A vast rubbish dump in southern Spain has become a magnet for ornithologists as thousands of storks, black kites and vultures make a stopover to feed on food waste before beginning their journey across the Strait of Gibraltar.
“It’s especially useful for carrying out a census, as with so many birds in one place it’s easy to count them and to read their rings,” said Jesús Pinilla of SEO/Birdlife in Andalucía.
It is easy for the birds to find food among the mountains of garbage in the dump at Los Barrios, near Cádiz, that receives the waste of 400,000 people living in Gibraltar and the surrounding area.
Since July ornithologists have counted 210,000 kites and 125,000 storks crossing the strait.
But many Spanish and central European storks have chosen to stay in Spain, feeding from the dump, rather than make the long journey south. As a result, there is now a stable population of about 37,000 birds wintering there, according to SEO/Birdlife.
“It’s not new for birds to take advantage of this food supply, but what we are seeing is a change in migratory behaviour, especially in the case of white storks,” Pinilla said.
“Until relatively recently, all the European white storks wintered in Africa, and as the rubbish dumps have grown larger, the birds have learnt that if there’s enough food here there’s no need to make a 3,000km [1,850-mile] journey to sub-Saharan Africa to find something to eat.”
Strong crosswinds complicate the 8-mile journey and any bird that falls into the sea is likely to die there, as they are unable to regain flight, so the better fed they are before they set off, the better their chance of survival.
Researchers say that as well as food, the birds eat a lot of plastic, rubber and toxic substances on the landfills. There are also rats, which the kites hunt.
In an article published earlier this year in the journal Movement Ecology, Spanish researchers tracked the movements of white storks (Ciconia ciconia) on their journey from northern Spain to Morocco and found they made numerous stopovers at landfill sites en route.
The researchers found storks were capable of spreading toxins picked up from landfills, causing heavy metal and plastic pollution in agricultural areas, particularly in rice-growing regions.
However, by feeding in landfills on the way, storks expended less energy and had easier migratory routes, increasing their chances of survival. As a result, the European white stork population has increased dramatically since the 1980s.
The climate crisis is also driving birds, especially migratory species, to adapt. Two successive hot summers, combined with a prolonged drought in much of Spain, are forcing some species to change their habits.
“We don’t have all the facts, but it seems highly likely that, in the case of storks, which need wetlands to feed and to reproduce, they have probably changed their behaviour as a result of the drought and may have produced fewer young,” Pinilla said.