| Summary: | Management of colonial waterbirds at regional, national, and continental scales requires up-to-date monitoring information on breeding locations, population sizes, and trends. The large lakes of southern Manitoba, Canada (Winnipeg, Winnipegosis, and Manitoba), and the inter-lake region host nationally and continentally significant populations of colonial waterbirds, but the area has been surveyed only irregularly in the past. We conducted surveys in 2017 to produce updated information on population abundance and trends for the six most abundant species breeding on the lakes: Ring-billed Gull (Larus delawarensis), Herring Gull (Larus argentatus), Caspian Tern (Hydroprogne caspia), Common Tern (Sterna hirundo), Double-crested Cormorant (Phalacrocorax auritus), and American White Pelican (Pelecanus erythrorhynchos). We also examined the relationship between lake water levels and breeding pair abundance on Lake Winnipegosis. All species but American White Pelicans showed declines in recent years, and most notably populations of Caspian Tern and Common Tern were as low as or only just above 1970s levels, respectively. Higher water levels were associated with fewer breeding pairs of colonial waterbirds. The effects of water regulation in this region on colonial waterbirds and their habitat warrants further research, as does the degree to which the species use smaller lakes spread throughout the western boreal forest for breeding, especially during high water years on the lakes in southern Manitoba.
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