The kites, a protected species, were found dead along with four buzzards within two square miles to the south east of Conon Bridge near Dingwall in the Highlands.
Brian Etheridge, who has been with the RSPB for 27 years and marks 19 years as a red kite officer in the Black Isle this week, said he was reeling from the deaths of many birds he had worked with for more than a decade.
He said: "This has been the worst two weeks of my life. I have worked with all of the birds. Each one was ringed and tagged by me. I was there at the very beginning when they were only a few weeks old and I was there at the end when I went to collect their bodies.
"It's a huge mix of emotions; I've gone from being very, very angry to extremely sad. Some of these birds I've known very well and for a very long time."
The red kite was hunted to extinction in Scotland over a century ago but began to be reintroduced in Scotland over 20 years ago, including on the Black Isle, near where the dead birds have been found.
Mr Etheridge said one of the dead birds was a 16-year-old female that he had first tagged in 1998. She had been breeding in the Black Isle for 14 years and had raised between 25 to 30 young, one of which, an eight-year-old female, was also among the dead.
Mr Etheridge said he had gone to her nest every year since she first bred back in 2000. "Something like this can just wipe out so many birds and so many years of work. This is by far the worst example I've ever witnessed," he said.
Around 25 volunteers dedicate their free time at the Tollie Red Kite reserve near Conon Bridge, helping to feed the birds and speak to visitors.
Liz Rollinson, 66, from Contin, is a volunteer on the project. She said: "The skies are now empty and it just seems so sad, I cannot see rhyme nor reason for it. All that hard work and time and so much effort from an awful lot of people and now this has happened. It's devastating."
Tollie Red Kites is a partnership between RSPB Scotland and the Brahan Estate, where the centre is located. Alex Matheson, from the estate, said Brahan wanted to express its "total condemnation of any form of illegal poisoning".
"Responsible land managers across the country are working hard to stamp this sort of thing out and show that wildlife crime is just not acceptable," he said
RSPB Scotland has offered a £5000 reward for information that leads to the conviction of those responsible. The charity is working with police, the Scottish SPCA and experts from the Scottish Agricultural College.