Twenty birds of prey have now been found dead within a two-mile area in the past month, police said.
The total includes 14 red kites and six buzzards, all discovered near the Conon Bridge area in Ross-shire.
Tests revealed 12 of the birds had been poisoned.
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A reward for information has reached around £26,000 after RSPB Scotland offered a £5,000 reward, which was quickly doubled by an anonymous contribution. The charity then created a Justgiving site after being inundated with calls from members of the public, and more than 200 people have donated almost £5,000.
A pledge of £12,000 was also made by a group of around 15 farmers from the Moray Firth area.
Police Scotland thanked the public for their support as the investigation into the deaths continues.
Chief Inspector Ross MacKillop said: "I would like to take this opportunity to thank landowners, farmers and members of the local community in the Black Isle area for their patience, co-operation and assistance in dealing with this matter.
"All of our officers report that they received a great level of support and this is much appreciated. The most effective response to incidents like this is not just an appropriate police response but also a positive community response, and that is what we continue to receive."
All the birds were found to the south east of Conon Bridge around Conon Brae, Balvail, Leanaig and Alcaig. The last bird was found on April 9 and tests have shown nine red kites and three buzzards digested a poisonous substance. Post-mortem work into the other birds is ongoing.
Officers are continuing to appeal for information and advised anyone who finds any further dead bird of prey to note its location and inform them, without attempting to recover it.
A group of campaigners angered by the deaths held a protest in Inverness at the weekend.
Locals carried 19 white cut-outs of birds, called ''ghost raptors'', through the Highland capital before converging on the High Street, where they staged a demonstration.