[6] Rain is less abundant than in the nearby coast due to the low altitude of the islands, which represents a minimal obstacle for passing clouds.

Cortegada, due to its location in the inside of the estuary close to the coast, presents an oceanic climate, with a rainfall of 2000 mm/year.

The average annual temperature oscillates in a range of 13-15 °C with little seasonal variability, with dominant winds from the North in the summer months and from the Southwest in winter.

Among the bird species, those that are particularly significant due to their threat level are the European Storm Petrel (Hydrobates pelagicus), whose nesting colony could make up 1% of the Spanish Atlantic population,[5] or the overwintering Balearic Shearwater (Puffinus mauretanicus) and the Eurasian curlew (Numenius arquata) included in the Spanish Catalogue of Endangered Species as Endangered.

[5] In the amphibian family, the islands are home to smaller salamanders than in the nearby continent, with distinct reproduction and diurnal activity as an adaptation to insular conditions.

[13] Reptiles include Bedriaga's skinks (Chalcides bedriagai) and ocellated lizards (Timon lepidus), and a notable insect is the rare Spanish festoon butterfly (Zerynthia rumina).

The islands were occupied by various monastic orders during the Middle Ages, were owned by the Church, gauged by nobles of the time and attacked by invaders who used them as a base for raids on the coast.

[16] During the summer, daily boats link Cíes with the ports of Vigo, Baiona and Cangas, and Ons with Portonovo, Sanxenxo, Bueu, Marín and Pontevedra.

Due to successive fires and agricultural practice, the original forests of the islands eventually declined and reforestation attempts in the 70s and 80s of the 20th century were performed with foreign species, particularly Eucalyptus and Acacias.

[20] The park is gradually replacing these species with arbutus, pine trees and birches, a process expected to take 20 years to completion.

The population of lichens was completely wiped out in several locations and has repopulated only partially,[22] while the annual reproductive success of the European shag was reduced by more than half.