A critical fueling station
The Urdaibai estuary, a 220-square-kilometer UNESCO Biosphere Reserve on the coast of Biscay, is a stopover point for birds on the East Atlantic Flyway. This major migratory corridor connects breeding grounds in northern Europe and the Arctic with wintering areas in sub-Saharan Africa. The estuary's location, positioned between the Pyrenees mountains and the Bay of Biscay, funnels millions of birds into this feeding area. Over 245 bird species have been documented here, taking refuge in the extensive salt marshes and inter-tidal zones of the Oka river delta.
For many long-distance migrants, Urdaibai is the last chance to accumulate energy reserves before attempting to cross the Sahara Desert. The journey requires immense fat stores, and the estuary's nutrient-rich mudflats provide the perfect fuel. Small passerines and waders can increase their body mass by as much as 60% during a two-week feeding period. This process, known as pre-migratory fattening, primarily involves the accumulation of fat, with lean mass like flight muscles increasing to a lesser extent. The abundance of invertebrates in the wetlands allows for extremely efficient foraging. Eurasian Spoonbills (Platalea leucorodia), for example, achieve foraging rates more than double those observed in other parts of the estuary.