Cape St. Marys is located on the southwestern tip of the Avalon Peninsula at the entrance to Placentia Bay. The cliffs along the mainland rise to approximately 130 m above sea level, with grassy barrens being present on top. An isolated sea stack (Bird Rock) is located offshore. Colonial seabirds nest along approximately 4 km of mainland cliff and on the isolated stack. The site extends east and southwards out of the ecological reserve to include : Bull Island Point, the small islets of Bull, Cow and Calf islets, St. Marys Keys (Cays), and Lance Point.
Cape St. Marys supports a large colony of breeding seabirds. In all, over 30,000 breeding pairs are present. Common Murres and Black-legged Kittiwakes are the most abundant with their populations being conservatively estimated in the late 1980s at approximately 10,000 pairs each. This represents approximately 2% of the eastern North America population of Common Murres and 4 to 5% of the western Atlantic breeding population of Black-legged Kittiwakes. A large population of Northern Gannets is also present with breeding populations being estimated in the late 1980s at 5,485 pairs. This represents approximately 2% of the global population and as much as 12% of the North American population. Other seabirds nesting at Cape St. Marys include Thick-billed Murres, Razorbills, Black Guillemots, Herring Gulls, Great Black-backed Gulls, Great Cormorants, and Double-crested Cormorants.
The Cape St. Marys area also supports large numbers of migrant seaducks (Oldsquaw, scoters, eiders), including large numbers of the eastern population of Harlequin Ducks (nationally endangered). About 30 to 40 birds are reported in some years. This may be greater than 1% of the eastern North America population of Harlequin Duck.
BirdLife International (2023) Important Bird Areas factsheet: Cape St. Mary's. Downloaded from http://www.birdlife.org on 08/02/2023.