Habitat Changes at the Local Scale Have Major Impacts on Waterfowl Populations Across a Migratory Flyway

Migratory waterbirds are experiencing severe declines worldwide due to habitat loss. Their life cycles often span different countries and continents, highlighting the need for safeguarding wetland networks along migratory flyways. However, there are gaps in understanding how changes in specific sites can impact species at the biogeographical scale. Here we used a wetland of international importance (the Guadalquivir marshes, SW Spain) as a case study to investigate the causes and consequences at the flyway scale of annual changes in wintering waterbird assemblages. To do so, we combined 38 years of local and international waterfowl winter counts, environmental and remote sensing data encompassing 432 Ramsar sites, and a functional approach through structural equation modelling (SEM). We show that the environmental conditions experienced by wintering waterfowl in the study area were correlated with changes in their biogeographical populations in the East Atlantic Flyway. We found that during the last 40 years, the waterfowl assemblage wintering at the Guadalquivir marshes has shifted from a community composed mainly by herbivores and pre-Saharan dabbling granivores, to the current one dominated by Trans-Saharan dabbling granivores. Declines in 9/15 of the species studied were associated with the deterioration of the Doñana National Park natural marshes, whereas changes in the remnant six species responded mainly to global factors, such as the increase in winter temperatures in other areas of their distribution range. These results underscore the importance of considering global factors and flyway population data when interpreting regional trends of migratory animals. But also, that changes in specific wetlands can have measurable global impacts. Being that the long-term persistence of migratory animals in a changing world entails the protection and integrity of migratory flyways beyond national borders.