Colonization and Extirpation of Great Black-Backed Gulls (Larus marinus) Nesting on Lake Ontario and the Upper St. Lawrence River

Colonies of Herring (Larus argentatus) and/or Ring-billed Gulls (L. delawarensis) on Lake Ontario and the upper St. Lawrence River were searched annually, 19812018, for nesting Great Black-backed Gulls (L. marinus). Great Black-backed Gulls nested at nine sites where both Herring and Ring-billed Gulls nested, at seven sites with only Herring Gulls, and at one site where they nested solitarily. Nests per site ranged from 121 on Lake Ontario and …