Seeded ryegrass fills the late winter 'hungry gap' but fails to enhance local population size of seed-eating farmland birds

摘要

The abundance of many seed-eating birds in lowland temperate agricultural landscapes is thought to be limited by the availability of seed-rich habitats that can sustain birds through the winter. Late winter is a period of food shortage for seed-eating farmland birds, when established conservation measures such as sown mixes of seed-bearing plants are often exhausted of seed. Previous work has shown that ryegrass (Lolium) can provide abundant late-winter seed when protected from defoliation from mid-summer. Here we report an experimental study that assessed the utility to seed-eating farmland birds of unharvested cereal crops and seeded ryegrass in a grassland-dominated agricultural landscape that lacked alternative seed-rich habitats. Although both interventions produced abundant autumn seed, cereal plots were depleted by mid-January whereas ryegrass plots retained significant seed and sustained greater bird usage into late winter. Seed yield was higher on Italian L. multiflorum rather than perennial ryegrass L. perenne plots, and on plots that were protected from further defoliation from late May (perennial) or late June (Italian). Despite intervention plots accounting for 60% and 90% of all winter foraging observations of yellowhammer (Emberiza citrinella) and reed bunting (E. schoeniclus) respectively, and yellowhammer body condition being positively related to the proportion of ryegrass in the diet, provision of cereal crops with or without seeded ryegrass had no effect on local population size of either species. Seeded ryegrass constitutes a practical and widely available means of feeding granivorous birds during late winter but filling that 'hungry gap' may not always have the predicted impacts on farmland bird populations.