Population status

Common Tern
(Sterna hirundo)

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 Surveys conducted in 2017 suggest that the declines continue (A. McKellar, ECCC, unpublished data).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).

 

Population goal and acceptable levels of variation

Species/groupGoalLower levelUpper level
Common TernMean abundance (first 5 years of survey)Goal minus 25% Not applicable

Breeding Bird Survey (BBS) - Canadian analysis

Additional information on: Breeding Bird Survey (BBS) - Canadian analysis

Table 1: Population trends by geographic area
Geographic areaTime Period Table 1 - footnote 1 Annual trend Table 1 - footnote 2 Limits
LowerUpper
Canada Select to view graph of the geographic area: Canada 1970-2016; Breeding Bird Survey (BBS) - Canadian analysis1970-2016-2.5-5.30.6
Boreal Taiga Plains Select to view graph of the geographic area: Boreal Taiga Plains 1970-2016; Breeding Bird Survey (BBS) - Canadian analysis1970-2016-3-6.4-0.2
Prairie Potholes Select to view graph of the geographic area: Prairie Potholes 1970-2016; Breeding Bird Survey (BBS) - Canadian analysis1970-2016-3.9-6.1-1.2
Lower Great Lakes / St. Lawrence Plain Select to view graph of the geographic area: Lower Great Lakes / St. Lawrence Plain 1970-2016; Breeding Bird Survey (BBS) - Canadian analysis1970-2016-2-4.40.8
 

Great Lakes Decadal Colonial Waterbird Census

Additional information on: Great Lakes Decadal Colonial Waterbird Census

Table 2: Population trends by geographic area
Geographic areaTime PeriodAnnual Trend Table 2 - footnote 1 Standard error
LowerUpper
Great Lakes Select to view graph of the geographic area: Great Lakes 1976-2009; Great Lakes Decadal Colonial Waterbird Census1976-2009   
 

References