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Moulting Dunlin Calidris alpina ssp. staying for three weeks in the Öresund area, S. Sweden |
| For readers from other continents: The Dunlin Calidris alpina is an Arctic species with an almost circumpolar distribution, it winters in W Europe, Africa, the Mediterranean, the Persian Gulf, China, Japan, North America etc. All conceivable moult strategies occur in different populations; birds visiting W Europe migrate unmoulted or in fresh plumage or with minute remige growth after having exchanged part of their remiges on breeding-grounds. |
In all papers dealing with the late summer migration of adult Dunlins in the South Baltic area, there is consensus of opinion at one point: adult birds never linger in the Baltic, e.g in order to complete the growth of remiges shed on breeding grounds. Brenning (1987) reports an average "Verweildauer" of 2.9 days in 62 retrapped adults at Langenwerder, Germany, and Gromadzka (1983) remarks that particularly July migrants have a very brief stopover in the Bay of Gdansk, Poland. The highest primary moult scores in migrating Dunlin at Ottenby were c. 35 (primaries 6 - 8 growing), 2y birds being slightly more advanced than 3y+ birds. Such high scores were recorded only in August, not in July birds (Holmgren et al. 1993).
Adult Dunlins with growing remiges occur rarely on Falsterbo peninsula in the south Öresund area till the end of October, the extreme case being a 2y bird with primary score 36 at the spits of Skanör (55.26 N, 12.51 E) on 21.10.96. In addition many late adults still carry 25 - 50 % of the (nuptial) belly-patch and old scapulars, and adults belonging to the wintering population may turn up with P9 - 10 and an unknown number of secondaries + all scapulars suspended. Since many 1y birds will stay in the same area for 1 - 2 1/2 months (September - November), moulting into winter plumage, the possibility that at least a few adults might take the same option and complete their remige moult must be taken into consideration.
During the course of a study of wintering Dunlins in the Öresund area some attention was given to this problem - particularly since we were keen on establishing the moulting-grounds of the wintering birds. But throughout the early nineties the findings of e.g Brenning 1987 were confirmed by our own results; moulting adult birds never stayed for more than 3 days, and the progress of moult was always extremely slow. Two cases, given in full, illustrate this (scores [P + S]):
The emphasis seems to be on minimal secondary growth during this stage - and still both birds were losing weight! The likely explanation is, that water levels were extremely elevated during the days in question, adult birds occupying an islet of decaying seaweed and spending much energy on keeping 1y birds away. Finally, by the first week of October, the area was evacuated by Dunlins and we could not catch a single bird for the rest of that autumn. In 1998, however, there was a first record of a moulting adult staying for almost three weeks. So, the need for at least a marginal revision of the established picture becomes a distinct possibility. In full, score [P+S]:
In this case conditions were reversed as compared to 1997; the water slowly being swept away by rising air pressure and easterly winds after a maximum water level of +41 cm on September 1st. When the above bird was retrapped nineteen days later, the wind flats surrounding Skanör had just emerged from the sea, fresh and unused after a summer characterized by almost constant high-water. For all we know this Dunlin may have completed its moult in our area. Still, the Öresund area, and probably Kattegat as well, have offered too undependable foraging possibilities in recent years to attract large amounts of moulting waders. Exceptions may coincide with a high "parking" over the South Baltic area for longer periods in August or September, creating wind flats and thus tempting migrating adult Dunlins to halt and complete their moult.
The moult cards were recorded by Peter Olsson and Christer Persson, this note published on the web on 29.10.98