A video has emerged apparently showing an armed man using a decoy bird to attract a rare hen harrier on land owned by the National Trust.
The film was shot on land leased for grouse shooting in the Derbyshire Peak District in February.
Conservation group Raptor Persecution, who published the footage on Tuesday, has called for action to be taken.
Derbyshire Police investigated the footage but have decided not to take any further action.
However, the National Trust said it was taking the incident "very seriously" and has started an investigation.
The footage was captured by birdwatchers with a digiscope about a kilometre away and was released after police had investigated.
A spokeswoman for Raptor Persecution said, while nothing illegal happened, it appeared the decoy bird had been made to look like a male hen harrier, possibly in order to attract another of the birds.
She said: "Male hen harriers are notoriously territorial and will attack what they think is an intruder.
"While the bird is distracted with mobbing the decoy, [the] gamekeeper takes aim from his hiding place nearby."
The RSPB said while decoy hawks and owls could be used for the lawful control of crows, it was "worried" the footage may have shown an "illegal attempt to target [hen harriers] or other birds of prey".
The Peak District National Park's chief executive Sarah Fowler described the video as "alarming and suspicious" and said she would support the investigation.
The Moorland Association condemns all acts of wildlife crime but said no crime had been committed in this instance.
A spokeswoman said: "Making judgements based on assumptions of the content of this clip, or indeed the intentions of those who have produced it, would be pure supposition and not something we are going to enter into."
Derbyshire's Police and Crime Commissioner Alan Charles said: "A recent sighting of a man with a gun and a plastic model of a hen harrier bird on a Peak District grouse moor is a sharp reminder of the continuing efforts needed to protect our wildlife.
"Although the man was caught on camera by two observers, they were, unfortunately, too far away to obtain film evidence of sufficient quality to enable a prosecution."
According to the RSPB, hen harriers are the most "intensively persecuted" birds of prey in the UK and its predation of grouse is a source of conflict on moors used for shooting.
In 2015, five male hen harriers disappeared from sites across England leading to the collapse of nests.
It is still not clear what happened to the birds.