Dedicated waterbird surveys that target individual colonies provide the best monitoring data for the Caspian Tern, although coverage of colonies is incomplete. Surveys throughout the Great Lakes have documented moderate increases in abundance overall (see graph below), and large increases in some areas. For example, 47 nests were estimated to be present in Lake Ontario in the late 1970s; there were 1,541 nests in 2009 (C. Weseloh et al., unpublished data). Increases were also observed at what are perhaps Canada’s largest colonies, in Lakes Winnipeg and Winnipegosis (Cuthbert and Wires 1999). In 2012, all colonies on the large lakes in southern Manitoba were surveyed. The survey revealed evidence of a long-term increase since the 1970s, but with a possible decline since the 1990s (Wilson et al. 2014). However, the small Atlantic coast population may be experiencing a decline (Wires and Cuthbert 2000), and the species has been listed as threatened in Québec since 2009, since it now only breeds in the province sporadically (Gouvernemement du Québec 2010). Results from the Breeding Bird Survey (BBS) suggest a large increase in abundance in Canada relative to about 1970 but, even in regions well covered by the BBS, colonies may be poorly monitored by this roadside survey because the largest colonies tend to be in the middle of large lakes (all Bird Conservation Regions with reasonably reliable results are shown below). However, BBS results for both Canada and North America provide corroboration that the population within Canada is not decreasing. Considering all available information, the Caspian Tern appears to have shown a moderate increase in abundance in Canada since about 1970, but incomplete information from prairie colonies and regional variation means that this assessment is considered to be of medium reliability. Although the Great Lakes Decadal Colonial Waterbird Census is a regional survey, it currently represents the best available information for this population; the national population goal is therefore based on it (see graph below). This goal may be revisited if regional populations begin to show differing trends. The Caspian Tern is at an acceptable level relative to its national population goal.
Additional information on: Great Lakes Decadal Colonial Waterbird Census
Additional information on: Breeding Bird Survey (BBS) - Canadian analysis
