Friday, April 12, 2013

Yellow Wagtail - a complex species


Yellow Wagtail Motacilla flava are without doubt a complex species and cause of much confusion when determining race. Here in the southern most reaches of the Iberian Peninsula we are able to witness several races, both during spring and autumn migration. The fact we observe so many races does not lessen the complicated process of correctly identifying the geographical races and of course the added complexity of hybrids. Certainly I find these wonderfully colourful and attractive birds a mix of the curious and a compelling subject for study.

Smallest, most compact of west Palearctic wagtails, with form and silhouette more like a large pipit. Plumage of both adult and (most) 1st-winter birds basically yellow below and on patterned edges of wing-feathers. Adult breeding ♂♂ of the many races differ in pattern of green, bluish grey and black crowns, variable yellow or white supercilia, and variable white or yellow chins and throats. ♀♀ and immatures may show hints of ♂♂’s pattern but many racially are inseparable. In winter even ♂♂ may lose racial characters. Identification also complicated by frequent occurrence of racial hybrids and unusually pale birds.

Yellow Wagtail is a wagtail with complex systematics and morphology, subject to confusion with other species and doubts in racial identifications. The British and extreme west European race flavissima most distinctive, with green and yellow head, but blue- and grey-headed forms are less easy to separate. It is unlikely to be mistaken for other wagtail species except for the risk of confusion with Citrine Wagtail in both adult and sub-adult plumage.


Most populations are migratory, wintering in the Afrotropics, India, and south-east Asia. The Egyptian race is largely resident, and some parts of the breeding range in north-west Africa and southern Spain occupied throughout the winter, with the possibility that some individuals are resident.

Several factors make this a particularly well documented migrant: large populations; conspicuous (mostly diurnal) movement; use of huge communal roosts, both on migration and in winter, facilitating ringing; assumption by ♂♂ of racially distinct breeding plumage shortly before spring migration. On the other hand, confusion can arise through racial intermediates and disjunct pattern of geographical variation. Precise wintering areas of the various races are not well established, but largely lie between south-east and south-west of their respective breeding areas.

Movement is broad-front in both spring and autumn, with numerous sightings of migrants at sea in all areas. Autumn passage in Switzerland has been noted as early as late July, but main passage begins during the second half of August and peaks through September, usually to end abruptly in early October, though individuals have been noted still passing in the first third of November. At the Strait of Gibraltar, passage extends from early August to early November peaking mid-September. Arrives in Afrotropics in late September, further south in October. Movement north in spring, after build-up of fat just south of Sahara, is also on a broad front, starting in March and extending to early May. ♂♂ reach breeding grounds before ♀♀; arrivals are from late March in south, west, and much of central Europe, from mid-April in Moscow area, and from early May or early June in Lapland.


Many records occur of birds resembling a particular race well outside that race’s normal range, but some (at least) of these are part of the species’ normal variability and do not necessarily indicate vagrancy. Birds showing the characters of several races have been recorded in Britain, for example, mainly in spring and sometimes well outside their normal range: continental nominate flava occurs regularly and has bred occasionally.

Geographical variation is both marked and complex; mainly involving colour of ♂ breeding plumage, less so other plumages; also (but scarcely) size. 2 complexes recognized, often considered separate species: (1) lutea complex (lutea, flavissima, extralimital taivana); (2) flava complex (all other races). Every member of the lutea complex overlaps partly or fully with flava complex, apparently with limited interbreeding, though some gene-flow between the complexes occurs, and members of lutea complex are perhaps not closely related to each other as measurements and structure of each are closer to the neighbouring member of flava complex than to other members of lutea complex. The flava complex is subdivided into 3 groups: grey-headed thunbergi group in north, blue-headed nominate flava group in mainly temperate latitudes, and black-headed feldegg group in south, from Balkan countries to eastern Kazakhstan. Each of these groups are sometimes considered a separate species also, but as they are connected by hybridization zones of variable width, are better combined into a single highly polytypic species. Breeding ♂♂ of all races readily separable in colour, apart from birds of unstable local populations in hybridization zones.


The following races breed in west Palearctic: lutea (basin of lower Volga north to Kazan’ and Perm’, eastwards); flavissima (Britain and locally on continental coast of north-west Europe); nominate flava (most of Europe); cinereocapilla (Italy and north-west Yugoslavia); iberiae (south-west France, Iberia, and north-west Africa); pygmaea (Egypt); thunbergi (Norway east to northern Russia); feldegg (Balkans east to Caspian); beema (lower Volga).

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4 comments:

Oretani Wildlife said...

Great article Pete. It's always exciting here in La Mancha to see one of the different types passing through.
Regards,
Mike
Oretani Wildlife.

Peter Jones said...

Thanks Mike, pleased you liked the mini article. They are an exciting migrant in all their many forms. Be good to know which sub species you see this spring? Peter

eileeninmd said...

Great post! And a beautiful bird. I love your photos.

Peter Jones said...

Thanks Eileen and much appreciate your very kind comment... Peter