Improving the value of field margins as foraging habitat for farmland birds.
Uncropped field margins are important foraging habitats on farmland for many declining bird species and are a key component of Agri-Environment Schemes across Europe. Maximizing the value of foraging habitats requires detailed knowledge of the factors influencing habitat selection and food availability. We firstly conducted an observational study of foraging habitat selection by breeding yellowhammers Emberiza citrinella L. on lowland mixed farmland, in relation to underlying invertebrate and vegetation characteristics. There was a clear seasonal shift in the relative use of field margins and cereal crops. Margins were used less than crops in late summer, despite supporting higher invertebrate abundance relative to cereals. Seasonal increases in vegetation height were most marked in margins, suggesting the seasonal decline in margin use may reflect reduced food accessibility. In the second phase of the study, field margins were cut experimentally to create short, open patches within taller margin swards. The use of cut patches by foraging yellowhammers increased significantly between early and late summer, and patches were used more frequently with increasing height of adjacent uncut margin. These findings strongly support the theory that tall vegetation reduces margin accessibility in late summer. Synthesis and applications. Provision of invertebrate-rich field margins is a core component of Agri-Environment Schemes, but current prescriptions may result in them having limited value in late summer. More effective management, such as more frequent cutting, may be required to maximize the benefits for foraging birds by creating short, open vegetation patches. Measures to increase accessibility to invertebrates on farmland are likely to benefit a range of bird species across a variety of crop types.