Fears have been raised over the safety of female hen harrier Mabel, one of four chicks that fledged in July.
Police have launched an appeal for information about the whereabouts of the bird, which was fitted with a satellite tag by Natural England.
Hen harriers are among the UK's rarest birds, with just three successful nests recorded in England last year, and are legally protected under the Wildlife and Countryside Act.
North Yorkshire Police said the last signal from the tag was received from the Little Smale Gill area in Cumbria, north of Ravenseat Moor, on October 2.
Sergeant Stuart Grainger, of North Yorkshire Police’s Rural Taskforce, said: “I was lucky enough to spend some time monitoring Mabel’s nest site, and, despite following birds all my life, this was the very first time I’d ever seen a male or female hen harrier because they are so rare.
“It is therefore particularly upsetting that one of the fledglings from the nest has inexplicably gone missing. I would urge anyone with information to come forward.”
David Butterworth, Chief Executive of the Yorkshire Dales National Park Authority, said there was joy when the chicks fledged but it is now feared that the missing bird has been harmed.
He said: "There was delight from all right minded people when four hen harrier chicks fledged earlier this year in the National Park. That has been matched by the despair felt that one of the birds, Mabel, has now gone missing.
“The North of England has an unenviable reputation for criminal activity in terms of raptor persecution, so it’s hard not to think the worst in this case."
A spokesperson for Natural England added: “The sudden disappearance of the hen harrier, Mabel, is a matter of grave concern. We urge anyone with information to get in touch with North Yorkshire Police.”
Anyone with information should contact North Yorkshire Police on 101 of the RSPB's Raptor Crime Hotline on 0300 9990101.