Increased abundance and range expansion of harlequin ducks Histrionicus histrionicus wintering in Eastern Canada

The eastern population of harlequin duck Histrionicus histrionicus in Canada has been designated a species of special concern since 2001 and as endangered from 1991 to 2001, largely due to low and declining wintering numbers detected in the 1980s. Our objectives were to summarize the current state of knowledge of harlequin duck abundance and distribution in Eastern Canada, and assess trends in wintering abundance across regions over the past 30+ years. Abundance estimates were generated from targeted surveys with thorough spatial coverage of major wintering areas on insular Newfoundland (NL), Nova Scotia (NS), and New Brunswick (NB) between 2015 and 2018, complemented by regional trends estimated from Christmas Bird Count (CBC) data collected at 12 sites across the same wintering range between 1988 and 2021. Overall, targeted surveys indicated that numbers have increased and distributions have expanded beyond historically surveyed regions, particularly in NS. This was supported by trends from CBC data, with steep average increases (mean λ > 1.05) on most CBC circles in Eastern Canada since 1988 or later. Using localized annual rates of mean population change from CBC data to project counts from dedicated surveys, we suggest wintering areas in the provinces of NL, NS, and NB may currently be supporting a combined total of 5682 birds (95% CI 5065-6354) in 2022, well exceeding the recovery target of 3000 individuals listed in the 2007 management plan. Our findings indicate that the eastern population is in recovery, but also emphasize that a consistent, dedicated monitoring program would enable managers to confidently evaluate management actions, and to respond most effectively should regional trends slow or reverse.