Forårstrækket af Ederfugle gennem Femern Bælt 2009-19:Trækkets forløb og udviklingen i antal og kønssammensætning

• Main Authors: Berg, Preben, Bregnballe, Thomas
• Format: Article in Journal/Newspaper
• Language: Danish
• Published: 2020
• Subjects: Ederfugl; træk; jagt; Danish Island; Kattegat; Main Passage; Common Eider; Melanitta nigra; Somateria mollissima
• Online Access: https://pure.au.dk/portal/da/publications/foraarstraekket-af-ederfugle-gennem-femern-baelt-200919(1717503c-9b5a-4cd9-9c9a-8bf4d47cd386).html; https://pure.au.dk/ws/files/191749866/Eiders_DOFT_2020_Spring_migration_Fehmarn_Belt.pdf; https://pub.dof.dk/artikler/1288/download/doft-114-2020-42-55-foraarstraekket-af-ederfugle-gennem-femern-baelt-2009-19

Spring migration of Common Eider Somateria mollissima through Fehmarn Belt: Timing of migration and changes in numbers and sex ratio The Fehmarn Belt (the part of the Baltic Sea that lies between the German island of Fehmarn and the Danish island of Lolland) forms a corridor though which thousands of Common Eider pass when returning from their winter quarters to their breeding grounds farther east and north in the Baltic Sea. The migrating Eiders are birds that have wintered in the Wadden Sea and the westernmost part of the Baltic Sea, as well as birds that in late winter have moved southwards from Kattegat into the Little Belt and Great Belt prior to starting their spring migration onwards (Noer 1991). 54 Forårstræk af Ederfugle i Femern Bælt Here we present annual observations of this Eider migration made at the southernmost point of the Danish island of Lolland (54°37’ N, 11°27’ E) during 2009-2019. Specifically, we describe 1) seasonal and diurnal migration patterns, 2) annual changes in sex ratio and total numbers, and 3) the proportions of immature males observed. Given the nature and position of the observation point, we believe that these observations record a large proportion of the Eiders passing through Fehmarn Belt. Observations were usually performed from just before sunrise until midday, in the early afternoon or (from end of March onwards) during all daylight hours. The migration recordings started in February and continued daily (or almost daily) up to 8 June (see Appendix 1, Tab. 1; on average 108 days per season). On mornings with particularly intense migration, data collection followed a fixed schedule switching between three protocols: (i) sampling sex ratios of Eider flocks, (ii) counting migrating Eiders, and (iii) counting Common Scoters Melanitta nigra (Fig. 1). Sex-ratio determinations were performed on all observation days during 2013-2019. On days when it was not possible to sample sex-ratios from all flocks, ratios were sampled from large numbers of flocks, considered to be representative of those that passed through that day. The sex-ratio of migrating Eiders varied within season (Appendix 1, Fig. 1) and between years (Tab. 3). The total number of birds and the total number of females passing per day were calculated by correcting for the numbers of hours and days with incomplete coverage (Appendix 1, Tab. 1), taking into account the differences between the sexes in relation to the times when the majority of individuals passed the observation site during the day. While there was considerable variation in the date when the first 10% of all migrating Eider passed during 2009-2019 (6-31 March; Fig. 3), the date when 50% had passed varied little (28-30 March in eight of eleven years but delayed in the late springs of 2010, 2013 and 2018 when this date was 2, 7 and 5 April, respectively; Fig. 3). Average daily numbers of Eiders migrating are shown in Fig. 2, highlighting the delayed migration in 2010, 2013 and 2018 which resulted in migration peaking during 13- 17 April. Days with > 30 000 Eiders migrating through Fehmarn Belt ranged between 28 March and 7 April (Tab. 1), with the highest on a single day being 54 336 (30 March 2009). Across all years, the first 10% and 50% of all males passed one day before females, but the date when 90% of both sexes passed was the same (Tab. 2). Migration typically started about 30 minutes before sunrise and continued throughout the day; on most days, however, movements were most intense around sunrise and the intensity of the passage diminished progressively from one hour after sunrise until 12:30 (Fig. 4). The intensity of migration was affected by wind direction, with a preference for westerly winds when wind speeds exceeded 5 m/s (Fig. 5). The estimated annual totals (corrected for missing observation periods) declined from c. 410 000 Eiders in 2009-2012 to 228 000 in 2017 (Fig. 6; Appendix 1, Tab. 2), with the most dramatic reductions recorded in 2015 and 2016, when numbers decreased by 13.5% and 23.3%, respectively, compared to the previous years. Overall, numbers passing per year at Hyllekrog decreased by 37.4% from 2009 to 2019 which corresponds to a decline of 36.7% over the same years at the coastal locality Kåseberga in southeast Scania (Fig. 6). The annual percentage of females observed decreased from 34.0% in 2013 to 30.5-32.2% in 2015-19 (Tab. 3). On average, the proportion of females increased during the season to reach a peak briefly at 34-45% in the first half of May and declined again in late May and early June, typically to 18-27% (Fig. 7). During the period 6 March – 20 April, when 90 % of the males and females migrated, the average daily proportion of females ranged from 23 to 39% (Fig. 7). Immature males were widely distributed throughout the migration period. Between 2014 and 2015, when immature male migration was systematically recorded (12 828 and 5730 birds, respectively, corresponding to 5.6 and 2.9% of all migrating males) there was little correspondence in the timing of their passage (Fig. 8). The lowest proportions of immature males occurred during the main passage period in March (Fig. 8). The main conclusions from the study were (i) that the proportion of females among Eiders migrating through Fehmarn Belt has decreased, (ii) that in the last five years, fewer Eiders have migrated through the Fehmarn Belt than in previous years, and (iii) that the phenology and intensity of the migration varies between years, partly depending on winter severity. The increased male sex bias probably reflects a continued increase in mortality among breeding females (cf. Ramula et al. 2018, Tjørnløv et al. 2019). The observed decline from 2009 to 2019 in total numbers of migrating Eiders passing the south coast of Lolland was comparable to a decline in numbers recorded at Kåseberga in southeast Scania (37% at both sites). Falling records of migrating Eiders at these two sites probably reflect a continuing decline in breeding Eider populations along the Baltic Sea coasts. However, we cannot rule out the possibility that these decreasing records of migrating Eiders partly reflect overwintering of an increasing proportion of the flyway population further to the north along the east coast of Sweden and/or along the Baltic coast of Poland and Germany.