Over the last twenty years, the Natural Processes Monitoring Team (ICTS-RBD) of the Doñana Biological Station carried out the mid-January census of wintering birds in the Guadalquivir Marshes (Doñana Natural Space, adjacent non-protected areas and Brazo del Este Natural Site).
The total number of birds counted has been much more modest than last year, 491,557 compared to 674,974 at the time , which was the highest number in the last 15 years. However, two species reached their historical wintering record in the area, such as the White-Headed duck (Oxyura leucocephala) and the Slender-Billed gull (Chroicocephalus genei).
The White-Headed duck is a threatened species, in the category of "Endangered" at worldwide and Spanish level. This year 503 individuals of this species were counted, exceeding the highest figures recorded in January 2003 with 332 birds. Most of them were in Veta la Palma estate (La Puebla del Río, Doñana Natural Park), where they were counted 449 specimens distributed among different fish farm ponds. In addition, 44 were counted in the Tarelo lagoon (Cádiz Sector of the Natural Park), while the remaining 10 were in the nearby Bonanza lagoons, three small ponds surrounded by crops without any specific protection.
Moreover, the Slender-Billed gull is a species not threatened worldwide but "Vulnerable" according to the Red Book of the Birds of Spain. In this census 377 specimens were counted, an exceptional peak that doubles the maximum number -175 birds- recorded in the survey of January 2016. Gulls were located mainly in the saltpans of Cádiz included in the Natural Park of Doñana -368 birds-, and 9 birds in “El Codo de la Esparraguera”, transformed marshes in the municipality of Trebujena and adjacent to the Natural Park.