Type of agricultural drainage modifies the value of fields for farmland birds

The study compares populations of two typical open farmland bird species, the skylark and the meadow pipit, on fields with open in-field and subsurface drainage systems in Finland. Data are derived from the national monitoring of farmland birds for 2002–2009 and are based on territory mapping. Generalised additive mixed models with repeated measures were used to investigate territory density and temporal population trends of the species on crop and grassland fields with two types of drainage. The results provided evidence for a higher habitat value of fields under open drainage (ODF) than fields with subsurface drainage (SDF) for the meadow pipit at any given year. Both species’ trends were significantly positive on ODF while trends on SDF were not significantly directional. This strongly suggests that foraging resources and nest survival of individuals breeding in ODF habitats rise above those of SDF habitats. The retention of the remaining ODF as part of the agri-environmental and relevant land-use policies could contribute to stabilisation and enhancement of populations of farmland birds for regions in Europe and elsewhere with the need for drainage.