Merlins breed in a circumpolar distribution extending over North America and northern Eurasia. Most populations are migratory, moving south in winter as far as northern South America, North Africa and south-central Asia. In Great Britain they breed in Scotland (including the northern and western islands) and in upland areas of northern England and Wales, with a small outlying population on the moors of north Devon/ northwest Somerset. In winter the British population moves to lower levels and becomes more widespread, augmented by migrants from Iceland.
    In Wiltshire Merlins occur as winter migrants, arriving from August onwards and remaining until April. In the partial winter survey for Birds of Wiltshire Merlins were recorded in 15 tetrads (mostly in the upland areas of the Marlborough Downs, Salisbury Plain and the downs of southwest Wiltshire) equivalent to about 38 tetrads if scaled up to cover the whole county. The comprehensive winter coverage for WTA2 recorded the species in 94 tetrads, including many in lowland areas such as the Cotswold Water Park and along the valleys of the Salisbury Avon and its tributaries.
    There have been occasional records of Merlins being seen in Wiltshire in summer, most notably in the 1970s when there were second-hand reports of pairs in 1971 (taking over an old Carrion Crow's nest and producing eggs, subsequently taken by a collecter), and in 1973, 1974 and 1975. The only accepted breeding record however dates from July 2005 when a newly fledged juvenile was seen at an undisclosed location with an adult female in close attendance.

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.