The killing of five red kites in the space of a month has been described as a "serious setback" to efforts to re-establish one of Scotland's rarest birds.
There are 53 breeding pairs of red kites in Scotland
Four of the birds were poisoned with an unapproved pesticide and the fifth was shot.
RSPB Scotland said there were only 53 breeding pairs of red kites north of the border.
Alan Stewart, a wildlife and environment officer with Tayside Police, said it was "absolutely disgraceful" that poisons were being used.
"Poisoned baits are totally indiscriminate and red kites, being scavengers, are likely to be the first victims," he said.
RSPB Scotland's Duncan Orr-Ewing added: "When is enough, enough?
"We need tougher action against the perpetrators of these crimes and more
full-time police Wildlife Liaison Officers."
He urged the parties contesting this week's Scottish Parliament election to strengthen the laws on the possession and abuse of agricultural pesticides.
Conservationists have warned that efforts by RSPB Scotland, Scottish Natural Heritage and local landowners to re-establish a healthy population in several areas are being placed at risk.
Research by the RSPB has suggested that more than a third of the red kite population in the north of Scotland had been illegally poisoned between 1989 and 1998.
In the latest cases, three of the birds were found poisoned near Laurieston in Dumfries and Galloway.
Osprey eggs are being protected by volunteers
A police inquiry has been launched into the incident.
A dog walker at Gargunnock, near Stirling, found a fourth dead kite and reported the incident to RSPB Scotland and Central Scotland Police.
It was later confirmed that this bird had been also been poisoned.
The discovery of a fifth dead bird on a farm near Crieff was reported to Tayside Police.
A veterinary examination concluded that the lower half of the beak and tongue had been had been shot away, leaving the bird unable to feed. The bird eventually starved.
Meanwhile, more than 70 volunteers are mounting round-the-clock surveillance in Perthshire to protect the eggs of another rare bird of prey.
Security is tight at one of the best-known nesting sites for the osprey.
Enthusiasts are aiming to deter egg thieves after two nests in the area were targeted last year.
They plan to continue their vigil until next month, when it is hoped that the eggs will have hatched. There are only 60 pairs of the bird in Scotland.