MIGRATIONAL DRIFT and the YELLOW WAGTAIL COMPLEX by KENNETH WILLIAMSON (Fair Isle Bird Observatory)

MIGRATIONAL DRIFT AND THE YELLOW WAGTAIL COMPLEX BY KENNETH WILLIAMSON (Fair Isle Bird Observatory) INTRODUCTION FROM time to time grey-headed yellow wagtails which appear to be identical with Sykes's Wagtail (Motacilla flava beema) of western Siberia have been observed or captured in the British Isles. The frequency of their appearance has puzzled ornitholo gists and in the absence of a more satisfactory explanation of their origin they are now generally regarded as aberrant examples of the typical race, the Blue-headed Wagtail (Motacilla f. flava) of middle Europe, or even of the British Yellow Wagtail flavissima. The current view is reflected in the decision of the late British Ornithologists' Union List Sub-Committee to consider such speci mens as belonging to the race flava (Ibis, vol. 92, pp. 140-141) so that in consequence beema has failed to find a place in the Check list of birds of Great Britain and Ireland (1952). The fashion of regarding such birds as variants or mutants of flava, whilst it owes its origin to the work of Stresemann (1926), received a great impetus as a result of the policy of the Editors of The Handbook of British Birds (1941, vol 1, p. 215) and of the journal British Birds in the treatment of such records from 1935 onwards (see editorial note, antea, vol. xxix, p. 200). So far as this country is concerned, the seal of authority was placed on this theory by the writings of Harrison (1945), Tucker (1949) and Smith (1950). Stresemann's view is that mutants arise in the various yellow wagtail populations showing extraordinary con vergences, so that birds very like each other may be found in widely-separated breeding-areas. That this is a common situa tion throughout the range of these birds is well-known, and Gladkov (1954), presumably writing from Russian experience, says: "There is to be noted a considerable overlapping of the areas of the separate subspecies and numerous occurrences in the depth of an area of one subspecies of specimens not distin guishable from another." Harrison sought support for Strese mann's view by plotting the cline of crown-colour against the east- to-west distribution of the birds, but there is no simple cline affect ing all the races—as will be seen from the taxonomic survey below —and characters other than crown-colour are also involved in the variation. Tucker concluded his study with the remark: "The races of the yellow wagtail present some evolutionary problems of great interest and there is reason to believe that they are actually in a state of more than ordinary genetical instability." Smith (p. 113) expressed the same opinion. The view that this is 382 VOL. XLVIII] YELLOW WAGTAIL COMPLEX 383 the case has gained, and now enjoys, a wide popularity; but there are a number of inconsistencies in the theory when the group is studied as a whole which leave one unsatisfied as to its validity, and invite a re-examination of the problem. Before proceeding with this examination it is necessary to have a full picture of the nature of the variation within this extensive group, and of the summer and winter distribution of the different forms. RELATIONSHIP AMONG THE YELLOW WAGTAILS The yellow wagtails have been reviewed in recent years by Johansen (1946), who recognizes too many subspecies; by Grant and Mackworth-Praed (1952), who recognize too many species; and by Meinertzhagen (1954), whose analysis is more conserva tive but avoids discussion of the possible evolutionary trends within this complex assemblage of birds. Whereas Johansen, using quadrinomials, admits 22 forms, Meinertzhagen recognizes only 14 subspecies. Grant and Mackworth-Praed entirely ignore the new biological approach to taxonomy which has sprung from the work of Mayr (1942) and others, and raise no fewer than 7 species embracing 21 forms in all. I cannot agree with Meinertzhagen that in this complex array we find "the reverse of normal variation in that it is not clinal" (p. 148), for there are regions in which primary intergradation is clearly discernable, although the most complicated situations are found in what appear to be "hybrid zones" of secondary inter gradation. It is in these latter regions that, as he truly remarks, the variation is not always constant "owing to gene leakage or even gene-flow, which must be expected in a species which sinks its racial identity in its winter quarters". This last, as I shall attempt to show, has an important bearing on the question of variation as a result of the displacement of individuals on spring migration, but it is of much less importance, of course, than the intermingling of stocks which takes place over comparatively large areas within the total breeding-range. Johansen offers a hypothesis of the development and spread of the various forms, regarding the yellow-headed populations as the most primitive, and the dark and