There are actually two hides, plus the big building.Excellent thank you. Good for cross-checking. Presumably these are a combination from the Centre building and the hide one can see from the webcam.
Several bird of prey species have been seen from the Center which I hope might show up over or near to the lagoon, though because of the distances they might be difficult to see properly. I can imagine Black Kites, for instance, coming down to water (they occasionally fish).There are actually two hides, plus the big building.
Most of the sightings registered as "Urdaibai Bird Center" (yep, they used the American spelling) have been made from the building itself, that username is used by the staff.
The other two hides offer different viewing angles. An advantage of the main building is a high tower (between 6 and 12 m, I don't remember).