566 Land-Use Issues Modeled pre-agricultural land cover (a) Current land cover (b) Cropland Wooded tundra Temp deciduous forest Pasture/grazing land Boreal forest Warm mixed forest Scrubland Savanna Ice Cool conifer forest Grassland/steppe Tropical woodland Tundra Temp mixed forest Desert Tropical forest Figure 14 Preagricultural land cover as modeled by the BIOME model (Prentice et al., 1992) and current land cover as modeled by the IMAGE 2.4 model (Bouwman et al., 2006) and fertilizers (2) Removal of hedges to increase field size (3) Plowing in late summer after harvest rather than waiting until the following spring (4) Draining wetlands to increase arable and pasture lands (5) Conversion of mixed farms that produced a variety of animal and plant products to monoculture cereal farms (6) Earlier harvest dates (7) Intensive grassland management to increase grass growth (8) Increased stocking density of sheep in hill country These changes in agricultural practices cause declines in bird distribution and density for three main reasons (Newton, 1998): (1) chemicals and early harvests reduce breeding success and survivorship of birds; (2) breeding and foraging habitat is reduced by hedge removal, land drainage, late summer plowing, intensive grassland management, and increased grazing; and (3) habitat diversity and resulting bird community diversity are reduced when mixed farms are converted to monocultures The results of these changes have been dramatic It is suspected that intensification has affected 42% of bird species of conservation concern in Europe Nearly 90% of the 26 farmland birds in Britain decreased their range of occurrence from 1970 to 1990, and those species using pastures were negatively affected by increasing grazing intensity Specific aspects of agricultural intensification affect particular species, but on the whole this form of habitat degradation threatens a significant portion of the common, as well as the rare, European avifauna