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