Evidence of continued declines in overall abundance and proportion of females among spring migrating Baltic-breeding Common Eiders 2009–2024

Abstract

Declines in the Baltic/Wadden Sea Common Eider Somateria mollissima flyway population prompted development of an International Single Species Action Plan (ISSAP), requiring previously unavailable annual assessments of abundance and sex ratio. We assessed whether daily spring counts of Common Eiders at Hyllekrog, a migration bottleneck watchpoint in southern Denmark, could track annual changes in abundance and sex ratios breeding “upstream” in the Baltic Sea. Standardised counts declined by 65.2% from 404,845 in 2009 to 140,820 in 2024, matching similar trends (45.2% decline) at Kåseberga, Sweden, 185 km northeast. How much this is due to true population decline and/or to short-stopping in milder winters remains unknown. Proportions of females in migrating flocks at Hyllekrog fell from 0.340 in 2013 to 0.296 in 2024, consistent with declines at Hanko Bird Observatory (Gulf of Finland) which fell from 0.396 in 2009 to 0.321 in 2024. Truncating the observation period at Hyllekrog by 61% to 6 March–20 April reduced the count totals by 8.9%. Counts from this reduced period were highly correlated with the total counts (r = 0.996), and annual trends almost identical (–7.7% and –7.5%). Subsampling showed observations at two–, four– and eight–day intervals produced weaker correlations and much reduced precision in abundance and sex ratio estimates. We therefore recommend continued daily spring counts of migrating Eiders and sex ratio sampling between at least 6 March and 20 April as the most effective means to fulfil ISSAP monitoring requirements.