Yellow Wagtails breed from North Africa and western Europe right across Eurasia to western Alaska. They winter in Africa (mostly south of the Sahara), the Indian subcontinent  and southeast Asia down to north Australia. Not surprisingly, given their wide distribution, there are many subspecies (Handbook of the Birds of the World - Lynx Edicions - lists 17). Most of those seen in Britain belong to the race Motacilla flava flavissima which are also found in coastal areas of mainland Europe from Norway to France. The nominate race, M. flava (" Blue-Headed Yellow Wagtail"), the main continental race, also occurs increasingly in Britain, interbreeding with M.f.flavissima. There has been at least one British record of a pure bred pair successfully nesting.
    In Britain they are found mainly in a few scattered areas on the east coast of southern Scotland and then down the eastern side of England with the densest concentrations in East Yorkshire, Lincolnshire, the Fens, the Norfolk Broads and the coastal marshes of Essex and Kent. Elsewhere there are smaller populations on the Welsh borders and across central England. This follows a significant range contraction in the last half-century - Bird Atlas 2007-2011 recorded a 31% reduction since the 1968-72 Breeding Atlas, mainly from northwest England, the West Midlands, parts of East Anglia and the southern counties from Devon across to Sussex. This range reduction has been accompanied by a 72% population reduction mainly attributed to habitat loss caused by changes in agricultural methods.
    In Wiltshire, Yellow Wagtails were reportedly common in the 19th century, but only "fairly common" in the first half of the 20th century and "far from common" in the 1950s. Birds of Wiltshire recorded them in 127 tetrads, almost all to the north of Salisbury Plain, with breeding reported from 64 tetrads, WTA2 recorded them in 108 tetrads, still almost entirely in the north half of the county, with breeding in only 43

.References
The following references are used throughout these species accounts, in the abbreviated form given in quotation marks:
1968-72 Breeding Atlas” – Sharrack, J.T.R. 1976:  The Atlas of Breeding Birds in Britain and Ireland. T. & A. Poyser
1981-84 Winter Atlas” – Lack, P.C. 1986:  The Atlas of Wintering Birds in Britain and Ireland. T. & A. Poyser
1988-91 Breeding Atlas” – Gibbons, D.W., Reid, J.B. & Chapman, R.A. 1993: The New Atlas of Breeding Birds in Britain and Ireland 1988-91. T. & A. Poyser
Birds of Wiltshire” – Ferguson-Lees, I.J. et al. 2007 : Birds of Wiltshire, published by the tetrad atlas group of the Wiltshire Ornithological Society after mapping fieldwork 1995-2000. Wiltshire Ornithological Society.
Bird Atlas 2007-2011” – Balmer, D.E., Gillings, S., Caffrey, B.J., Swann, R.L., Downie, I.S. and Fuller, R.J. 2013: Bird Atlas 2007-2011: the Breeding and Wintering Birds of Britain and Ireland
WTA2” – ("Wiltshire Tetrad Atlas 2 ") the present electronic publication, bringing together the Wiltshire data from “Birds of Wiltshire” and “Bird Atlas 2007-11”, together with data from further fieldwork carried out in 2011 and 2012.
"Hobby" - the annual bird report of the Wiltshire Ornithological Society.