A wetland is an area of flat ground which undergoes regular periods of flooding. The soil becomes saturated, removing the oxygen and creating a hybrid system in which an interaction is created between an aquatic area and a terrestrial area. These tend to be endorheic areas, not very deep and stable, comprising lagoons, ponds, plateaus, etc. They are fundamental enclaves for different birds. They can be grouped into three categories, according to the wetland's occupation period: hibernating birds, breeding birds and migratory birds.
Hibernating birds constitute the most attractive group, comprising different species, mainly anatid and geese, as from the month of October.
There tend to be groupings of numerous individuals, the most common ones being the greylag goose, common teal, mallard, northern shoveler, stork, combatiente, common redshank and northern lapwing.
Within the area of Castilla and León it is worth mentioning the lagoons of Villafafila, La Nava and Boada.