Continent-wide gradients in open-habitat insectivorous bird declines track spatial patterns in agricultural intensity across Europe

Aim To investigate spatial gradients in population trends of European birds in relation to their association with farmland, dietary dependence on insects, and the intensity of agricultural practices. Location Europe. Time period 2001-2012. Major taxa studied Birds. Methods We collected population trends for 197 species in 32 European countries. For the same time period, we used agricultural variables (annual yields of five major crops and the per-hectare application of pesticides and fertilizers) to express the agricultural intensity level (mean values of variables over the years) and intensification rate (slopes of agricultural variables over the years) in each country. We employed spatial generalized additive mixed models accounting for the effects of 11 species' traits and phylogeny to test for spatial gradients in bird population trends in relation to species' associations with farmland and their diet dependence on insects and the interaction of these traits with agricultural intensity levels and intensification rates. Results Open-habitat insectivores showed the strongest spatial gradient in population trends, from insignificant trends in south-eastern Europe to steep declines in north-western Europe. Insectivorous species breeding in semi-open habitats showed very similar but weaker spatial gradient in trends. More negative bird trends were related to higher mean crop yields and fertilizer amounts across countries, whereas the temporal trends in yields and fertilizer amounts, and the mean and temporal trend in the amount of pesticides, were unrelated to bird declines or showed the opposite patterns. Main conclusions Our results indicate that high agricultural intensity levels are stronger drivers of spatial gradients in population declines of insectivorous farmland birds than intensification rates. Therefore, approaches to the conservation of farmland birds may differ regionally: introducing more management for birds and insects into highly intensive agriculture in north-western Europe and preventing further intensification in south-eastern Europe.