Altering perceived predation risk and food availability: management prescriptions to benefit farmland birds on stubble fields.
European farmland bird populations have fallen dramatically and sympathetic management of key habitats is one crucial way to help boost these populations. Maximizing the value of habitats for foraging birds has largely focused on practical measures to increase food abundance, but energy intake, the key determinant of habitat quality, is also affected by food accessibility and perceived predation risk. In an experiment conducted in England during 2004 and 2005, we evaluated the importance of manipulating perceived predation risk and access to food on the distribution of birds on stubble fields, a key wintering habitat for many UK species and used by many species in different parts of the world. Recent evidence suggests simple reductions in vegetation height alter perceived predation risk for some species. Light cultivation, by scarification of the soil surface, could potentially alter both perceived predation risk (via changes in vegetation structure) and food availability (by opening up the soil and bringing seeds to the surface) and, thus, be a single solution to enhancing suitability of stubble fields for birds. In experiment 1, we investigated the effects of changing vegetation height (via topping) and scarification on vegetation structure, seed density and distribution of farmland birds, using a 2×2 factorial within-field design. In experiment 2, we tested the temporal effects of scarification on bird distribution using a similar within-field design. Scarified plots supported higher abundance of invertebrate feeders (thrushes, Turdus spp., for example). Plots that were scarified within the last 1-13 days were used more by invertebrate feeders and granivores (such as yellowhammer, Emberiza citrinella) than plots scarified 2-4 months ago. Both results were probably a consequence of food availability being temporarily increased by scarification, making prey more accessible. Granivorous passerines (Passer sp.) and invertebrate feeders preferred plots with shorter stubble, while the abundance of skylarks (Alavda arvensis), partridges (Perdix spp.), pigeons and meadow pipits (Anthus pratensis) was higher on plots with taller stubble. This was probably the result of differing anti-predation strategies. Prescriptions that generate fine-scale heterogeneity should benefit a range of species. Although our work was confined to stubble fields, the importance of simultaneous consideration of predation risk and access to food is likely to apply across European farmland landscapes and elsewhere, and could apply to other arable crops and potentially to grassland systems. On stubble fields specifically, topping of part of the field in the autumn could be combined with successive strip scarification treatments throughout the winter to provide optimum conditions for a range of species. This could be incorporated as a management option in agri-environment schemes, such as the English Environmental Stewardship Scheme.