They are formed by masses of deep and dynamic waters, with absence of trowel vegetation. They are a good alternative for many migrating aquatic birds as well as migrating birds during drought periods, in which the wetlands and other systems lack water. In Castilla and León we can find the reservoirs of Cuerda del Pozo, Santa Teresa, Ricobayo, Ebro and Porma. Here we find species like the Great Crested Grebe, Red-crested Pochard, Common Pochard, Eurasian Coor, etc. In some areas, it is possible to find reed beds of considerable sizes housing species such as the Black-necked Grebe, the Little Bittern, Purple Heron and Reed Bunting. This is the case in the San José del Duero and Monteagudo reservoirs.