There are multiple strands of evidence, both correlational and experimental, that point to changes in agricultural practices as the primary, but not the only, cause of farmland bird declines in Europe (for full review and supporting literature see: Wilson et al., 2009). One strand of evidence comes from detailed studies of individual bird species. Some carried out over many years have pinpointed agricultural change, sometimes with other factors, as a major cause of population decline and range contraction. Farmland birds in this category include Grey Partridge Perdix perdix, Stone Curlew Burhinus oedicnemus, Corncrake Crex crex, Lapwing Vanellus vanellus, Turtle Dove Streptopelia turtur, Skylark Alauda arvensis, Whinchat Saxicola rubetra, Cirl Bunting Emberiza cirlus, and Corn Bunting Miliaria calandra, to name a few where several different organizations have carried out the research. In each case, specific changes in agricultural operations or practice have been identified as the main cause of decline (Wilson et al., 2009).
To take an example, Hole et al. (2002) worked on the House Sparrow Passer domesticus, whose populations have declined sharply in Western Europe in recent decades. They used a combination of field experimentation, genetic analysis, and demographic data to show that a reduction in winter food supply caused by agricultural intensification was probably the principal explanation for the widespread local extinctions of rural House Sparrow populations in southern England.
In a wider study, Chamberlain et al. (2000) examined agriculture changes through time (1962-95) quantitatively for 31 variables representing crop areas, livestock numbers, fertilizer application, grass production, and pesticide use in England and Wales. The period 1970-88 saw the most intensification, characterized by increases in the area of oilseed rape, autumn-sown cereals, and the use of pesticides and inorganic fertilizers. Spring-sown cereals, bare fallow, and root crops declined. Indices of relative population change between 1962 and 1996 were determined for 29 bird species over the same area. The ordinations of agricultural change and bird population change matched each other with a time lag in birds’ response. The authors conclude that large shifts in agricultural management are a plausible explanation for the decline in farmland bird populations.
Donald et al. (2001) showed that population declines and range contractions of farmland birds 1970-1990 were significantly greater in countries with more intensive agriculture and significantly higher in the European Union than in former communist countries. Cereal yield alone explained over 30% of the variation in population trends. The results strongly suggest that recent agriculture trends have had deleterious and measurable effects on bird populations on a continental scale. Donald et al. (2006) went off to show that of the 58 species classed by an independent assessment as being primarily birds of farmland, 41 showed negative overall mean trends across Europe in 1990-2000, 19 of them significant. There was a significant negative correlation between mean national trends of all farmland species and national agricultural intensity indices. This relationship strengthened when the 19 declining species were considered alone and were not apparent when only non-declining species were considered. Population trends of terrestrial non-farmland bird species over the same period were unrelated to agricultural intensity. The results support earlier evidence that population trends of farmland birds across Europe can be predicted from gross national agricultural statistics.
Many other peer-reviewed studies from the UK (Aebischer et al., 2000; Vickery et al., 2004) and across Europe have explored links between birds and agriculture and alternative drivers of population trends. Although the individual bird species, bird communities, and farming systems vary across Europe, change in agricultural systems is the predominant and recurrent factor implicated in population declines of breeding birds (see more info on online BOU conference proceedings, Lowland farmland birds III).
A final compelling strand of evidence comes from cases where limiting factors on farmland have been identified and practical remedies put in place to bring about the recovery of bird populations. Individual bird species in this category include stone curlew, corncrake, skylark, and cirl bunting in the UK. The RSPB´s Hope Farm in Cambridgeshire shows how wildlife-friendly measures can bring about the rapid recovery of farmland birds and, in this case, increase farm profits at the same time.