Urdaibai Estuary

UNESCO biosphere reserve at the mouth of the Oka river

The Oka river opens north of Guernika into a wide tidal estuary, designated by UNESCO as a biosphere reserve in 1984 — the only one in the Basque Country. The mosaic of saltmarsh, mudflat, reed bed, and beech forest supports more than 300 plant species and is one of the most important waterbird sites on the Cantabrian coast.

Three visitor centres explain the ecology, and walking trails leave from each of the surrounding villages. The Urdaibai Bird Center at Gautegiz Arteaga is the best place to spot rarities, particularly during spring and autumn migration.

things to do sight
A story
When the tide goes out in the Urdaibai, a 12-square-kilometre expanse of mudflat appears between the dunes — one of the largest in the Bay of Biscay. The estuary is a key staging point for migratory waterbirds: in winter you can see ospreys, spoonbills, and tens of thousands of waders.