In the old town of Donostia / San Sebastián, the province capital of Gipuzkoa, this black eagle calls for independentzia:
The symbol with the black eagle, arrano beltza in the local lingo, goes back to Sancho VII, king of Navarre 1194–1234. However, nowadays it’s mostly used by the Basque left-wing independence movement.
It might seem surprising that a socialist group uses a medieval royal symbol, but this is far from the strangest thing that has happened in the Basque Country. For example, at Basque festivities there are usually some very popular dressed up characters called buruhandiak (cabezudos in Spanish) that run around and mistreat children as could be seen in this Youtube clip.
Compared to that, it seems perfectly normal and rather reasonable that a left-wing movement has an old heraldic emblem as one of its symbols.