Effects of the proportion and spatial arrangement of un‐cropped land on breeding bird abundance in arable rotations

Abstract

Summary1. The response of bird abundance to the proportional availability of un‐cropped land (i.e. land that could be cultivated, such as fallows, grass–flower or wild bird areas) is under‐studied but of considerable significance for managing declining populations on farmland in western Europe.2. In this study, bird abundance was examined at a scale consistent with many national monitoring schemes. Birds were counted on 28 farm sites of c. 100 ha, representing cereal‐based and organic rotations. Sites were surveyed in summer, from 2007 to 2010, to assess the effect of the percentage cover and spatial arrangement of un‐cropped land on bird abundance, with data analysed at the whole‐farm (not patch) scale.3. Un‐cropped land area had significant effects on the abundance of key species (those with a high dependency on farmland) when controlling for effects of semi‐natural habitats and management. On farms with <3% of their total area as un‐cropped land, the densities of birds were significantly lower than on farms with >10% area of un‐cropped land.4. Positive, significant effects of the percentage area of un‐cropped land were detected for lapwing, skylark, linnet and yellowhammer and for all highly farmland‐dependent species combined. The relationship between un‐cropped land and bird abundance was stronger on conventional compared with organic farms, suggesting a greater importance of un‐cropped land on conventional farms.5. Un‐cropped land patch arrangement was significant for skylark and linnet abundance but generally weak amongst species compared with the availability of un‐cropped land. Skylarks were positively associated with a larger relative edge effect amongst patches, whereas linnets were more associated with larger blocks of contiguous habitat.6. Synthesis and applications. This study provides important evidence for a proportionate effect of habitat provision on farmland bird abundance. The relative area of un‐cropped land had the strongest effect on bird abundance. Sites with <3% (and, to a lesser extent, <5%) un‐cropped land were highly under‐populated. A two‐fold increase in the area of un‐cropped land was associated with an average 16–53% increase in the relative abundance of key species, which has implications for the contribution of un‐cropped areas towards population stabilization amongst farmland birds in Europe.