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Length 15‚Äď16¬∑5 cm (6-6¬Ĺ in), weight 11‚Äď26 g
A wagtail with many plumage variations but all show uniform yellow breast and vent, most of the variation is to the head. A relatively short tail allows for differentiation of the grey headed varieties from the similar Grey Wagtail. Males are brighter than females, and juveniles duller still, often lacking any strong yellow tones.
M. f. thunbergi Photo by Macswede Fulufj√§llet National Park, Sweden, July 2008
The races which have grey heads may look superficially like Grey Wagtails but they usually have greener or browner backs. The shorter tails and different calls of the Yellow Wagtail is always conclusive. Eastern Yellow Wagtail, formerly considered conspecific, is virtually indistinguishable with similar variation in head patterns; genetic tests are generally required for identification to be confirmed.
The main differences between the various races are described in the Taxonomy section. These differences are obvious in spring males, while autumn males, females, and juveniles can be very difficult or impossible to assign to subspecies. This Birdforum thread discusses the identification of females.
North African populations are resident or only partial migrants, but most birds migrate to winter south of the Sahara Desert in Africa, and east to India. Small numbers may also occur in winter in southern Spain. Present on breeding grounds April-May until August-October, rarely later.
Eastern Yellow Wagtail from eastern Asia was formerly included in this species; genetic tests however show that western and eastern birds are respectively more closely to western and eastern populations of Citrine Wagtail than they are to each other.
Breeds in Great Britain (north to southernmost Scotland), with a few pairs on the adjacent continent from northern France to the Netherlands, and had bred rarely in Ireland; migrates through northwest Africa. Yellow head with greenish tone to ear-coverts and crown. Note that M.f. flava and M.f. flavissima form a narrow hybrid zone in northern France. Birds from this zone vary in appearance, but one type, which resembles nominate M. f. flava except that the blue tones to the head are paler and more mauve and the white of the head is more extensive, particularly on the throat, ear-coverts, and supercilium, is colloquially referred to as Channel Wagtail.
M.f. cinereocapilla (Ashy-headed Wagtail) breeds in Italy, Sardinia, Sicily and parts of east Adriatic coast, migrates through north Africa. Vagrant to Britain, though some records may refer to aberrant M.f. thunbergi. Grey crown darker than flava, supercilium faint of absent; throat white.
M.f. iberiae (Iberian Yellow Wagtail) breeds Iberian Peninsula and southern France, northwest Africa, migrates through North Africa. Grey crown darker than flava, narrow white supercilium, mainly or entirely behind eye. []
M.f. thunbergi (Grey-headed Wagtail) breeds in central and northern Scandinavia and Russia, migrates through north Africa and Middle East. Dark grey crown, black ear-coverts, no supercilium, chin and throat yellow (occasionally white, and then easily misidentified as M.f. cinereocapilla).
M.f. feldegg (Black-headed Wagtail) breeds Balkans, Turkey and the Near East, migrates through Algeria, Libya and Egypt, Arabia and Middle East. Vagrant to Britain (<10 records). Crown and ear-coverts black, no supercilium, underparts entirely yellow. Sometimes considered to merit full species rank. []
M.f. pygmaea (Egyptian Wagtail) resident in Egypt. Small size, greenish-grey crown, blackish-green ear-coverts, white chin, dark blotches on breast-sides and little or no supercilium.
M.f. melanogrisea breeds in south Russia and migrates south-east through Iran. Head black, olive back paler than feldegg, paler yellow below, chin white. 
M.f. lutea (Khirgiz Steppe Wagtail), breeds in southern Russia from Lower Volga eastwards, migrates through Arabia and adjacent Middle East. Head yellow with olive ear-coverts, upperparts olive-green, entirely yellow below.
M.f. beema (Sykes's Wagtail) breeds in Russia, migrates through Arabia the Middle East to India. Ash-grey crown and ear-coverts, broad white supercilium.
M.f. leucocephala (White-headed Wagtail) breeds in Mongolia, rare visitor to the Middle East, head white, ear-coverts very pale grey.
A couple of other forms are sometimes given names that look similar to subspecies but without formal recognition as such. These include "M. f. dombrowski" and "M. f. superciliaris".