This ecological park is, together with Urdaibai, one of the most important avifauna areas on the entire Basque coast. There are many the bird species that can be observed in Plaiaundi, but we have to stand out the number of wading birds, marsh birds and seabirds that use this area as a resting and feeding place during their migratory movements, their wintering period and, to a lesser extent, during their breeding season.
Their strategic situation between the sea and the mountain barrier of the Pyrenees contributes undoubtedly to the richness of species that frequent this location during the migration steps, turning Plaiaundi into a privileged place for watching numerous species.
Besides the birds, other groups of fauna find here a suitable habitat, being outstanding the presence of the Natterjack Toad, which has a population in the park that is isolated from the rest of its distribution area.