Red-breasted Mergansers have a circum-polar distribution across northern Eurasia and northern North America. They nest by shores and rivers of the arctic and boreal zones and winter mostly along temperate coasts within or to the south of their summer range. In Britain their breeding range was confined to northwest Scotland until the mid-19th century, since when they have expanded southwards into northwest England and Wales.
    In Wiltshire there were only five records, all of single birds, during the 19th century and no records at all in the 20th century until the unusually harsh winter of 1947 when a group of three were seen on the Salisbury Avon near Charlton-All-Saints. Thereafter they began to appear more regularly in winter, with a further 26 records by the end of the century, mostly at the Cotswold Water Park, including in every winter from 1990-91 to 1999-2000. This was followed by a period of scarcity with only two records, both of singletons, in the next seven winters, before regular winter appearances resumed from winter 2008-09 onwards.


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.