Deelder C.L. (1949) On the autumn migration of the Scandinavian Chaffinch (Fringilla c. coelebs L.). ARDEA 37 (1-2): 1-88

Observations made on the sex-ratio of Chaffinches, caught at Dutch fowling-yards, indicate differences in the autumn migration-routes of ?? and ?? Chaffinches. The population under discussion breeds in Norway and Sweden and winters, according to L. Tinbergen (1941), mainly in Great Britain and Ireland, but also in Scandinavia, Denmark, NW-Germany, Holland, W-Belgium and N-France. 2) The ?? of the Scandinavian population seem to migrate to more distant winter quarters than the ??, just as do the ?? of the German population (cf. VON Bachmann 1934, Niethammer 1937). While in Scandinavia (Kalthoff and Jagerskiold 1898), Holland and S-England (Charteris 1938, table page 8) ?? are more abundant than ?? during winter, in Ireland there is a surplus of ?? (cf. Yarrell 1882, Ussher and Warren 1900 and Patten 1912). So it seems certain that the ?? have a stronger migratory impulse than the ??. 3) Moreover the ?? migrate on the average earlier in the season than the ?? (cf. Thienemann 1931, Drost 1940 and the present data, page 6). 4) As a rule there is an excess of males in Holland during autumn. This is shown by the catches at the fowling-yards (page 13), as well as by field observations (page 11). Only at Reeuwijk and once at Wassenaar have exceptions been found. During autumn there is a surplus of ??on Heligoland (DROST 1940). 5) In interpreting these results, it is desirable to know the sex-ratio of the Scandinavian population in summer, but counts are not available. For the Finnish and Dutch populations, however, reliable counts have been made; Palmgren (1933) and Tinbergen (1946) finding 54% ??. As these figures are similar, it is assumed that the Scandinavian population has a similar small excess of ??. 6) As Scandinavia has a surplus of ?? during winter, it is at first sight puzzling that the flocks which pass over Holland also contain a high percentage of ??. 7) This can be explained in the following way: The West-coasts of Scandinavia and Denmark, the North-Frisian coast and the West-coasts of Holland and Belgium are reached during autumn by a 'broad-front-migration' (cf. Geyr von Schweppenburg 1933) of Chaffinches, which has a WSW direction. Van Dobben and Makkink (1934) have described the behaviour of such migrants at the moment that they reach a coastline. Some of them put out to sea in the direction in which they have been travelling; the rest turn and follow the coast in the direction which most closely approaches to their original line of flight. The latter birds form a 'concentrated-migration' along the coast and they tend to leave it and cross the sea only when they reach a very sharp angle in the coast-line or at the 'end' of an island. This phenomenon may be explained through their desire to avoid water, and it may be supposed that more ?? Chaffinches than ?? cross the sea, as the former have a stronger migratory impulse (see section 2). So on the coast there is a partial separation between the sexes: a broad but diffuse flight, consisting mainly of ??, crosses the sea, and a narrow but concentrated migration, consisting mainly of ??, .follows the coast. This explanation is supported by the observations of Drost (1940), that in Heligoland ?? are always in excess during autumnal migration. 8) For the present, it remains doubtful what happens to the Chaffinches which follow the W-coast of Norway and reach the SW-part of this country. According to the principles of Van Dobben and Makkink (1934), they ought to cross the sea in a WSW direction. However Tinbergen (1941) showed, that some of the Norwegian Chaffinches pass Heligoland in the autumn; cf. also Geyr von Schweppenburg (1941) and Van Dobben (1944). 9) Indirect confirmation of the conclusions in section 7 is provided by the catches of the fowling-yards at Monster, Ockenburg and Wassenaar (fig. 5, page 6). These stations were in action from 1933 to 1947 inclusive. Four years with a very strong Chaffinch-migration occurred ,in this period, viz. 1934, 1935, 1938 and 1942. These years were characterised by a comparatively small excess of 00, A strong migration along the coast presumably means that there was a weak migration across the sea. So we may expect, that in those years proportionately many more ?? than usual migrated along the coast-instead of crossing the sea-thus causing only a small ?? surplus. On the contrary years with an abnormally weak migration (1939, 1940, 1941, 1944 and 1946) were characterised by a high ??-surplus. Moreover, at the Zuider Sea direct confirmation was obtained as we were able to make special sex-ratio counts of the migrants. An examination of the data of the other Dutch fowling-yards gives further support too. 10) In Chapter VI, page 28, the age-ratio's of the Chaffinches caught at Monster are discussed. It appears that old ?? and old ?? show the greatest contrast, in the strength of their migration-impulse and in their tendency to avoid flying over water. First-year birds behave intermediately. This conclusion is strengthened by the aberrant data of 1942. The attempt to explain the figures on age-ratio at Heligoland 1939 communicated by Drost (1940) has discovered no consistent explanation. 