Tests carried out on the 14 red kites and five buzzards discovered near Conon Bridge in Ross-shire have revealed 12 were poisoned.
A police spokeswoman also said: "Following analysis of the birds' remains, 12 have been confirmed as having digested a poisonous substance, nine red kites and three buzzards. Post-mortem examinations and toxicology work continues into all the birds seized."
Local landowners and farmers added £12,000 to the £14,000 reward already being offered for information leading to the successful prosecution of those who killed the birds.
The £14,000 is made up of £5000 offered by RSPB Scotland, £5,000 from an anonymous donor, and well over £4,000 from around 170 members of the public.
A Police Scotland spokeswoman said: "We are currently conducting searches in the Conon Brae area as part of ongoing enquiries into a wildlife crime investigation regarding the death of 19 birds of prey, the red kites and buzzards. Landowners in the area are co-operating with the searches."
The local members of Scottish Land & Estates and the NFU Scotland have condemned the deaths and offered assistance to police in the investigation as well as putting up the £12,000.
One of the landowners involved in collating the reward is Alex Matheson from the nearby Brahan Estate. It operates the Tollie Red Kites centre in partnership with RSPB giving the public a chance to see the birds that were re-introduced over 20 years ago having been hunted to extinction in Scotland at the end of the 19th century.
Mr Matheson said: "We would urge anyone who has any information to come forward."
NFU Scotland's Regional Board Chairman for the Highlands, Jim Whiteford, added: "I hope that those who are responsible are found and prosecuted."