Storks which nest on the Iberian Peninsula are increasingly rejecting the annual migration south to Africa in favour of spending the winter at their nesting sites, thanks to the ready availability of landfill "junk food".

Portugal, for example, currently has 14,000 overwintering white storks (Ciconia ciconia), which would normally arrive across the Strait of Gibraltar in spring and return to warmer climes in late summer or early autumn. That number is rising, according to Dr Aldina Franco, from the University of East Anglia's School of Environmental Sciences.

Franco led a UEA team which studied stork populations around five landfill sites in south-central Portugal. Forty-eight individuals were tagged with GPS/GSM tracker/logging units which reported their positions five times a day.

Keeping tabs: A white stork fitted with a GPS tracker. Pic: Aldina Franco

The data gleaned confirmed many white storks' permanent resident status. Franco said: "We found that the landfill sites enable year-round nest use, which is an entirely new behaviour that has developed very recently. This strategy enables the resident birds to select the best nest sites and to start breeding earlier.

"Having a nest close to a guaranteed food supply also means that the storks are less inclined to leave for the winter. They instead spend their non-breeding season defending their highly desirable nest locations."

However, even those year-rounders without a prime nesting spot close to a landfill are prepared to make a round-trip of up to 100km for a beak full of human food waste, demonstrating just how tempting this trash is to the white stork.

Unfortunately, the avian junk food addicts are about to be deprived of their free lunch. Franco explained: "Under new EU Landfill Directives, rubbish dumps sites in Portugal are scheduled to be gradually replaced by new facilities where food waste is handled under cover.

"This will cause a problem for the storks as they will have to find an alternative winter food supply. It may well impact on their distribution, breeding location, chick fledging success and migratory decisions."

The UEA's research is published today in Movement Ecology. ®