blue-headed forms as derivatives of inter-glacial and post-glacial origin whose subsequent spread has confined the yellow-headed birds to somewhat restricted ranges. According to his theory, the yellow-headed stock was ousted from continental Europe by the southern ashy-headed series (iberiae, cinereocapilla) and the blue-headed flava, the latter also expanding eastwards into Siberia and culminating in Outer Mongolia in the White-headed Wagtail leucocephala. With this I would agree, except that I believe the ashy-headed series repre sents a secondary invasion of later date into the range of flava, springing from black-headed stock. The west Asiatic yellow- headed lutea, in Johansen's view, gave rise to the black-headed 384 BRITISH BIRDS [VOL. XLVIII complex, whilst the east Siberian green-headed taivana produced the northern and eastern grey-headed populations, Gladkov departs from this view in considering that the Eastern Blue-headed Wagtail simillima belongs to the flava-beema series rather than to the group of northern populations; but the mor phological likeness of flava and simillima is not necessarily a proof of close relationship, and the fact that simillima intergrades closely with its neighbours to the north and west establishes its identity with the grey-headed series, as I have attempted to illus trate below. A completely satisfactory taxonomic study of these wagtails is made the more difficult because there are large tracts, particularly in Siberia, where very little is known concerning the breeding pop ulations. There are in the British Museum (Natural History) com paratively few birds taken as breeders in this vast region, and one's study of the eastern populations is necessarily based to a large extent on material collected on spring migration, and in winter-quarters from India eastwards to China and New Guinea. This collection, however, is very extensive. Within so widely- distributed and varied an assemblage so poorly represented by known breeders, so-called "splitting" serves to obscure rather than emphasize the evolutionary processes which appear to be at work, and the plethora of names bestowed on eastern forms (often on the basis of one or two specimens) only serves to con fuse the issue. I have found it best to follow Meinertzhagen in taking a conservative view, where possible using the concept of the cline as a means of reducing the number of acceptable forms. Accepting the principle that the genus should indicate relation ship, and the species distinctness, I see no reason for separating the Yellow from the Pied and White Wagtails in a different genus, Budytes Cuvier; on the other hand, I do regard it as a practical and desirable step to keep the yellow-headed group specifically distinct from the dark and blue-headed birds. The reasons for this will be discussed in detail later. Meanwhile, an examination of the large collection of specimens in the British Museum has led me to the following conclusions: A. There are three widely separated populations of yellow- heads, completely isolated from each other as breeders, and each sympatrie over the whole or a part of its range with one or more of the dark or blue-headed forms. They are located on the maritime fringe of Europe, the Kirghiz Steppes region, and far eastern Siberia. Hybridization with neighbouring forms occurs, but appears to be very rare. B. There are three major groupings of dark or blue-headed birds with a much wider continental distribution. Within each of these groups there is a considerable geographical variation, due in part to a clinal change in characters, and in part to the phenomenon of secondary intergradation where VOL. XLVinj YELLOW WAGTAIL COMPLEX 385 the borders of two of the major groups overlap. These occupy (i) the western and central Palaearctic, (2) the northern and eastern Palaearctic, extending to Alaska, and (3) the south- central Palaearctic, extending to the Mediterranean region. GROUP A—YELLOW-HEADED POPULATIONS Motacilla lutea lutea (Gmelin). Typically most of the crown and forehead are yellow, but in many the crown is greenish-brown, with the yellow confined to the forehead. In a small minor- ority the forehead also is greenish-brown. Complete yellow eye-stripe. Ear- coverts yellowish-green. Mantle bright greenish-brown but often darker, and in this as well as the head-characters many are quite indistinguishable from M. I. flavissima. Volga River eastwards across the Kirghiz Steppes to the head-waters of the Yenesei, reaching the shores of the Caspian and Aral Seas. Winters mainly in East Africa. NOTE: Rarely, interspecific hybrids with Af. /. heema are found (" Budytes perconfusus "); less rarely, hybrids with M. f. superciliaris (" Motacilla xan- thophrys"). Motacilla lutea flavissima Blyth. Typically, the forehead only yellow, the crown and mantle greenish-brown, the latter not usually so bright as in the nominate race.

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