Five red kites are thought to have been poisoned in Northern Ireland this year – including a mother who was incubating two eggs that subsequently failed to hatch.
The deaths mean that 30% of Northern Ireland's breeding population of the rare birds have probably been wiped out by poison in a single year, according to the RSPB.
One victim was found in the Castlewellan area last Wednesday and the RSPB is calling on the public to help uncover the cause of the deaths.
The remains of the bird found last week has been sent to the Agri-Food and Biosciences Institute for a post-mortem examination. However, it is strongly suspected that it and the four other birds were illegally poisoned.
Adam McClure, red kite officer for the RSPB, described it as a "very disappointing and very worrying" development.
"All birds of prey are protected under the law but this doesn't always mean that they are safe from poison," he said.