A review of the abundance and diversity of invertebrate and plant foods of granivorous birds in northern Europe in relation to agricultural change

Abstract

This study reviews the diet of 26 granivorous bird species of European temperate farmland, and evidence for the effects of agricultural practices on their invertebrate and plant foods, in order to assess whether the latter could have contributed to recent widespread population declines of farmland birds. Cereal grain and seeds of Polygonum (knotgrasses and persicarias), Stellaria (chickweeds) and Chenopodium (goosefoots) are important for the bird species considered. Seeds and green material of Asteraceae, Fabaceae, and Brassicaceae are also widely used, the seeds of Asteraceae particularly by cardueline finches. Declining bird species are not associated with particular plant foods, but reductions in overall diversity and abundance of food plants have taken place in intensively managed arable land. Grassland intensification has reduced floral diversity, and the quantity and diversity of grass and broad-leaved seed produced, but some plant species of value to granivorous birds benefit from high-nitrogen environments and may increase in availability (e.g., Stellaria – chickweeds). During the breeding season, Acrididae (grasshoppers), Symphyta (sawflies), Araneae (spiders), Chrysomelidae (leaf-beetles), Curculionidae (weevils), Lepidoptera (butterflies and moths and their larvae), Aphididae (aphids) and Tipulidae (crane-flies and their larvae) are important foods. The first four are associated with the diet of declining bird species, and all are sensitive to insecticide applications. Herbicide applications, increasing specialisation of farmland, loss of uncultivated field margin habitats, and ploughing are also associated with generally detrimental effects on invertebrate groups in arable habitats. In intensively managed grassland, loss of grasshoppers, ants, spiders and lepidopteran larvae removes an important source of food for younger chicks of a wide range of species. Some phytophagous taxa and predators, however, may be more abundant due to the greater standing biomass of plant material. Overall, intensification and specialisation of arable and grassland systems is likely to have reduced the availability of key invertebrate and seed foods for birds. However, there is also evidence that reversal of intensification, especially in arable systems can result in rapid recovery of these resources. In intensively managed farmland, uncultivated field margins, hedgerows, ditches and road verges are likely to become increasingly important sources of seed and invertebrate food for birds.