The Chausey Islands archipelago is a vast ornithological reserve. Grande Île, on its own, is home to most of the Normandy bocage birds (243 species). The great cormorants and crested cormorants nest from mid-January to mid-August, while the roseate terns change islands every year. The archipelago is also the only crested merganser nesting site in France. The Groupe Ornithologique Normand (Norman Ornithological Group) conducts numerous censuses on the reserve throughout the year.