The gull populations of the Great Lakes have increased enormously in the last decade. During this 6-year survey, 1960-1965, herring gulls (Larus argentatus) increased from 24,000 to 43,000 breeding pairs in Lakes Huron and Michigan, and ring-billed gulls (Larus delawarensis) increased from 27,000 to 99,000 breeding pairs. Apparently herring gulls of Lakes Huron and Michigan have produced the birds which are nesting there on their own, but ring-billed gulls have, in addition to raising more gulls in the Lakes Huron and Michigan colonies, recruited many adult gulls from Lake Ontario. The Lake Ontario ring-bill population exceeds 100,000 breeding pairs. Both of these populations fit models which have a 10 or 12% annual adult mortality and 60 to 70% loss of fledglings prior to first breeding. Similar population growth was noted in the caspian tern (Hydroprogne caspia). Alewives (Alosa pseudoharengus) make up 50 to 85% of these birds’ diets while resident on Lakes Huron and Michigan. Apparently this new food resource has provided the gulls with more food when needed to raise young and thus underpinned these populations’ growth. Factors threatening these growing gull populations include a shortage of nesting space, pesticides in the food chain (particularly in Lake Michigan), and botulism. Increasing gull populations seem to be providing, with no increase in death rate, many more gull carcasses on the beaches. Clearly, the gull dieoffs are much more complex than owing to a single cause, botulism. As yet, these dieoffs have had little effect on the populations.