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La Tour du Valat

Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	The ratio of females and immature males among migrating Common Eiders Somateria mollissima in southern Denmark, in spring 2013 and 2014
Auteurs :	Preben Berg, Auteur
Type de document :	Article
Dans :	Dansk Ornitologisk Forenings Tidsskrift (108(3) 2014)
Article en page(s) :	207-210
Langues:	Anglais
Résumé :	The present survey was made as a supplement to other surveys which show a declining ratio of females in the Baltic flyway population of Common Eiders (see Lehikoinen etal. 2008, Ekroos etal. 2012). During 27 February -8 June 2013 and 10 February -8 June 2014 we recorded 334658 and 342 501 Eiders, respectively, migrating eastwards at Hyllekrog, the southern tip of Lolland (54 37'N, 11 2T E) in Denmark.The field work was conducted during two very different years with 2013 being a cold spring, while 2014 had an early and warm spring. Hence, in 2013 a lot of Eiders were delayed because of a rather severe late winter period from 10 to 26 March, where the migration almost stopped (Fig. 1). In 2013, 58681 of 325803 recorded Eiders were sexed (i.e. 18.0%), while in 2014, 64492 of 342501 Eiders were sexed (i.e. 18.8%). The fluctuations of the female percentage were followed on a daily basis throughout the spring and the percentage typically varied between 20 and 45% (Fig. 1).The final percentage for the whole spring period was 33.9% in 2013 and 33.1% in 2014.Separation between immature and adult males was not part of the original project in 2013, but later on, a total of 4498 males were aged during 27 April - 8June 2013.1n 2014, a total of 44218 males were aged during 10 February - 8 June. The highest ratios of immature males were recorded from mid-April onwards, while low rations of generally a few percent were found during the main passage in March 2014. For the entire 2014 season, 5.6% of the males were immatures passing more or less distributed over the entire spring migration period (Fig. 1).The main migration took place during 6-7 weeks in MarchApril (Fig. 1), where it consisted of several waves. Two primary migration types were noticed during this study. The presumably paired birds were seen migrating in small flocks with rather equal numbers of males and females. These flocks were predominantly seen in the first half of the migration period. The other and far more conspicuous type was large flocks of presumably unpaired males mixed with relatively few females.There were two important factors that influenced the final estimated female ratio. Firstly, in 2013 it was estimated that about 45000-50000 Eiders passed unseen primarily in the evenings of the first 1.5 month, making up about 12-13% of the spring total. In 2014, it was estimated that 27000-29000 birds passed unseen, i.e. approximately 7-8%. In general, the female percentage was lower in this early part of the spring, and if corrected for these missing birds, the final female ratio only drops slightly to 33.4% in 2013 and 33.0% in 2014.The second factor was the large number of unsexed birds. The large flocks mainly of males and the weight of these against the flocks with paired birds could to some degree be a source of error, but a high number of randomly picked flocks in both categories have probably minimized this problem.The grand totals (seen and unseen) in the years 2010-2014 were estimated to approximately 410000 in 2010 and 2012 (about 359000 counted each year), 380000-385000 in 2013 and 370000-372000 in 2014 - i.e. a declining trend. This may be related to a similarly declining trend in the ratio of females to males, in that there were more females than males in the Baltic up until around 1980 (Lehi koinen et aL 2008, Ekroos et aL 2012).

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