Changes in sheep numbers in Britain: implications for bird populations

Abstract Large increases in sheep numbers occurred in most regions of Britain, both upland and lowland, between the mid 1970s and late 1980s, followed by a drop or levelling in numbers. In the mid 1990s numbers of sheep generally remained far higher than in the mid 1970s. Wales stands out as showing an increase throughout the period since 1950 and as a region of exceptionally high sheep density. These broad trends conceal much local variation. There is concern about the implications for bird populations, especially in the Welsh uplands, and the mechanisms are reviewed by which the increase in sheep may have affected birds, especially in semi-natural upland habitats. Grazing can potentially affect bird populations through a large number of mechanisms but principal ones are likely to be: (1) loss of preferred vegetation types, (2) alteration of food supplies and (3) alteration of predation pressure. The recent increases in sheep have almost certainly caused reduction in habitat quality for ground-nesting birds such as grouse and waders in some regions. However, firm evidence linking declines in upland birds to increases in sheep is largely unavailable because (a) there is inadequate monitoring of upland birds, (b) habitat-specific information on changes in grazing pressure is generally lacking, (c) there has been little relevant research on mechanisms, (d) effects of severe grazing cannot be readily isolated from other factors such as acidification and changes in predation pressure. It is suggested that the apparent collapse in Welsh upland bird populations may be a response to this recent increase in grazing pressure superimposed on a long-term reduction in carrying capacity.