The vets who had to put down a buzzard that was found shot in Sywell Country Park say it was hit with a shotgun pellet days before it was brought in for treatment.
The wild animal was found alive by a park ranger on November 29 and was taken to the Raptor Foundation - who specialise in rescuing birds of prey.
An x-ray revealed the pellet lodged next to the bird's spine and a broken wing but due to the extent of the injuries, it had to be euthanised.
The vet’s report estimated that the bird had been shot several days prior to being found.
Jenny Shelton from the Royal Society for the Protection of Birds' investigation unit said: "Buzzards are superb birds, often seen perched in trees at the side of the road or wheeling high in the sky emitting their signature, cat-like ‘mewing’ call.
"Sadly however there are those who intentionally harm buzzards and other birds of prey, and our team deals with incidents like this every week.
"Bird of prey persecution is one of the UK’s hidden crimes, and the issue is not going away.
"It’s horrific to think that someone has fired a gun at this buzzard. What a sad end to the year."
All birds of prey are protected by the Wildlife and Countryside Act 1981.
To kill or injure one is a criminal offence and could result in an unlimited fine or up to six months in jail.
PC Chloe Gillies, of Northamptonshire Police rural and wildlife crime team said: "We have opened an investigation and are appealing for any information or witnesses to come forward.
"We take wildlife crime very seriously and will deal robustly with any suspects identified.
"This bird was left to suffer unnecessarily due to someone’s irresponsible actions."
Anyone with information relating to this incident should call the police on 101.
Earlier this year a red kite was found shot near Corby, and in 2014 a red kite was found shot near Wellingborough.