The movements of greater spotted eagles being tracked by a team in Abu Dhabi suggest two separate populations may visit the UAE each year.
Every autumn, the birds of prey migrate from their breeding ground in southern Russia and northern Kazakhstan to Europe, North Africa, the Gulf, and sometimes beyond, to north east Africa.
Environment Agency Abu Dhabi has been monitoring six greater spotted eagles over the past several years.
One, known as 296, was tagged in late 2015, providing scientists with more than four and a half years of data, which is highly unusual. Most tags last a couple of years, at most.
“Once you track a species for such a long time you get some fantastic information,” said Dr Salim Javed, the agency's acting director of terrestrial biodiversity.
“It helped us to understand there is possibly a migratory divide along the Caspian Sea. It is part of a population that comes from northern Kazakhstan and Russia. There is another part of the population that moves around the West coast of the Caspian Sea.”
Once the birds reach the Caspian Sea, they split into two directions.
“One to the east side and one along the West side,” he said.
Bird 296, which is believed to be male, was tagged in Al Wathba Wetland Reserve in 2015.
Since then it has travelled 100,000km, visiting 10 countries. It is not known how far they travel in their lifetime.
Understanding migratory routes and resting sites is critical to the survival of the species. There are believed to be fewer than 10,000 mature individuals left across the world. The birds are classed as vulnerable to extinction by the International Union for Conservation of Nature.
Number 296 has carried out four winter and five spring migrations, and is currently spending the summer in southern Russia and northern Kazakhstan, its breeding ground.
It will return to the UAE for the winter.
“They start in mid or late September and by the third week of October they are here in the UAE. From that time until the mid or end of March they are here in the UAE,” said Dr Javed.
“The last time this bird left was on March 25.”
The birds tend to arrive in Al Wathba, a beauty spot which was declared a reserve in 1998 by Sheikh Zayed, the Founding Father.
“They come to Al Wathba, but they keep moving around,” said Dr Javed.
“All the four autumn migrations we have tracked they have come there, but then they go to the Northern Emirates and Ras Al Khor and Umm Al Quwain and all those areas, and maximising their opportunities to feed.”