The trees of Spider island have now fallen over or been washed away and no permanent vegetation is known on Gravel Island. The refuge is home for to a wide array of bird species that either use the islands as nesting grounds or as a place of shelter. The great black-backed gull ("Larus marinus") is known to have a small breeding colony on Gravel Island, and historically on Spider Island. In 1994 the species was discovered on Spider Island, making it the westernmost breeding on record at the time. Large colonies of herring gulls ("Larus smithsonianus") and double-crested cormorants ("Phalacrocorax auritus") are found on both islands, while a colony of Caspian terns ("Sterna caspia") can be found on Gravel Island. Scattered populations of waterfowl nest on Spider Island; these include the Canada goose ("Branta canadensis"), American black duck ("Anas rubripes") and mallard ("Anas platyrhynchos").