The bird was discovered on the wall, next to a layby on the Kildale to Commondale Road, near Percy Rigg, in January.
But police have today (Monday) launched an appeal for information after toxicology tests revealed it died after being poisoned.
According to the tests, it was "likely" killed by the chloralose pesticide, commonly used on mice.
Sergeant Stuart Grainger, of North Yorkshire Police’s Rural Taskforce, said: “North Yorkshire is known for its wonderful countryside, which is home to many species of birds, including protected birds of prey.
"Sadly, as a county, we have more confirmed incidents of raptor persecution than any other county in England – a situation North Yorkshire Police is absolutely determined to tackle.
“It is saddening that this magnificent bird has been poisoned."
A North Yorkshire Police spokesman said it was unlikely that the bird died where it was found, but that it appeared to have been placed onto the wall deliberately.
Anyone with information is asked to call police on 101, quoting reference number 12180127114.