The effect of the spatial distribution of winter seed food resources on their use by farmland birds

1. Agri-environment measures providing winter seed are central to current management activities aiming to reverse granivorous farmland bird declines. Previous research has considered the effectiveness of particular agri-environment options, but the influence of their distribution in the landscape, an important factor in determining cost-effectiveness, has received less attention. We used a large-scale field experiment in eastern England, featuring 10 replicates of a spatial arrangement of seven artificial feeding stations separated by different distances, to investigate bird movements between discrete winter food resources. 2. Feeding sites were established and bird use monitored at least weekly over two winters (November-March). Habitat type in the areas surrounding the feeding sites was also recorded. Two measures of feeding site use were analysed as functions of the distance from the nearest alternative feeding site, controlling for ambient resource availability, for 11 passerines (including chaffinch Fringilla coelebs, reed bunting Emberiza schoeniclus and yellowhammer Emberiza citrinella and non-granivores such as blue tit Parus caeruleus). 3. Feeding site use varied significantly with separation distance for nine species. For most, both maximum counts and bird minutes were higher at more isolated sites, but the opposite was true for yellowhammer and reed bunting. Most relationships incorporated significant thresholds at separations of around 500 m, with low site use below the threshold and high above it (or vice versa). 4. The results suggested that species such as chaffinch and blue tit made disproportionately more use of food at isolated sites than at clumped ones, but that reed buntings and yellowhammers used food in proportion to its availability, moving freely between more clumped patches. In both cases, birds tended to share resources separated by 500 m or less. At greater separations, resources tended to be used by discrete groups of birds. Radio-tracking and colour-ringing work produced corroborative evidence. 5. Synthesis and applications. The response of birds to the experimental food resource distribution implies that creating resource patches more than 1 km apart should be most cost-effective, a recommendation that can inform agri-environment scheme planning directly. This distance may represent a minimum, however, because birds might locate standing crops more readily than inconspicuous artificial food patches.