A conservation charity has expressed concern over the disappearance of four tagged hen harriers in recent months.

RSPB Scotland said the birds were last located over land managed for grouse shooting when their satellite tags suddenly stopped transmitting.

A chick from a nest in Northumberland whose last known position was on a grouse moor near Grantown-on-Spey in Inverness-shire has not been seen since August 16.

Two birds tagged on the Mar Lodge Estate in Aberdeenshire this summer were last located near the Aberdeenshire/Moray border on August 29 and near Ballater on September 3 respectively.

The fourth missing bird was last recorded on a grouse moor north of Glenalmond in Perthshire on September 24.

Ian Thomson, head of investigations for RSPB Scotland, said: "In common with so many previous disappearances of satellite-tagged birds of prey, each of these missing birds was last known to be on a moor managed for driven grouse shooting before its transmitter suddenly stopped.

"The picture is becoming ever more clear - in almost all cases when a tagged bird dies naturally we are able to recover its remains. If it disappears over a Scottish grouse moor, it's never seen or heard of again."

The charity's Hen Harrier LIFE Project works cross-border to secure a future for the threatened bird of prey.

Running until 2019, the project combines satellite tagging, on-the-ground monitoring, nest protection, investigations work, and awareness-raising.

It also works with volunteer raptor field workers, landowners and local communities to protect hen harriers across northern England and southern and eastern Scotland.

Cathleen Thomas, who manages the project, said: "To have more hen harriers disappear, including three of this year's youngsters, is devastating for all of us involved in monitoring these hen harrier chicks."

The Scottish Gamekeepers Association said there could be several factors at play.

A spokesman said: "Until the findings of satellite tags are monitored by independent experts or bodies, we will never fully understand what happens when tags lose transmission nor will anyone be any closer to being able to do anything about it.

"There could be many factors at play. Our understanding is that the majority of the seven tagged hen harrier chicks at Mar Lodge this year have died in some circumstance or another, with one tag going off radar for some days before signalling again, so we are not going to speculate on cases.

"Around a quarter of 'suspicious' tags studied in SNH's satellite-tagged eagle report lost transmission away from grouse moor areas, including islands, yet the tags themselves were never recovered."

A spokesperson for Scottish Land & Estates (SLE) said: "Anyone with information on the location of these hen harriers should contact Police Scotland as soon as possible.

"Neither Scottish Land & Estates, nor its members, were aware of these cases.

"Where a satellite tag stops transmitting, we cannot instantly conclude that the bird has been killed or harmed. Land managers across many farms and estates would have been willing to assist the search for these birds had earlier notification been provided.

"If any of these birds have been intentionally killed then that is unlawful and we fully support the full weight of the law being brought to bear on the individuals responsible."

Anyone with information is asked to contact 101 or the RSPB's raptor crime hotline on 0300 999 0101.