Richard B. Bradbury

The second Silent Spring

The drive to squeeze ever more food from the land has sent Europes farmland wildlife into a precipitous decline. How can agricultural policy be reformed so that we have fewer grain mountains and more skylarks?

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.

Habitat characteristics affecting use of lowland agricultural grassland by birds in winter

Almost a third of the bird species designated as Species of European Conservation Concern exploit agricultural grasslands, yet few studies have focused on their use as foraging habitats for birds. This study investigated the influence of variation in sward structure, grassland management and landscape variables on the use of 77 grass fields by 14 field-feeding bird species wintering on lowland mixed farmland in southern England. Multiple logistic regression was used to model the proportion of bird-count visits in which each species was encountered as a function of the recorded habitat variables for each field. Variation in sward height and density were associated with frequency of occurrence for 12 bird species and larger areas of bare earth and occurrence of winter grazing by stock animals were correlated with greater frequency of occurrence by 11 bird species. Two rapidly declining species, skylark (Alauda arvensis) and yellowhammer (Emberiza citrinella), were recorded more frequently on fields with higher numbers of seeding grasses. We suggest that mosaics of fields managed as short-term leys and permanent pastures with low-intensity cattle grazing over the autumn and winter would provide the combination of heterogeneous sward structure, areas of bare earth and presence of some seeding plants necessary to maximise the range of bird species able to use a given area of agriculturally improved grassland throughout the winter.

Protected areas facilitate species’ range expansions

The benefits of protected areas (PAs) for biodiversity have been questioned in the context of climate change because PAs are static, whereas the distributions of species are dynamic. Current PAs may, however, continue to be important if they provide suitable locations for species to colonize at their leading-edge range boundaries, thereby enabling spread into new regions. Here, we present an empirical assessment of the role of PAs as targets for colonization during recent range expansions. Records from intensive surveys revealed that seven bird and butterfly species have colonized PAs 4.2 (median) times more frequently than expected from the availability of PAs in the landscapes colonized. Records of an additional 256 invertebrate species with less-intensive surveys supported these findings and showed that 98% of species are disproportionately associated with PAs in newly colonized parts of their ranges. Although colonizing species favor PAs in general, species vary greatly in their reliance on PAs, reflecting differences in the dependence of individual species on particular habitats and other conditions that are available only in PAs. These findings highlight the importance of current PAs for facilitating range expansions and show that a small subset of the landscape receives a high proportion of colonizations by range-expanding species.

Foraging habitat selection by yellowhammers (Emberiza citrinella) nesting in agriculturally contrasting regions in lowland England

Fine-scale habitat use by yellowhammers (Emberiza citrinella) searching for food to provision nestlings was compared in three agriculturally contrasting regions of lowland England. Log-linear modelling was used to test for significant overall variation in habitat use and significant differences in relative use between pairs of habitats. Yellowhammers provisioned nestlings non-randomly with respect to habitat availability. Habitat selection was found to be generally consistent across the three regions; field boundary structures and barley crops were the most selected foraging habitats, while intensively-managed grass fields were avoided relative to virtually all other habitats. The observed patterns of habitat selection are likely to result from an interaction of food abundance and varying accessibility to food, mediated by sward structure. The geographical generality of the results allows reasons to be suggested for the recent rapid population decline of the species and general conservation recommendations to be made with respect to lowland farmland.

Widespread local house-sparrow extinctions

House-sparrow populations have declined sharply in Western Europe in recent decades, but the reasons for this decline have yet to be identified, despite intense public interest in the matter. Here we use a combination of field experimentation, genetic analysis and demographic data to show that a reduction in winter food supply caused by agricultural intensification is probably the principal explanation for the widespread local extinctions of rural house-sparrow populations in southern England. We show that farmland populations exhibit fine-level genetic structuring and that some populations are unable to sustain themselves (sinks), whereas others act as sources.

Improving bird population models using airborne remote sensing.

This work was undertaken to devise a technique to measure the height of crops in farmland fields through remote sensing. Crop height is a useful spatial variable which, when measured by ground-based manual survey, has proven to be an important predictor of bird species population. An airborne scanning laser system capable of measuring topography to a height accuracy of better than 10 cm was used to acquire height data over a region of farmland near Oxford, UK. A scanning laser was pulsed from an aircraft at the ground, measuring the time between transmission and receipt of the last significant return signal. Differential Geographical Positioning System (GPS) and onboard attitude sensors were combined with these delay times to construct a set of spot heights through the region. Crop height was also measured from the ground. Pulses were returned from mainly within the crop, rather than predominantly the canopy or ground, so an algorithm to measure the variation of the returned height, after detrending the heights for topography, was developed. A simple relationship was found between the mean crop height and the standard deviation of detrended return heights within a field. This relationship could be used to derive crop height from Light Detection and Ranging (LiDAR) data with an accuracy better than 10cm.

Wildlife-friendly farming benefits rare birds, bees and plants

Agricultural intensification is a leading cause of global biodiversity loss, especially for threatened and near-threatened species. One widely implemented response is ‘wildlife-friendly farming’, involving the close integration of conservation and extensive farming practices within agricultural landscapes. However, the putative benefits from this controversial policy are currently either unknown or thought unlikely to extend to rare and declining species. Here, we show that new, evidence-based approaches to habitat creation on intensively managed farmland in England can achieve large increases in plant, bee and bird species. In particular, we found that habitat enhancement methods designed to provide the requirements of sensitive target biota consistently increased the richness and abundance of both rare and common species, with 10-fold to greater than 100-fold more rare species per sample area than generalized conventional conservation measures. Furthermore, targeting landscapes of high species richness amplified beneficial effects on the least mobile taxa: plants and bees. Our results provide the first unequivocal support for a national wildlife-friendly farming policy and suggest that this approach should be implemented much more extensively to address global biodiversity loss. However, to be effective, these conservation measures must be evidence-based, and developed using sound knowledge of the ecological requirements of key species.

Predicting population responses to resource management

The use of limited funding to optimal effect in conservation depends on the costs and benefits of different approaches to predicting population responses to management action. Resource management based on predictions from various classes of population-level model has had success in increasing populations of at-risk species. However, such models might not meet policy demand for more accurate predictions of the extent of population recovery. This is because, by necessity, they often extrapolate from known data to predict the effect of new environmental conditions. Behaviourally structured population models could deliver greater prediction accuracy because they can truly predict population-level responses to novel situations. If such an approach can be applied to new situations, it could play an increasing role in the prediction of population recovery following management.

Use of field margins by foraging yellowhammers Emberiza citrinella

Abstract Some agri-environment schemes promote the creation and management of a variety of non-crop habitats on farmland in the UK, yet there has been relatively little monitoring to assess how species, particularly birds, use these habitats. The present study deals with a declining UK farmland bird species, yellowhammer Emberiza citrinella , and considers to what extent grass margins of arable fields are used as a foraging habitat when feeding nestlings. Studies were carried out in lowland mixed farmland in southern England. Grass margins and other non-crop field boundary habitats, such as hedgerows and ditches, were selected relative to cropped areas by yellowhammers. No significant difference was found between use of cut and uncut grass margins. Studies have shown that grass margins support high densities of invertebrates and their provision at the edge of arable fields would benefit yellowhammers during the breeding season both as habitat for prey and as nesting habitat. During the breeding season from May to August, management should create cut and uncut grass margins in close proximity to each other. This could be achieved by cutting only the outer edge of the grass margin, maintaining cover next to the hedgerow. Cut areas would provide easier access to food resources for birds and prevent weed encroachment to the crop, whilst adjacent uncut areas would maintain invertebrate sources and provide nesting cover for yellowhammers.