11) The Dutch fowlers have long known that autumns with many E and NE winds are characterised by. a weak migration of Chaffinches along the coast. This is also demonstrated in table 3, page 26, where direction frequencies of winds for the month of October are compared with the total catches at the fowling-yards at Monster, Ockenburg and Wassenaar in different years. E and NE winds in Holland are often the result of an anticyclone over Scandinavia, which also causes E winds in SW-Scandinavia and SE winds in the islands N of Scotland. It may be supposed, that these weather-conditions are favourable to a strong migration across the sea. Possible sup~ port of this view comes from various observers (ef. Hinxmann, 1894; Morris, 1895 and Eagle Clarke, 1912) who have recorded a strong migration across islands N of Scotland with a SE wind. However it is also possible, that these are birds which have been driven off their normal course by the wind. Palmer (1897) states that there is a strong immigration of Chaffinches into Ireland when there is an anticyclone over Scandinavia, while it is known that under these conditions few Chaffinches pass through Holland, and DROST (1940) mentions the same for Heligoland with E winds. Moreover, it is obvious that a period with SW winds originates a strong concentrated migration along the coast. 12) In order to obtain more data about the correlation of wind-direction and migration-direction special observations were made in 1945, 1946 and 1947 with the help of collaborators. The purpose was to verify if E and NE winds originate a diffuse large scale broad-front migration, running over the North Sea, while winds from the SW quarter on the contrary originate a concentrated migration along the coast. Besides that we intended to examine whether there would run a diffuse broad-front migration over the North Sea together with a+ concentrated migration along the coast during years with 'normal' weather and to test the assumption that the sea-migrants would be mainly ??. Since with winds from the NW quarter practically no migration occurs, only SE winds remain to be considered here. We were not able to determine the sex of the flying Chaffinches, except in one instance already mentioned before. For the rest all the questions raised have been answered in the affirmative. 13) Other phenomena also have been observed and discussed. The seaward migration with winds from the NE quarter cannot be observed properly under all circumstances owing to its great altitude. Usually the birds fly at a height of several hundred metres when passing the coast. To my opinion it is for this reason that this high migration has not been observed up till now (except by Kramer, 1931). Finally it may be mentioned that the direction of the coast exerts an important influence upon the behaviour of the Chaffinches, migrating along it. We already know that the more this direction deviates from the standard-direction the more the Chaffinches are willing to cross the sea. It became clear that even a slight turn of the coast may induce many Chaffinches to cross the sea. 14) Deelder and Tinbergen (1947) have already shown that the altitude of migrating Chaffinches is highly affected by 1) the direction and the strength of the wind and 2) the sort of terrain which is crossed. (It appeared that a greater wind-resistance will induce a lower flight, while a bare country will induce a higher flight than a less bare one). Moreover we received the impression during our observations that near the coast the birds will gain height before crossing the sea. This has been observed many times in concentrated migrations from which flocks separated themselves. In order to determine whether this same principle is at work in the case of a diffuse broad-front migration special observations were made by two persons, one standing at the coast and one nearly in the forest. It appeared that under these conditions also a considerable increase in height can be observed. So it is clear, that the height of the birds depends also upon their decision of whether or not to cross the sea. The factors affecting this decision have not previously been discussed, It may be doubted whether wind-direction itself plays a part. For in this case the fact that winds from the NE quarter cause a much stronger seaward migration than winds from the SE quarter would be inexplicable, since for a WSW migrating Chaffinch both come obliquely from behind. Perhaps some weather-conditions stimulate the migratory urge of the birds, so that strong stimulus will result in a large scale migration over the sea, while a small stimulus will cause only those birds which already had a great migratory urge to cross (??); but no confirmation of this supposition has been obtained.