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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 wagtails are always conclusive. There are many races shown in the gallery.
North African populations and those from the Caucasus are resident or only partial migrants but most birds winter in sub-Saharan Africa. Small numbers may also occur in winter in southern Spain. Present on breeding grounds April-May until August-October, rarely later.
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 Blue-headed Wagtail (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. [Link to Channel Wagtail image]
M.f. beema, Sykes's Wagtail, breeds in Russia, migrates through Arabia and the Middle East. 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.
M.f. cinereocapilla, Ashy-headed Wagtail, breeds Italy, Sardinia, Sicily and parts of east Adriatic coast, migrates through North Africa. Vagrant to Britain. Grey crown darker than flava, supercilium faint of absent.
M.f. iberiae, Spanish Wagtail, breeds Iberian Peninsula and southern France, North-West Africa, migrates through North Africa. Grey crown darker than flava, narrow white supercilium, mainly or entirely behind eye. []
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. thunbergi, Grey-headed Wagtail, breeds in northern Scandinavia and Russia, migrates through North Africa and Middle East. Dark grey crown, black ear-coverts, no supercilium, chin and throat yellow.
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. 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. flavissima, Yellow Wagtail, breeds Britain and coastal north-western Europe, migrates through North-West Africa. Crown and ear-coverts olive-green, forehead and supercilium yellow, underparts entirely yellow. The bird pictured is of the subspecies flavissima.
A couple of other forms are sometimes given names that look similar to subspecies but without formal recognition as such. These include "dombrowski" and "superciliaris".
Clements, J. F., T. S. Schulenberg, M. J. Iliff, D. Roberson, T. A. Fredericks, B. L. Sullivan, and C. L. Wood. 2014. The eBird/Clements checklist of birds of the world: Version 6.9., with updates to August 2014. Downloaded from http://www.birds.cornell.edu/clementschecklist/download/