Farmland birds and arable farming, a meta-analysis

Summary

Declines in farmland bird populations have been principally attributed to the intensification of agriculture. In response, agri-environmental schemes and organic farming have been introduced with the aim of making farmland better able to support wildlife populations. These “bird-friendly” agricultural practices include using more diverse crop rotations, stopping the use of pesticides, and creating more heterogeneous landscapes and are expected to create more food resources and nesting habitats for birds. Many studies have been published that evaluate the success or failure of agricultural practices to increase bird abundance. While many studies have found that most organic farming practices are beneficial to birds, other studies have found that some organic farming practices, such as using increased tillage passes, are not beneficial to birds. We conducted a search of the literature and used a meta-analysis approach to analyze the relationship between farming practices and bird populations. We first tested whether organic agriculture is more favorable to farmland birds of Europe and North America compared to conventional agriculture. We used data from 16 experiments and six publications that fulfilled fixed criteria for inclusion in 34 the meta-analysis. We found that organic agriculture had a global positive effect on bird abundance compared to conventional agriculture. However, this effect was significant in only five out of the 16 site*year combinations tested. We also found that the effects varied with the bird species. 10 out of the 36 species tested show a significant higher abundance value in organic agriculture. When the ratio was significantly different from zero, the abundance was 1.5 to 18 times higher in organic systems in comparison to conventional systems. We also tested the effect of crop type on the territory abundance of the most studied species in the literature, the skylark (Alauda arvensis). Using data from six publications, we found two times more skylark territories in set-aside and legume fields during the breeding season than in other crop types. One of the problems we encountered in our meta-analysis was that many different bird metrics were used in the studies, and these data were often reported without standard deviations or other measures of variability. We describe problems encountered with the data found in the literature and provide recommendations to prevent these problems.