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.
Feather forensics
Scientific analysis of feathers collected from migratory birds confirms the estuary's importance. Feathers are metabolically inert once fully grown, meaning their chemical composition provides a snapshot of the bird's diet and location during the time of feather growth. Researchers use stable isotope analysis to read this chemical record.
The ratios of stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes in a feather are directly linked to the food the bird consumed. Different environments and food webs have distinct isotopic signatures. By analyzing the isotope ratios in feathers, scientists can differentiate where a bird has been feeding. This technique has shown that birds arriving in Urdaibai carry the isotopic signature of their northern breeding grounds, and that their time spent feeding in the estuary prepares them for the next leg of their migration. This method is a tool for tracing migratory pathways without relying on direct tracking of individual birds. The research is spearheaded by institutions like the Aranzadi Society of Science, which manages the Urdaibai Bird Center.