Data from the North American Breeding Bird Survey indicate a survey-wide decline in Black Terns (Chlidonias niger) at an average rate of 3.1% annually during 1966-1996. Black Terns in Canada decreased at an average annual rate of 3.5% during this interval, while the United States population showed no significant trends. The long-term declines largely reflect trends prior to 1980, when the continental, Canadian, and United States populations decreased at average annual rates of 7.5%, 5.6%, and 11.9%, respectively. Most population trends were reversed during the 1990s, causing trend estimates over the 1980-1996 interval to become more positive. Associations between patterns of change in Black Terns, Mallards (Anas platyrhynchos), and numbers of ponds in the northern Great Plains suggest some relationships exist between habitat availability and the population trajectories.
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Population trends of black terns from the North American Breeding Bird Survey, 1966-1996