Breeding Bird Survey (BBS) results suggest a large decrease in the abundance of Common Terns in Canada relative to about 1970, though the precision of the trend estimate is only moderate, and geographic coverage of the species' breeding range is incomplete, particularly across the northern portion. The three Bird Conservation Regions for which there are reasonably reliable BBS results are shown below; all three suggest large decreases. Additional information is available from dedicated monitoring of tern colonies in Atlantic Canada, Quebec, the Great Lakes, and the Northwest Territories. Results from surveys in the Great Lakes suggest substantial decreases in abundance; the population in Canadian waters and connecting channels has declined by more than 40% since about 1970, from a high of 16,000 nests (Courtney and Blokpoel 1983, Morris et al. 2010; see also graph below). Trends elsewhere in the range are variable (Morris et al. 2012). In 2012, results from surveys of the large lakes of central Manitoba (Lakes Winnipeg, Winnipegosis and Manitoba) suggested that the abundance of Common Tern had increased relative the 1970s but showed a 57-67% decline in the population over the last 20 years (Wilson et al. 2014). Surveys conducted in 2017 suggest that the declines continue (A. McKellar, ECCC, unpublished data). These data help fill an important gap in central/western Canada and suggest that declines are not restricted to the Great Lakes. Overall, the abundance of Common Terns in Canada has most likely decreased since about 1970, but the magnitude of that decline and whether it is range-wide is unclear, so the reliability of this assessment is considered to be low. The Common Tern is currently below the lowest acceptable level relative to its national population goal (see Canada graph below).
Additional information on: Breeding Bird Survey (BBS) - Canadian analysis
Additional information on: Great Lakes Decadal Colonial Waterbird Census