Abundance of invertebrate prey for birds on organic and conventional arable farms in the Netherlands

As a result of agricultural intensification, populations of farmland birds have been in steep decline for several decades. Reduction in food abundance has been mentioned as one factor behind these declines. Extensive farm management, such as use of organic methods, is expected to provide more food for birds. In this study we compared invertebrate prey abundance for birds during the breeding season between organic and conventional arable farms. We made comparisons for three different groups of birds: (1) birds feeding on soil-living invertebrates (earthworms), (2) birds feeding on ground-dwelling invertebrates and (3) birds feeding on aerial invertebrates. Invertebrate abundance was compared between organic and conventional farms, crop and non-crop habitats, and between crop and non-crop habitats under the same farm management. On organic sites, earthworm abundance was 2–4 times higher than on conventional sites, but no differences were found between crop types. Total abundance of ground-dwelling invertebrates did not differ between organic and conventional sites, but positive effects were found for several individual taxonomic groups, such as carabid beetles and spiders. On organic farms, invertebrate abundance was higher in carrots, cereals and onions compared to other crops; on conventional farms this was true for onions. When compared with most crops, ground-dwelling invertebrate abundance was low in uncropped field margins and on ditch banks. On organic farms, aerial invertebrate abundance was approximately 70% higher than on conventional farms. On cereal fields, aerial invertebrates were especially abundan