Could the fact that the Spain/Portugal border is influenced by geography also be a major factor? Both the border and the birds' flight patterns are likely heavily influenced by geography
It's not just the first tweet, read the whole thread.
> The Portuguese-Spanish border follows river valleys and is not associated with abrupt changes in climate or land-use.
> The reasons for the mismatch are historical. In 2001, Europe’s answer to the mad-cow disease (BSE) crisis was banning the abandonment of dead livestock. [...]
[Conservationists] convinced European Union legislators to delegate the choice to member states. Some countries, like Spain, resumed cattle abandonment under special conditions. Portugal never changed its laws.
> The Portuguese-Spanish border follows river valleys and is not associated with abrupt changes in climate or land-use.
> The reasons for the mismatch are historical. In 2001, Europe’s answer to the mad-cow disease (BSE) crisis was banning the abandonment of dead livestock. [...] [Conservationists] convinced European Union legislators to delegate the choice to member states. Some countries, like Spain, resumed cattle abandonment under special conditions. Portugal never changed its laws.