A protected red kite found dead in Aberdeenshire hatched three chicks last year, RSPB Scotland said.
The bird of prey's carcass was discovered by a walker in woodland near Aboyne on April 6.
A post-mortem examination showed that the bird did not die of natural causes.
PC Mike Whyte, Police Scotland's local wildlife crime liaison officer, said inquiries have so far failed to identify a suspect.
"We are appealing to the public for information that may assist us in our inquiries," he said.
Known as Red/Blue 44 after the colour combination and number on its wing-tags, the female kite was three years old and fledged from a nest in Perthshire in 2010. It had bred successfully in 2012, raising three chicks, only a few miles from where it was found dead.
Ian Thomson, head of investigations at RSPB Scotland, said the red kite is still rare in Scotland, despite a reintroduction programme that has been running for the past 24 years.
"It is one of our most spectacular and unmistakable birds of prey, with its main diet being carrion, insects or small mammals," he said.
"It is unforgivable that someone could deliberately target such a fantastic bird, and I ask anyone who may be able to assist in bringing the perpetrator to justice to contact the police."
Jenny Lennon, RSPB Scotland red kite project officer for north-east Scotland, said the team is "deeply saddened" by the death, the first known case of illegal killing in the area since the project began six years ago.
"This female was of great importance to our young population with only a handful of our 20 or so breeding pairs on Deeside itself," she said.
"We hope the three 2012 offspring of red/blue 44 will thrive in Aberdeenshire and contribute further to the Scottish red kite population".
Anyone with information is asked to contact Police Scotland on 101.