One of Scotland's premier birds of prey is facing almost terminal decline because government measures meant to protect it from persecution are failing.

The hen harrier, which used to be a familiar feature of the moorlands, is disappearing so fast that experts fear for its future, and have castigated the government's wildlife agency, Scottish Natural Heritage (SNH), for its "pathetic" response.

Scottish environment minister Michael Russell has said he is "horrified" by the decline. SNH has defended itself by promising renewed action to crack down on wildlife crime and encourage better behaviour by landowners.

The plight of the hen harrier is nowhere better illustrated than across a vast swathe of southwest Scotland between Kilmarnock, New Cumnock and the M74. The area, known as Muirkirk and North Lowther Uplands, was declared a Special Protection Area in 2003 because of the richness of its hen harrier population.

In the 1990s the area supported 29 breeding females as part of one of the largest populations in Britain. By 2004, however, the number of breeding pairs had dropped to 21. The latest counts have found no more than 14 pairs.

Worse are the statistics for the number of young that the birds have successfully managed to fledge. The figure has plummeted from 44 in 2004 to just five in 2008.

Across Scotland, the number of home ranges occupied by hen harriers has fallen from 417 in 2004 to 264 in 2007, and the number of successfully fledged young from 630 to 383.

The figures were provided to the Sunday Herald by Scottish Raptor Study Groups, a network of dedicated experts who regularly monitors birds of prey across the country. They have few doubts over who is to blame.

Kenneth Sludden, secretary of the South Strathclyde Raptor Study Group, ascribes the "alarming" and "almost terminal" decline in Muirkirk and North Lowther mainly to "a concerted cull of raptors by gamekeepers, condoned by factors and landowners".

He pointed out that SNH had a statutory duty to protect the birds in the area, and accused it of abjectly failing to do so. "The response from SNH management is cosmetic, condescending and pathetically inadequate," he said.

According to Sludden, landowners were paid £100,000 or more of taxpayers' money to help conserve hen harrier habitat. But he said the payments were having little effect, and that the monitoring of the work they were meant to fund was "haphazard, and at worse non-existent".

Meetings with landowners had "descended into farce" after one apologised for arriving late because he had been "shooting a couple of hen harriers", Sludden said. A gamekeeper also allegedly expressed hatred for the birds, describing them as "rats with wings".

SNH's local area manager, Ross Johnston, said he shared Sludden's concern about the drop in the number of hen harriers. He promised to work with all those involved to develop a "local action plan" for Muirkirk and North Lowther.

"This will focus on raising awareness of the issues, funding positive action to help the hen harriers, carrying out surveillance, and conducting crime-analysis work and enforcement," he said.

Environment minister Michael Russell said: "I was horrified to hear about the apparent decline in hen harrier numbers at Muirkirk and would be interested to hear more details about potential reasons as to why this has happened."

The Royal Society for the Protection of Birds (RSPB) pointed out that across Scotland the hen harrier population was significantly lower than habitat and food supply suggested it ought be. "Human interference, especially illegal killing, is known to be the main factor causing this," said RSPB Scotland's Duncan Orr-Ewing.

"The Scottish government, SNH, the police and other agencies must redouble their efforts to identify and prosecute the culprits - while encouraging good land management practice."