Vultures can fly huge distances over the course of a single day - but not if they have to cross an international border. In just one day vultures can fly 300 to 400 kilometers in search of food. However scientists in Spain have discovered that Spanish vultures don't like to fly into Portugal.
Spanish biologists fitted 71 Griffon and Cinereous vultures in the Portuguese-Spanish border region with GPS trackers. They discovered that the birds hardly ever crossed the border into Portugal. This border region consists largely of river valleys and there are no geographical or climatic reasons why the birds should stop at the border. There must therefore be another reason why the vultures prefer to remain in Spain.
It is true that most vultures don't carry passports, but that isn't the reason why they avoid Portugal. The reason is the availability of dead livestock. There just happens to be more dead animal carcasses lying in Spanish fields than there are in Portugal (where the authorities collect and dispose of livestock). After the mad cow disease scares of the 1980's & 1990's the European Union introduced directives that required the incineration of cow carcasses. This law was passed to ensure that when cows died they were not allowed to decompose naturally. This is not good news for vultures who like nothing more than snacking on the dead bodies of other animals.
Partly because of the environmental effects of the directive on the disposal of cow carcasses and partly due to the containment of the disease the EU has since relaxed the rules on the disposal of dead livestock. Member states and their own regulatory bodies now have more independence as to how farmers should dispose of dead cows. In Spain the authorities now allow dead cattle to decompose naturally in authorized areas. In Portugal the national government has done nothing to change the regulatory system of disposing of dead cows since the 1991 EU directive. Therefore Portuguese farmers must still incinerate dead cattle.
It appears that vultures have quickly learnt that they are unlikely to find food in Portugal. They therefore avoid the country and dine instead on the more readily available Spanish carrion.
From Invisible barriers: Differential sanitary regulations constrain vulture movements across country borders, Eneko Arrondo et al. Via El Pais
