Ardea
Official journal of the Netherlands Ornithologists' Union

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Vleugel D.A. (1948) Enkele waarnemingen over 'vorstvlucht' en 'randtrek' in het Sloe-Schengengebied tijdens de winters van 1935/36 en 1936/1937. ARDEA 36 (3): 143-162
In the winters of 1935/1936 and 1936/1937 the writer investigated the numbers of Lapwing, Golden Plover, Common Redshank, Spotted Redshank, Avocet, Shelduck and Coot in the Sloe- and Schengendistrict, a part of the estuary of the river Schelde, in the province of Zeeland (see curves in Fig. 1 and 2), with the following results: . 1. Lapwing and. Golden Plover disappeared almost entirely in each of the four frost-periods, even in that occurring early in March 1937. After every frost-period they reappeared almost immediately in hundreds (up to 1000 birds of each species were to be observed again in the area). 2. A very small number of Avocets remained during the winter of 1935/1936, but there was some flux and reflux owing to the frost-periods. In the winter 1936/1937 the species was absent from 23 Dec. 1936 till 26 Febr. 1937 owing to the longer and more severe frost-periods. 3. A small number of Spotted Redshanks remained in both these winters, but more in that of 1935/1936. There was also a flux and reflux depending on the weather conditions (frost or otherwise). The Common Redshank, as is well-known, winters farther north than the Spotted Redshank. Consequently there was an increase of the former during some frost-periods. 4. Shelduck and Coot very much increased in some frost-periods, and again there was an important decrease after the frost-periods. 5. Some other birds also increased in or after frost-periods: Green Sandpiper, Lapland Dunlin, Turnstone, Grey and Ringed Plover. In 'Trekvogels 1936 and 1937' (Ardea) we find some other apparent examples of birds increasing in or after frost-periods: Mute Swan, Ruff, Woodcock and Oyster-Catcher. In December 1943 the writer observed in Zeeland early return migration of grey geese (Anser species), Wigeon, Meadow-Pipit, Sky-Lark and Redwing after a frost-period. 6. Two weather-movements are to be distinguished: (a) one in a south-westerly direction associated with falling temperature, especially when frost sets in. The writer has given the Dutch name 'vorstlucht' (frost-flight) to this movement, as one had not yet been given. (b) one in a north-easterly direction associated with rising temperature, especially after frost-periods. To this the writer gives the Dutch name 'randtrek', as there had not previously been a name for this kind of migration (the literal translation in German would be Randzug, in English perhaps border-migration). He has chosen this name because this, northward movement takes place chiefly on the northern border of the winter area of the species concerned. Both movements were also observed directly in a few cases. 7. The name 'randtrek' can be used only when frost-flight occurs in the species concerned in the particular year. It is still a matter for investigation whether there is a clear break between the 'randtrek' and the normal spring migration in any year. 8. It is not necessary to give the late frost-flight in March and April a new name as German ornithologists have done, namely 'Rnckzug' (reverse migration). The reasons are (a) that theoretically there is only a slight difference between the early and late frost-flights, and (b) that although the participating populations are different in many cases, it is mostly impossible to decide whether the migrating birds are wintering ones or newcomers from the south which retreat under the influence of the frost. 9. It seems better to distinguish not only 'voorjaarstrek' (spring migration) and 'najaarstrek' (autumn migration), for these names refer only to the time of migration and in this way we cannot give a clear survey of all the phenomena of bird migration. This is possible only if we classify in another way. Then we can distinguish: I. 'Wegtrek' (emigration), divided into (a) pre-migratory movements, (b) autumn migration (in restricted sense), and (c) frost-flight (weather movement). II. 'Terugtrek' (return migration), divided into (a) Cursus retroversus (Koch), (b) border-migration, and (c) spring migration (in restricted sense). Phenomena such as Zwischenzug (literally 'between migration'), dispersal (Van Oordt) and moult migration (Hoogerheide) and Kraak) need further investigation before we can adequately classify them. 10. In a theoretical section the idea is developed, that the investigated birds belong to the group of 'Wetterv÷gel' (Weigold). As the writer has some objection to the names 'Instinktv÷gel' and 'Wetterv÷gel', he classifies the birds as shown in Fig. 3. Every bird or species can in this way be given a place on the vertical line. This place is not fixed, but can change, as has already been said by Weigold. Apparently the investigated birds, which the writer reckons among the extrovert migrants and vagrants, change their position on the line and become more or less extrovert residents in proportion to the severity of the winters in which they live. 11. In accordance with McDougall's theory of instinct, the writer explains the border-migration instinct as the result of an increase in energy (cf. Tollenaar) caused by better circumstances of life and temperature. The frost-flight instinct he considers to be the result of a strong feeling of distress due to unfavourable life and temperature conditions.


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