A RARE golden eagle has been poisoned on the eve of the Scottish shooting season, in an attack which has been branded as "sickening" by an animal welfare group.

The female bird was found dead on Sunday in the Borders near a grouse moor, coinciding with yesterday's official start of the grouse-shooting season.

The Royal Society for the Protection of Birds Scotland (RSPB) said the bird and her mate were the only breeding pair of golden eagles remaining in the Borders, an area it described as a "black hole" in Scotland's efforts to reintroduce the species into the wild.

The pair were raising a chick this year whose future was now in jeopardy, the RSPB said. The dead bird was nine or 10 years old and is thought to have been killed with a poison involving the banned substance carbofuran.

An reward of £1000 is now being offered by the charity to help catch those responsible, with wildlife crime police leading a joint investigation. Also involved in the investigation are RSPB Scotland, the Scottish Society for the Prevention of Cruelty to Animals and the Scottish Executive.

Bob Elliot, investigation officer with the RSPB, said the poisoning was "absolutely tragic". He added: "The Borders is an ideal area for golden eagles and there should be more than one pair there.

"It makes you want to weep that one of the last pair has been killed for some unexplained reason by an illegally used pesticide. This bird was 10 years old but could easily have lived for another 15 years."

A survey of illegal poisoning incidents published by the Scottish Agricultural Science Agency found 39 confirmed cases of deliberate abuse - a 12-year high. However, the RSPB considers the real figure is much higher.

There are 420 breeding pairs of golden eagles in Scotland, mostly in the north-west but there was only one in the Borders.

Mr Elliot yesterday linked the poisoning of birds of prey to the presence of grouse moors.

"Despite being excellent golden eagle country, the Borders has been a real black hole in terms of golden eagle breeding in Scotland," he said.

"Sadly, illegal persecution of birds of prey continues to be a shameful fact of life in parts of Scotland in the 21st century, and unfortunately the evidence shows that there is a correlation between the location of grouse moors and the incidence of raptor poisoning."

A spokesman for the RSPB and also for the Lothian and Borders Police said they would not be naming the country estate where the bait was found as investigations into the incident were ongoing. Investigators have also asked that the exact nest site is not identified to protect the remaining eagle and chick.

The poisoned eagle had been using a long-established nest site. Although the chick has fledged, it would expect to be fed by both its parents and there are question marks over its long-term survival.

The maximum penalty, if found guilty, for killing an eagle is six months imprisonment and/or a £5000 fine.

Constable Mark Rafferty, wildlife crime officer with Lothian and Borders Police, said: "This incident will be thoroughly investigated. It is a tragedy that this has happened. There are no excuses for this kind of activity and those responsible will be brought to justice."

Scottish SPCA chief superintendent Mike Flynn said: "Scotland has lost one half of the only breeding pair in the Borders and this could ultimately result in a second tragedy as it is unclear if the chick will survive."

The League Against Cruel Sports believes there should be a ban on snares, the equipment which is commonly used by estate owners to protect grouse and other game birds from predators such as foxes.

A Scottish Gamekeepers' Association spokeswoman said modern snares had a grip on them which held animals in a more humane way.