poisoning, particularly in the north of Scotland, appear to be preventing the population of kites from increasing as they should."