At least three Griffin vultures have been poisoned in the Kressenski Prolom, or the Kressenski Gorge of the Strouma River in Pirin Mountains, Bulgarian media reported on June 14 2010.

It is believed that the three birds that died were poisoned with bait laced with toxic chemicals. The exact nature of the poison is yet to be established.

The white headed vulture, or the Griffin vulture (Gyps Fulvus), had made a historic return to the Rila and Pirin mountain ranges in Bulgaria, territories which were once his hunting grounds. In the 1960s, the vultures had disappeared from these ranges thanks to the indiscriminate use of pesticides and poison aimed at mice, rodents and wolves.

Spain had recently donated a total of 26 griffins, the Bulgarian news agency (BTA) reported on February 18 2010.

The birds had just passed their acclimatisation period following their arrival from Spain and had started feeling comfortable in their new surrounding. The birds were "acclimatised" near the village of Rakinta, a process which reportedly lasted anything from five to 12 months. According to media reports, they had even managed to reproduce.

The Griffin vulture reaches up to 110cm in length with a 270cm wingspan, and weighs between six and 13kg.

Like other species of vultures, the Griffin is a scavenger, feeding mostly off carcasses of dead animals which it finds by soaring over open areas, often moving in flocks.

It breeds on crags in mountains in southern Europe, North Africa, and Asia, laying one egg. Griffin vultures may form loose colonies. The population is mostly resident.

The birds were donated to Bulgaria by the Global Relief Foundation (GREFA), an NGO.