Instead of Summers in Northern Europe and Winters in Africa many storks are living year round in Spain and Portugal. Feeding in garbage dumps may be one factor. I wonder why the article didn't mention drought in Africa as another possible cause.
... since the mid 1980s increasing numbers of storks have stopped their annual migration from Northern Europe to Africa for the winter.
Instead, many are living in Spain and Portugal the whole year round -- feeding on 'junk food' from rubbish dumps.
I would think weather would be a reason - climate change.
"They are also breeding in new areas in the north of the country -- and we think this may be because the climate has changed to become more suitable for them. So climate change is another likely factor."
Exactly my hypothesis - then I found it mentioned in the article.