Long-term population trends for the Least Bittern in Canada are difficult to quantify despite the fact that the species is concentrated in southern Ontario, a region that is well-covered by the Great Lakes Marsh Monitoring Program, the Atlas of the Breeding Birds of Ontario, and the Breeding Bird Survey. The species’ very low detectability, frequent turnover of breeding sites, and the possibility that marsh monitoring data may underestimate abundance all limit the accuracy of population trend estimates (Woodliffe 2007b, Poole et al. 2009). Results from the Marsh Monitoring Programs in both Ontario and Quebec suggest fluctuating but overall stable populations (see below), but these results are short-term. Results from the Ontario Atlas, which only cover the 20-year period between the mid-1980s and mid-2000, suggest a moderate decline in the population during that period. The Breeding Bird Survey (BBS) suggests an increase in abundance relative to 1970 both at a national and continental scale, but the sample size is low and the BBS is not ideally suited for monitoring marsh birds. Recent information from the Quebec (Atlas of the Breeding Birds of Quebec 2017) and Maritimes (Makepeace 2015a) atlases also show increases in the number of squares recording the species. Interestingly, all but two sites changed between the first and second Maritimes atlas (Makepeace 2015a), underscoring the difficultly of monitoring this species over the long-term given its dynamic site selection. Anecdotal observations suggest historical decreases in numbers across most of the species’ range (COSEWIC 2009c). Extrapolating from available data, the species is perhaps best assessed as having shown little overall change in population since 1970. However, the paucity of knowledge of this species’ biology, dynamic site selection, and secretive nature have resulted in an absence of long-term trend estimates and thus this assessment is considered to be of low reliability. The development of a National Least Bittern survey protocol for the breeding season will hopefully inform future assessments. The Least Bittern is currently at an acceptable level relative to its national population goal, which is based on the Great Lakes Marsh Monitoring Program (see graph below), and is consistent with the goal stated in the Recovery Strategy (Environment Canada 2014f).
Additional information on: Great Lakes Marsh Monitoring Program
Additional information on: Quebec Marsh Monitoring Program
Additional information on: Breeding Bird Survey (BBS) - Canadian analysis
Additional information on: Atlas of the Breeding Birds of Ontario