THE MIGRATION OF EUROPEAN THRUSHES: A COMPARATIVE STUDY BASED ON RINGING RECOVERIES

SUMMARY1 A full summary and discussion of each of the six species considered in this paper will be found at the end of its own section.Song Thrush. Most Song Thrushes from the northern and eastern parts of the European range migrate for the winter, while the western populations contain a high proportion of permanent residents. Most of the migrants move roughly S.W. in autumn, although many birds from the Low Countries, in particular Holland, go west to Great Britain. Many Scandinavian birds on their relatively long migration to Spain “leap‐frog” those which breed in Southern England. The migrants that winter furthest east in Europe tend to come from the more eastern breeding populations, and those furthest west from the more western populations. But birds from a very wide region of Europe may be found together in S.W. France and Iberia in winter. Dutch migrants moving west cross Norwegian birds moving S.W. at an angle of about 65°. Drift occurs during migration over the North Sea and possibly over the Baltic. Apparently individuals which have moved from their breeding area to a wintering ground do not subsequently move on during the winter, but a few individuals have been found in different countries in successive winters. Blackbird. Except in the far north of the breeding range, some Blackbirds in all European populations remain resident throughout the winter. The direction of autumn migration is between west and south. In certain northern and western populations, some birds are permanent residents, some migrate westward or north of west and others S.W. or even south. It is possible that some Swedish birds that move west and north of west into Norway may later move S.W. to the British Isles. There is no evidence of “leap‐frog” migration although the wide spread of migration directions from any one area suggests that birds from different populations cross each other's tracks in autumn. Reversed autumn movement evidently occurs on occasions in the North Sea. Individual birds may migrate in one year but not in another and also may migrate at very different dates in different years; and some movement occurs throughout the winter. Redwing. The Redwing is a total migrant throughout most of its range. There is no separation of the more eastern and western breeding populations during the winter and most of the European birds migrate between W.S.W. and S.S.W. in autumn. Migrants may be drifted off course when over the North Sea and possibly also over the Baltic. Individuals have been found in successive winters in widely separated regions, and there is a suggestion that Redwings may occur in greater numbers in particular regions in some years than in others; it is evidently nomadic in winter. Fieldfare. Fieldfares leave the more northern parts of the breeding range for the winter but the southern part of the breeding range overlaps with the winter range. There is an autumn movement between S.W. and N.W. of Swedish and Finnish birds into Norway, which is possibly followed by a movement into the British Isles during the winter. There is no evidence of drift while over the sea. The Fieldfare, like the Redwing, is nomadic during the winter and individuals have been found in widely separated countries, both during the same winter and in successive winters. Ring Ouzel. The Ring Ouzel is a total migrant throughout most of its European range; relatively few have been recovered. The direction of migration in autumn is apparently about south, and it is probable that drift of Norwegian birds occurs over the North Sea. Mistle Thrush. Mistle Thrushes leave the more northern parts of the breeding range for the winter, although many are resident further south. The direction of migration in autumn is mainly S.W., although some British birds move south to the Continent. Birds from Holland and Belgium do not move very far, and there is a tendency for the Swedish birds to “leap‐frog” them and spend the winter further south. The distribution of recoveries throughout Europe is much the same in all the winter months.2 The Redwing and Fieldfare are almost totally synhiemic while the Song Thrush and Blackbird are partially allohiemic. Some populations of one species of thrush overlap with another species during only part of the year and thus the competition that they are likely to encounter is probably different in different seasons. 3 No migratory Redwings and only a few migratory Fieldfares come into competition with permanent residents of their own species during the winter, whereas many migratory Song Thrushes, Blackbirds, Mistle Thrushes and a few Fieldfares spend the winter in areas where others of their kind are resident. Song Thrushes, Blackbirds and probably also Mistle Thrushes from different breeding populations tend to concentrate in different regions in winter and presumably are adapted to conditions in the particular regions where they winter. On the other hand, Fieldfares and Redwings tend to be nomadic, and individuals from each population may spend the winter in almost any part of the total winter range. 4 There is evidence that Song Thrushes, Redwings and Ring Ouzels may be able to redetermine their directions after being drifted off course while over the North Sea, and in the case of Song Thrushes and Redwings, also the Baltic Sea. Only for the Blackbird is there any ringing evidence for reversed movements. 5 Probably most wintering areas in Europe which are potentially suitable for Turdus spp. are exploited. This is presumably because overcrowding in the optimal areas must tend to affect their advantages in relation to less suitable but less crowded areas. In cold winter weather both interspecific and intraspecific competition are presumably severe, and local residents may be at an advantage with greater knowledge of local food sources. 6 Certain populations of Song Thrushes, Blackbirds and Fieldfares contain a proportion of birds that move westward in autumn, some that go in a more S.W. or S.S.W. direction, and in some cases some that remain resident. The Swedish and Finnish Fieldfares probably move on southwards, but it is unlikely that other westward moving birds later go southwards. The proportion of birds remaining resident, and of those going westward or S.W. varies from population to population and from species to species. 7 The difference in winter conditions from year to year is probably responsible for maintaining the migration “polymorphism” of partial migrants; residents may be favoured in one year and migrants in another. Some hard‐weather movements may be composed of birds which do not normally migrate, but have been stimulated to migrate, although late, by cold weather in a particular year. The “polymorphism” in the direction of migration in certain populations is probably also maintained by variation in winter conditions.