Changes in sheep numbers in Britain: implications for bird populations

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 dro p or levelling in numbers. In the mid 1990s numbers of sheep generally rema ined far higher than in the mid 1970s. Wales stands out as showing an incre ase throughout the period since 1950 and as a region of exceptionally high sheep density. These broad trends conceal much local variation. There is co ncern 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 c an potentially affect bird populations through a large number of mechanisms but principal ones are likely to be: (1) loss of preferred vegetation type s, (2) alteration of food supplies and (3) alteration of predation pressure . The recent increases in sheep have almost certainly caused reduction in h abitat quality for ground-nesting birds such as grouse and waders in some r egions. However, firm evidence linking declines in upland birds to increase s in sheep is largely unavailable because (a) there is inadequate monitorin g of upland birds, (b) habitat-specific information on changes in grazing p ressure is generally lacking, (c) there has been little relevant research o n 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 ma y be a response to this recent increase in grazing pressure superimposed on a long-term reduction in carrying capacity. (C) 1999 Elsevier Science Ltd. All rights reserved.