Losses and gains of fallows impact farmland bird populations over three funding periods of the EU Common Agricultural Policy

1. Fallow land provides habitat for threatened and declining farmland biodiversity. Policy change under the EU’s Common Agricultural Policy (CAP) has been driving the area of fallows over the past decades and influenced trends in farmland biodiversity. 2. We analyzed the relationships between fallow land area across Germany over three CAP funding periods and species richness and abundance of farmland birds. We examined whether the strength of the relationships with fallow land area were moderated by species habitat preferences and landscape configurational complexity (edge density). We combined spatial data on fallow land area from three agricultural censuses in Germany (2007, 2010 and 2016) with country-wide farmland bird monitoring data. 3. Farmland bird species richness and the abundance of the majority of the studied farmland bird species were positively related to increases in fallows across three CAP funding periods. The relationship of fallows with bird richness was strongest at intermediate levels of edge density. There was generally little support for a moderating effect of edge density on the relations between fallows and bird abundance. 4. We conclude that the loss of fallows in the period 2007 to 2016 resulted in strong declines of farmland birds. We predict that a future increase of the proportion of fallow land to 4% of the arable land, as envisaged in the German 2023-2027 CAP strategic plans, or to 10%, as foreseen in the EU Biodiversity Strategy, will lead to increases in farmland bird species richness and abundance depending on the landscape context and species-specific habitat preferences. Policy implications Increasing the proportion of fallow land will likely be a key lever to stabilize and revert negative farmland bird population trends. An increase of fallow area in all but the least complex landscapes will boost farmland bird richness and abundance. Increasing the proportion of fallow land to 4% should bring farmland bird richness and abundance back to the levels observed in 2007 acknowledging that farmland bird populations were already severely depleted in 2007. A more ambitious expansion of fallow land towards 10% should be targeted towards areas that experienced the strongest loss of fallows and towards landscapes with intermediate levels of edge density. ### Competing Interest Statement The authors have declared no competing interest.