Contrasting patterns of change in the distribution and abundance of farmland birds in relation to farming system in lowland Britain

Patterns of change in distribution (presence/absence) and abundance since t he late 1960s were examined in 20 species of farmland bird in southern Brit ain in predominantly arable (eastern), predominantly mixed (central) and pr edominantly grassland (western) regions. Comparisons were made between chan ges in distribution and in abundance to determine whether these measures sh ow similar relationships to environmental change. Local extinctions of sele cted species and reductions in species richness were significantly greater in the predominantly grassland region. Decreases in abundance were greatest in seven species in the predominantly arable region, two in the mixed regi on and nine in the grassland region. Changes in distribution and abundance showed consistent patterns in three species, turtle dove Streptopelia turtu r L., yellow wagtail Motacilla flava L. and reed bunting Emberiza schoenicl us L. In another four species, grey partridge Perdix perdix L., lapwing Van ellus vanellus L., tree sparrow Passer montanus L. and corn bunting Miliari a calandra L., decreases in abundance were greatest in the arable region, y et declines in distribution were lowest. For other individual species, chan ges in distribution were too small to draw any conclusions in relation to f arm type. We suggest that modern grassland systems are suboptimal habitats compared to arable or mixed agricultural land for many farmland species tha t occur at relatively low density in the more western, grass-dominated regi on. Declines in abundance are therefore more likely to lead to local extinc tion in these areas than in eastern areas where abundance is higher. Howeve r, the role of changes in grassland management on bird populations requires further research. It is suggested that conclusions drawn from changes in d istribution alone, in the absence of supporting data on changes in abundanc e, may be misleading where the aim is to assess how large-scale spatial dyn amics of populations relate to environmental change.