Aarhus University Seal / Aarhus Universitets segl

No. 336: Status of wintering velvet scoters and long-tailed ducks in Danish waters

Petersen, I.K., Sørensen, I.H., Nielsen, R.D., Fox, T. & Christensen, T.K. 2019. Status for overvintrende fløjlsænder og havlitter i danske farvande. En analyse af bestandsudviklingen og årsager til forandringer. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 52 s. - Videnskabelig rapport nr. 336


Between December 2016 and April 2018, DCE (Danish Centre for Environment and Energy), Aarhus University, in collaboration with the Danish Hunters Association, conducted a study of the status of Long-tailed Duck and Velvet Scoter in Danish waters, commissioned by the Danish Ministry of the Environment. The aims of the study were to describe changes in their distribution and abundance since the late 1980s, to determine their food choice and body condition through the course of the winter, as well as determine age- and sex ratios for each of the two species.

The winter distribution of Long-tailed Duck has not changed markedly between the 1980s and 2016. Rønne Banke, southwest of Bornholm, remains the most important wintering site for Long-tailed Ducks in Denmark. The total estimated numbers of Long-tailed Ducks wintering in Danish water have fluctuated markedly, although this may be partly due to changes in survey methods. In 2013, between 48,500 and 63,500 Long-tailed Ducks were estimated to winter in Danish waters, compared to 62,000 to 85,000 wintering individuals in 2016.

The wintering distribution of Velvet Scoter has changed to some degree since the 1980s, in that many more birds were recorded in the south eastern parts of the inner Danish waters in 2013 and 2016 compared to earlier years. Between 26,000 and 65,000 individuals were estimated to winter in 2013, compared to between 10,000 and 24,000 birds in 2016.

Analysis of the stomach contents of shot Velvet Scoters showed the diet to be almost entirely composed of bivalves and other benthic molluscs, while those of Long-tailed Ducks showed that species to have a much more varied diet. While bivalves and other molluscs also comprised an important part of the Long-tailed Duck diet, to a significant degree, crustaceans and small fish species also contributed to their diet.

Male (mean 1631 g ± 103.5 SD) and female (1472 ± 109.9) Velvet Scoters sampled during the current investigation weighed statistically significantly less than the means of historical samples (1707 and 1526, t122 = -8.1, P <0.001 and t30 = -2.7, P <0.01).  Male Long-tailed Ducks were statistically significantly heavier in the current sample (803 ± 60.2) than a historical mean (768, t49 = 8.5, P <0.001), but body mass of females did not differ (719 ± 69.6 and 720, t22 = -0.07, P >0.05).  However, in all cases, the percentage difference between recent and historical means was less than 5% (range 0.1% to 4.6%) and therefore is unlikely to be biologically meaningful.

The body condition of all dissected individual Long-tailed Ducks and Velvet Scoters was assessed to be moderate to high. Based upon a 0 to 9 scale body condition score (Franeker & Camphuysen 2007), the average condition index for Velvet Scoter was 5.3, while that for Long-tailed Duck was 5.4, which assigned individuals from both species to a category of “moderate condition”. The breast shape of the birds (an index of flight musculature volume) was classified according to a scale from 0 to 3. Ninety-two percent of the Velvet Scoters and 77% of the Long-tailed Ducks had breast shape scores in excess of 2. Because these body condition and breast shape indices have not previously been applied to classifying the condition of diving ducks, these results could not be compared to the results of previous studies. As such, these results should be regarded as base-line data, to serve for future comparison of body condition in these species.

There was a strong male bias in the sex ratio of both Velvet Scoters and Long-tailed Ducks. Annual evaluation of age ratios showed that juvenile percentages for both species was very low, both shown in the AU Wing Survey, from the birds collected for this project and from the retrospective analysis of image data, conducted under this project. Similar age and sex ratios were obtained to those from a similar survey of Long-tailed Duck wintering in Sweden. Despite large annual variation, the long-term trend in juvenile percentages in the wing samples between 1990 and 2016 was negative for both species and as low as 1 to 2 juveniles per adult female per year. The juvenile percentages for both species raise concern, not least since the long-term trend amongst both Velvet Scoters and Long-tailed Ducks wintering in Danish waters is negative.