Cuckoos breed in northwest Africa and right across much of Eurasia. They winter mostly in sub-Saharan Africa with a few also in southeast Asia. Their numbers generally are in decline: Bird Atlas 2007-11 recorded a 49% decrease in the summer population in the UK between 1995 and 2010, part of a 21% decline across Europe as a whole since 1980. The reasons for this are not clear: one, unproven, theory is that while the species they parasitise have been nesting earlier as a result of warmer winters, the Cuckoos have not advanced their arrival times to match and in consequence have found fewer suitable nests available. Other possible causes include reduced availability of prey species and unknown factors affecting their wintering grounds or migration routes.
    The situation in Wiltshire is in line with the national picture. Birds of Wiltshire recorded Cuckoos present in 594 tetrads, with confirmed or probable breeding in 115; WTA2 recorded them in only 287 tetrads with breeding in 48.

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.