Ardea
Official journal of the Netherlands Ornithologists' Union

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Deelder C.L. (1951) Enige beschouwingen over de vinkentrek in Nederland van omstreeks 1800. ARDEA 39 (1-3): 222-228
1. The present writer opposes the view of some ornithologists, chiefly that of L. Tinbergen, that ultra-high seaward migration in autumn is closely connected with tail-wind. By ultra-high seaward migration is here meant migration across the North Sea that cannot be seen with: the naked eye. The former stated before (1951) that this does not hold for the migration of Chaffinches (Fringilla coelebs L.) on a broad front over land. In this case there is even more migration with a head-wind. 2. In most years ultra-high seaward migration was observed to occur with winds from the Northeast and East indeed (e.g. in 1944 and 1946, cf. Deelder 1949). In other years, however, such migration took also place with SSE.-, S.- and SW.-winds and in one case even with a NW.-wind (e.g. 1945, 1947 and 1950). 3. Years with chiefly NE.- and E.-winds are hardly comparable to those with mainly SSE.- to SW.-winds because the latter are stronger as a rule and it is a well-known fact that strong winds are generally avoided by migrating birds. Besides, phenomena such as rain, mist and the like, which stop migration altogether, also occur much more in the case of western than in that of eastern winds. 4. In the opinion of the present writer it is not the direction of the wind but the weather condition that comes first. Birds tend to wait for anticyclonic conditions before crossing the sea. As good weather conditions mostly occur with winds from north-eastern and eastern directions, it would at first sight seem that these winds are preferred for migrating. But it has been found that in the case of good weather conditions with SSE.- to W.-winds Chaffinches cross the North Sea just the same as with NE.- to SSE.-winds. Some instances are given of this phenomenon. 5. The average figures for ultra-high migration across the North Sea per quarter of an hour were: In 9 cases the average was 17 Chaffinches with SSE.- to NNW.winds (head-wind). In 20 cases the average was 17 Chaffinches with NNW.- to SSE.winds (tail-wind). It is clear that the type of weather with eastern winds is preferred for crossing the sea, there being more migration by land and less by sea in the case of head-wind (d. Vleugel 1951). This may lead us to expect that more Chaffinches will hold the coast-line with western than with eastern winds. Observations have shown that this is indeed the case. Besides, many Chaffinches alight in the dunes or even take a north-eastern direction (reverse migration) when an eastern wind is blowing. 6. On nearly all days that sea-migration was observed, there were light winds (on 26 days). Oh only 3 days (cf. Deelder 1949) a slight sea-migration (ultra-high) was observed with more or less moderate winds. This occurred only on days with eastern winds. This again stresses the fact that Chaffinches prefer eastern winds for crossing the sea. It may be supposed that the birds know from experience or perhaps instinctively that on days with a western wind there is a greater chance of rain or mist or that the wind will increase in: strength. These circumstances adversely affect the migration of Chaffinches. When there is rain or mist they cannot alight as they do on land and with increasing winds they have to fly low, which at sea is dangerous for them. 7. The view that probably also in spring Chaffinches prefer to cross the North Sea with tail-wind is shown to be false. All instances which are given of sea-crossing and apparent sea-crossing in the spring occurred under anticyclonic conditions and with headwind, but again only because E.-winds are generally accompanied by good weather. But in the opinion of the present writer, migration from Great Britain to the continent may be expected with western winds, only if the weather be fine.


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