Farmland birds and late winter food: does seed supply fail to meet demand

摘要

Poor overwinter survival caused by low food availability represents the most likely mechanism behind the decline, and failure to recover, of many farmland passerine populations. Management to enhance food resources is now in place via agri-environment schemes (AESs) such as Environmental Stewardship in England, but the measures concerned probably fail to provide a significant seed resource in late winter. Landscape-scale experimental feeding over six winters was used to investigate the variation in demand for supplementary food from farmland birds between November and March. Quadratic modelling of the temporal variation in (semi-)weekly bird-use data from experimental feeding stations showed some evidence of curvilinear changes through the winter for all of the 11 granivorous or omnivorous species considered. For generalists and human-associated granivores, peak use of supplementary food was found in January or earlier, suggesting that harsh weather is the principal influence. However, peaks occurred in February or later for Yellowhammer, Reed Bunting, Chaffinch and Dunnock, suggesting an effect of ambient food availability. These species depend on seed in farmland in winter and three of them have declined, probably because of reduced survival (rather than poor breeding success). It is likely that current farming practices, including existing agri-environment prescriptions, do not provide enough food in late winter for such species and that the loss of set-aside will exacerbate this problem. Revisions of management guidelines and crop content for AES stubble and sacrificial crop prescriptions are needed to ensure late winter seed availability for farmland birds.