Storks normally migrate in the winter to sub-Saharan Africa, but changes in behaviour have seen them staying in Iberia over winter. More than 14,000 storks are overwintering in Portugal alone, living on open land fill sites just as seagulls do . The have been witnessed waiting for the rubbish trucks and descending on the trucks as they empty
Dr Aldina Franco, from the University of East Anglia, has studied this behaviour,  witnessing the change in migratory habits herself. White storks used to be wholly migratory, flying to sub-Saharan Africa to feed during the winter. In her study she shows that over the past few years more and more storks are staying in Portugal overwinter, feeding on the rubbish being deposited in landfill sites – mainly junk food. This is very different from their usual diet of small mammals, amphibians, small birds, reptiles, fish and insects. What is a diet of landfill doing to their physiology and health, what long term effects are there for storks that live on junk food? The newly released report already suggests that storks will fly 10o km round trips to eat at the landfill of their choice.
It also begs the question, do these particular birds migrate due to an innate urge or is it food dependant? If migration is innate to the storks then this is an unusual change in behaviour. It’s possible that some birds don’t have the genetic impetus to migrate; in the past not many would have survived or would have been rare, but with large open landfill sites they could survive and produce more offspring, out-competing the storks that do migrate because they’d have saved the energy of travelling across Europe and Africa. If the impetus to migrate is based on the availability of food why are some still migrating? Habit? Maybe a genetic survey of resident and migratory colonies would help with answering these questions? How old are the resident birds? Is there any evidence that they have previously migrated and now are not doing so?
The migratory behaviours of several species have changed in response to human changes to the environment and human behaviour. Landfill sites, the introduction of junk food and a throwaway attitude to food has provided resident and migratory species with new sources of food. These sites are going to have to close in the next few years due to EU legislation; this could cause some problems for the resident White Storks who will either have to learn to migrate using the thermals over Gibraltar or through the Levant to get them to Africa and India, or find alternative food sources in Europe. Alternative food sources might be hard to find which means storks that have never migrated before will be forced to. The long-lived nature of White Storks means that there are ‘resident’ birds who do not migrate, and migratory birds that still make the journey every year to sub-Saharan Africa in the colonies. If the birds have to migrate but have never made the journey they’d have to rely on older birds, or birds that have made the journey before, to show them the way, just as young birds have to follow their parents during a migration. Clearly they can learn and adapt their habits to prevailing conditions so it’ll be interesting to see how stork behaviour changes in the next few years.
 ‘Are white storks addicted to junk food? Impacts of landfill use on the movement and behaviour of resident white storks (Ciconia ciconia) from a partially migratory population’ is published in the journal Movement Ecology on March 16, 2016.