Police are investigating the discovery of five red kites and a buzzard, around Conon Bridge and Muir of Ord area in Ross-shire.
Red kites are on the amber, or moderate, list of birds of conservation concern because of the successful revival after being hunted to extinction in Scotland in the 19th century.
A reintroduction prog-ramme which began on the Black Isle more than 20 years ago has made progress.
But five fatalities in one area is seen as a highly significant loss. In 2012, there were thought to be only around 214 breeding pairs in the country.
The poisoning of red kites has been associated with land managed for driven grouse shooting in the past, but there are no grouse moors in the immediate area of this latest discovery. It is understood the birds were found on open farm land.
DI Scott McDonald who is leading the investigation said: "We are unclear as to what has caused the death of these birds but, given all the circumstances, we must assume at this stge some form of criminality."
Anyone finding further dead birds in the area are asked to note of the location and inform the police.
Duncan Orr-Ewing, head of species and land management at RSPB Scotland, said the charity was working with the police.
He added: "It is potentially a serious wildlife crime incident. To lose such a large number of red kites at one time from the small Black Isle population is a major setback to this important conservation programme for Europe's only endemic and vulnerable raptor species."
Scottish Gamekeepers Association chairman Alex Hogg said: "This is a hugely disappointing discovery and we would ask anyone who knows anything about this mindless act to contact Police Scotland and to assist with the investigation.
"The SGA condemns wildlife crime and advocates only legal means to solving conflicts."
Douglas McAdam, chief executive of the landowners' organisation Scottish Land & Estates, said: "Wildlife crime of any kind will not be tolerated by the responsible majority."