Linking population trends of farmland birds to changes in agricultural land use and farming systems

Agriculture has been, and still is, strongly shaping landscapes across Europe. A growing body of evidence suggests that agricultural intensification and land-use change are among the leading causes of population declines in common farmland birds (e.g., Berg et al., 2015). Policy reform has been considered a major driver of agricultural land use change. In recent decades, both the EU's Common Agricultural Policy (CAP) and bioenergy policy induced changes in crop and grassland management, spatial distribution of different crop types and crop specialization at farm and landscape level, thereby affecting farmland bird populations. Farmland birds have been acknowledged as good indicators of High Nature Value farmlands (HNVf; Morelli et al., 2014).For example, research conducted in France has shown that specialist bird species were significantly more abundant in HNVf than in non-HNVf areas (Doxa et al., 2010). Although relationships between population trends of farmland birds and changes in agricultural land use are well established in the literature, analyses are often hampered by the lack of suitable high-resolution, spatially explicit data on agricultural land use.
Here, we present results from a recent study where we linked temporal and spatial changes in agricultural land use at both the landscape and regional scale to population trends of farmland birds (Jerrentrup et al., 2017). Monitoring data of the Common Breeding Bird Survey of Lower Saxony, Germany, and high-resolution IACS (Integrated Administration and Control System) data on agricultural land use from 2005 to 2012 were used to calculate the temporal trends of farmland birds and agricultural variables at the landscape scale (1 km2). Agricultural variables comprised the area covered by maize, set-aside, permanent grassland as well as crop diversity. The number of territories were analyzed for all farmland birds together as well as for two subgroups (‘field nesters’ and ‘non-field nesters’). Additionally, we tested whether population trends of farmland birds differed between agricultural regions, characterized by different land-use change dynamics. Our results revealed that trends of farmland birds and non-field nesters were negatively related to increasing maize cultivation at the landscape scale. We further found that population trends of field nesters reacted region-specifically and performed worst in an agricultural region that was characterized by the strongest increase of maize and decrease of crop diversity, indicating a negative effect of broad-scale landscape homogenization. In a next step, we will analyze whether population trends of farmland birds are related to large-scale farming system patterns and farming system dynamics. This research is being carried out as part of the ongoing FARSYD project. Results and implications from this research will be discussed in respect to spatial targeting of conservation actions to halt or even reverse negative farmland bird population trends.