May 17 (UPI) -- For roughly a half century, the griffon vulture has been suspected missing in Bulgaria.
Sporadic sightings were reported, but the birds were considered extinct as a breeding species in the region.
Now, new surveys suggest the species -- one of Europe's largest birds of prey -- is making a comeback across Bulgaria's Eastern Balkan Mountains.
The survey results, detailed Monday in the journal Biodiversity Data Journal, helped confirm the effectiveness of a pair of conservation programs in Bulgaria.
As part of the conservation programs, researchers sourced captive-bred and recovered vultures from zoos and rehabilitation centers in Spain and France.
The birds were acclimated to Bulgaria's climate in special aviaries before being tagged and released at sites across the Eastern Balkan Mountains.
After a 50-year absence in Bulgaria, the first breeding griffon vultures was spotted in 2015.
Now, the Eastern Balkan Mountains are home to at least 80 griffon vultures, including 25 breeding pairs and, over the last five years, more than 30 chicks have been successfully fledged.
While not all the vultures introduced into the wild survive, the population is slowly rebounding, according to researchers.
"We have lost about a third of the vultures set free in that site, mostly due to electrocution shortly after release," researcher Elena Kmetova-Biro said in a press release.
"The birds predominantly forage on feeding sites, where the team provides dead domestic animals collected from local owners and slaughterhouses," said Kmetova-Biro, initial project manager for the Green Balkans NGO.
Researchers plan to begin focusing more on environmental protections to ensure the reintroduced griffon vulture populations will stabilize and continue to grow on its own.
"We consider the establishment phase of the reintroduction of the griffon vulture in this particular site as successfully completed," the researchers reported.
"The population is still dependent on conservation measures -- supplementary feeding, isolation of dangerous power lines and accidental poisoning prevention -- but the area of the Eastern Balkan Mountains can currently be regarded as a one of the only seven existing general areas for the species in the mainland Balkan Peninsula," they said.