If you’re a vulture soaring in the sky, flying 3-400 kilometers a day in search of carrion, the odds are you don’t give much of a damn about human borders. Yet somehow, vultures seem to love western Spain and usually stay well clear of Portugal. The reason for this, researchers found, is owed to a policy on carcasses.

A political border becomes an ecological border

Human borders don’t really follow environmental barriers. Sure, some borders follow rivers or mountainous massifs, but socio-economic factors typically define political borders, not geography. Yet, through policy, political national limits can also become environmental limits.

“Wildlife, especially highly mobile organisms, may encounter different degrees of human impact, disparate conservation regulations, and contrasting environmental policies within otherwise homogeneous ecological regions,” note the authors of a study that analyzed vulture mobility in the Iberian peninsula.

The researchers, led by Eneko Arrondo of Doñana Biological Station, equipped 60 griffon vultures (Gyps fulvus) and 11 cinereous vultures (Aegypius monachus) with GPS trackers. They found that while the vultures traveled extensively around Spain, they rarely ventured into Portugal. In fact, you can almost tell where the borders are by GPS tracking alone.

The reason for this is a different policy on cattle carcasses. In 2001, the European Union wanted to curb mad cow disease, so they published a directive that mandated the immediate burial or incineration of cattle found dead in the fields. Spain and Portugal (home to 90% of Europe’s carrion population) both implemented it.

But after a few years, Spain abandoned the directive, while Portugal kept it enforced. As a result, Spain now has a lot more cow carcasses than Portugal.

If you’re a vulture, you probably don’t understand much of human politics, but you do understand that more carcasses means more food for you, and that’s where you’d rather spend most of your time.

The carcass law is starving the vultures. Conservationists have raised the alarm before. Speaking to El Pais, Arrondo calls the measure “drastic” and “bureaucratic”. It’s a paradox of different policies, and vultures are paying the price.

“Our results should be seen as a warning signal to policymakers and conservation managers, highlighting the need for a stronger integration of sanitary and environmental policies at the European level.”

Unsurprisingly, vulture numbers in Portugal have plummeted. Meanwhile, on the other side of the border, the short-lived ban has also taken its toll. Near-starvation has led vultures to become more aggressive, sometimes even turning to live prey.

Among birds, vultures are one of the most threatened groups, exhibiting sharp population declines, particularly due to human activity. A separate study (also led by Eneko) showed that landscape anthropization can influence vulture populations: the main things that kill Iberian vultures are vehicles, electrocution, poisoning, and wind turbines.

“We always tell the joke that we don’t know any vulture that has died of old age,” says biologist Eneko Arrondo for El Pais.

Another problem vultures have to deal with is drugs — specifically, diclofenac, an anti-inflammatory drug used both in humans and in livestock. The drug causes kidney failure in birds and has already decimated vulture populations in Asian countries.

A number of different transboundary initiatives have been implemented in Europe, not necessarily for vultures, but for other species. In fact, this is one of the main purposes of the European Union: to drive a high degree of integrated policy across different countries. But for now at least, the EU doesn’t seem to radiate too many directives on international conservation.

“Biodiversity management is usually implemented at the regional and national scale,” the study reads. “Even inside a relatively homogeneous political entity such as the European Union, where all members comply with the same directives, national variations in policy implementation may still jeopardize large scale conservation efforts. Thus, trans-boundary biodiversity conservation in Europe would largely benefit from supervision by the EU Commission of local applications of general regulations.”

Vultures provide important environmental services. They’re nature’s “clean-up crew“, providing nutrient recycling, removal of soil and water contaminants and reducing the spread of disease to other facultative scavengers such as foxes.