BEINTEMA AJ (1980) The Smew Mergus albellus. LIMOSA 53 (1): 3-10.

In the past ten years very large concentrations (up to over 20 000) of Smews Mergus albellus are regularly wintering in the southern IJsselmeer. This fact has not yet been sufficiently recognized in international ornithological literature. Concentrations of over 10 000 Smews are only known from two other localities in the world: the Volga Delta in the Caspian Sea, and the Kuban Delta in the Azov Sea. There are four additional areas where concentations of many hundreds can be regularly met with: the Karakum Canal area in southern Turkmenistan, Apolyont Golli in Turkey, the Danube Delta in Romania, and the river system of Rhine and Waal in The Netherlands and Western Germany. The total wintering population of Europe, Turkey and Iran is estimated at 75 000 individuals, 57 000 of which winter within the USSR. Further east no large concentrations have been found, so far. The world population probably does not exceed 140 000 birds.
      Very large fluctuations in the numbers of Smews wintering in The Netherlands suggest the existence of an alternative, undiscovered site, probably somewhere in the Baltic region, The birds may hide at the same place during migration, since the species is completely missed during international spring and autumn counts. Breeding grounds of Dutch winter birds reach ?far into Siberia. Some Dutch rings have been recovered in areas from where the birds are supposed to winter in the Caspian Sea. This puts emphasis on the present importance of the IJsselmeer as a winter haunt for the species.
      The proportion of adult males in winter approximates 42%, with little annual variation. Within Europe, the percentage drops from north to south, but the differences are relatively unimportant, as practically all birds winter in The Netherlands. An assumed I : I sex-ratio thus leaves 16%juveniles for the known population. It can be calculated that this means either an extreme longevity of the species or large numbers of juveniles must winter elsewhere, as yet undetected.
      Smews in winter mainly feed on small fish (up to 10 em). All fish species of the right size class are taken, but there is a clear preference for pelagic fishes above bottom-dwelling species. In the IJsselmeer Smews feed almost exclusively on Smelt Osmerus eperlanus, a very abundant fish. They catch the fishes by pursuing them, swimming under water, in shallow horizontal dives, averaging about 15 seconds. Therefore, diving times are not related to water depth, as in other diving ducks, except maybe when fish supplies are poor.
      During 15-20 minutes Smews perform about three dives per minute, whereupon they rest and preen for about ten minutes. Diving and resting periods alternate regularly during the daylight period. Activity starts at dawn and may end well before sunset. Dutch Smews appear to make shorter days than German and Swedish ones, probably as a result of the extremely rich food supply in the IJsselmeer. Within flocks Srriews synchronize their diving. In a densely packed group of 1200 Smews it was observed that during several seconds 97% of the birds could be below the surface simultaneously. Groups behaving like this attract other Smews and fish-eating birds from great distances. Diving frequencies in dense groups seem to have increased slightly.
      In the IJsselmeer area Smews perform a regular daily migration between their feeding grounds, and a limited number of sleeping grounds. Sleeping grounds are small lakes, harbours, or inlets, adjoining the IJsselmeer, and separated from the open water by a single dike or dam. Individual Smews are not attached to one sleeping ground, but may switch from day to day. On each sleeping ground the numbers of birds arriving in the afternoon may fluctuate sharply from day to day. The sleeping ground to be used by a particular group of Smews may well be determined by the place where the birds have been fishing during the last part of the day. The southern IJsselmeer area, including the adjoining sleeping grounds, must be regarded as one single unit. Smew Mergellus albellus

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