Ardea
Official journal of the Netherlands Ornithologists' Union

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Hoogerheide C. (1950) De Eidereenden, Somateria mollissima L., op Vlieland. ARDEA 37 (3-4): 139-161
The increase and present status of the Common Eider colony on Vlieland are described, and also the courtship display of these birds. 1. During the last 42 years the Vlieland Common Eider colony has increased from a few solitary specimens to nearly a thousand couples. 2. The ?? remained timid longer than the ?? and ventured much later on land. 3. On Vlieland the ? has not yet been observed guarding the brooding female, as he does in Iceland and the Orkneys. 4. In 1948 the total Common Eider population of Vlieland consisted of roughly 4,200 individuals. S. The majority of these are found in the Wadden Sea, the remainder in the North Sea, where they limit their range of action to the neighbourhood of the surf. 6. During autumn and winter no Common Eiders are to be seen on Vlieland. 7. The Common Eider can be observed near the coast a few weeks before the first eggs are laid (early April). 8. In the spring communal life is not sacrificed to individual pairs. Courtship and most of the pairings take place in the community. It is possible to distinguish pairs in the flock, but while the ?? are brooding no segregated couples can be seen. 9. During the period when the ?? are brooding, the ?? stay in the Wadden Sea and the North Sea, where they court the remaining ??, who are always in the minority. It is not unusual to see 20 ?? crowding round one ?. 10. The most ardent courtships and the majority of the pairings take place during the latter half of April and the beginning of May, and in the early morning large groups of courting Common Eider can be seen on the ponds in the dunes. After this time the inland regions are much less frequented by the birds. 11. The egg-laying process is spread over a rather long period, so that the maximum number of ?? brooding at the same time is not found before mid-May (60 % of the ad. ??). 12. The ?? with young show a preference for grouping together, so that large troops of chicks with escorting :99 are formed. As a rule the number of the escorting ?? is much smaller than would be supposed judging by the number of chicks. . 13. Sometimes ?? without chicks join an existing group. 14. The death-rate among the chicks is very high, this being to a great extent due to the ravages of the Herring Gull. 15. The ?? show interest in the ?? with young, but take no active part in the upbringing of the chicks. 16. The number of year-old birds on Vlieland depends upon the death rate among the chicks hatched during the previous summer. In 1948 there were about 750 yearlings, in 1949 roughly 260. 17. The number of ?? along the coasts of the Wadden Sea and the North Sea diminishes rapidly at the end of May. During June the ??, many in eclipse plumage, may be seen at the end of a sand-dyke which stretches far into the mud-flats. 18. The ?? disappear at the end of June, but during July may still be observed at the end of the above mentioned sand-dyke. 19. How long these birds stay here, and where the Vlieland-population sojourns during the autumn and winter months, is so far unknown. One possibility is that they are dispersed into small+ groups and winter along the North Sea coast. 20. A number of movements by the ? are described, three of these being accompanied by the typical cooing cry. 21. The ? only makes one epigamic movement, i.e. jerking of the head, which is demonstrated on various occasions. 22. The exciting of the ?? 'still' consists of two distinct movements: jerking of the head alternating with threatening. It is not one smooth movement as with the ?? Anatinae and Tadorna. 23. With the Anatinae no movement has been observed which is homogeneous with one of the three courting motions; but this+ is well the case with the Nyrocinae and Mergus (jerking of the head backwards).


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