Five red kites and a buzzard were discovered in Ross-shire last week. Police said they suspected criminal behaviour. Another two red kites and three buzzards have since been found in the area around Conon Bridge and Muir of Ord.
Police have not established the cause of death but Detective Inspector Scott McDonald, who is leading the investigation, said "some form of criminality" is suspected due to the close proximity of the deaths.
A police spokesman added: "We are now investigating the deaths of seven red kites and four buzzards, all found around the Conon Bridge/Muir of Ord area.
"Inquiries are ongoing and we continue to appeal for any witnesses who have seen anything suspicious.
"The remains have been taken away for post-mortem [examinations] and we are awaiting the results."
Police advised anyone who finds a dead bird of prey to note its location and inform officers, and not to attempt to recover it.
Scottish Gamekeepers Association chairman Alex Hogg said: "The number of birds being found is hugely worrying and suggests something is very much amiss."
He urged anyone with information to contact police.
The findings came as it was revealed the number of birds of prey illegally poisoned doubled to six last year.
A report revealed a red kite, a golden eagle and four buzzards were poisoned in 2013; well below a peak of 30 in 2009, but up on 2012.
Figures from the Partnership for Action Against Wildlife Crime Scotland showed there were a total of 15 recorded crimes against birds of prey including shootings, trappings and nest destruction last year.
The RSPB said the figures are "very worrying" and show that birds of prey continue to be persecuted.
Two of last year's poisonings were discovered in Perthshire, two near Stirling, one in Angus and the other south of Edinburgh.
Reacting to the 2013 figures, Environment Minister and chair of PAW Scotland Paul Wheelhouse said he was "disturbed" to learn of the scale of the persecution against raptors.
Duncan Orr-Ewing of the RSPB said: "It is very worrying the number of detected illegal poisoning incidents has shown an increase on the previous year . The other reported criminal incidents show the deliberate targeting of vulnerable raptor species including hen harriers, red kites, and both golden and sea eagles."