A bustling avian metropolis
The granite cliffs of the Cíes Islands, part of the Atlantic Islands of Galicia National Park, host the largest known breeding colony of the yellow-legged gull (Larus michahellis) on the planet. This dense aggregation consists of approximately 22,000 breeding pairs, creating a noisy seabird equivalent of a major city. The gulls, with their distinctive yellow legs and powerful beaks, are opportunistic and intelligent. Their success here is tied to the protected status of the islands, which eliminates terrestrial predators, and the marine ecosystem of the surrounding Atlantic waters.
These gulls display complex social behaviors and have learned to exploit human activities for food. They often follow local fishing trawlers, feeding on discarded fish, which supplements their natural diet of fish, crustaceans, and mollusks. Studies of their behavior reveal adaptability; they can remember individuals, learn from experience, and even engage in kleptoparasitism—stealing food from other birds. On the islands, they build their nests on cliffs and even along the edges of the main walking tracks, especially on the ascent to the Cíes lighthouse.
Other seabird residents
The gulls are the most numerous residents, the Cíes are also a refuge for another major seabird: the European shag (Phalacrocorax aristotelis). About 1,000 to 1,500 breeding pairs nest on the islands, representing one of the largest colonies for the species in southwest Europe. These birds are specialist divers, with black plumage that has a greenish sheen in the sun. During the breeding season, adults develop a forward-curving crest on their heads.
Unlike the generalist gulls, shags are almost exclusively fish-eaters, pursuing their prey on dives that can take them deep beneath the waves. They are less adapted to a life on land, and their feathers are not fully waterproof, which aids in diving. This is why they are often seen perched on rocks with their wings spread out to dry. The specific subspecies found here is G. a. aristotelis. The park's ecosystem also supports other important species, including the European storm petrel (Hydrobates pelagicus) and peregrine falcons (Falco peregrinus), which sometimes prey on other birds. The high density of these seabird colonies has led to the islands' designation as a Special Protection Area for birds (ZEPA).