The hen harrier, named Arthur, had been tracked via a lightweight satellite tag since he hatched in the Peak District during the summer.
After initially staying there, Arthur moved to the Brecon Beacons in South Wales before flying to Nidderdale in mid-October.
On the morning of Friday October 26 he flew onto the North York Moors National Park and his last registered position was at 9.55am that same day when he was just north of Lowna Bridge, near Hutton-le-Hole.
Since then the body or the tracking tag have not been located despite extensive searches by the RSPB investigations team.
North Yorkshire Police believe the bird has been deliberately harmed as they are known to be subjected to human persecution and that his tag was destroyed.
Sgt Kevin Kelly, of North Yorkshire Police’s Rural Taskforce, said: “The sudden disappearance of Arthur is deeply concerning and we are working with the RSPB to establish the full circumstances surrounding the incident.
“This is a severe blow to the Hen Harrier project and an unwanted addition to the already concerning and ever-raising numbers of hen harriers that are just ‘vanishing’.
“I encourage any information that could help me forward this investigation. These rare birds are one of the jewels in the crown of the English countryside. We have been looking around last known locations but there has been no sign of Arthur.
“Anyone who may have information that could assist the investigation is urged to come forward as soon as possible.”
Call North Yorkshire Police on 101, select option 1, and speak to the Force Control Room, email RuralTaskforce[email protected] or contact Crimestoppers on 0800 555111 if you prefer to remain anonymous. Quote reference number NYP-31102018-0238 when providing information about this incident.
Anyone who finds a wild bird that is suspected to have been illegally killed is urged to contact RSPB investigations on 01767 680551 or fill in the online form: https://www.rspb.org.uk/our-work/our-positions-and-campaigns/positions/wildbirdslaw/reportform.aspx
Members of the public can also call the Raptor Crime Hotline on 0300 999 0101 and speak to someone in confidence with any information about people killing birds of prey.