A regional-scale study of associations between farmland birds and linear woody networks of hedgerows and trees

Farmland birds have declined throughout Europe over recent decades. Many farmland songbirds are associated with linear woody features on field boundaries, such as hedgerows and tree lines. Previous studies have assessed songbird associations with specific hedgerow and tree characteristics, and their landscape context, but large-scale assessments have been limited by difficulties in mapping linear woody networks over large extents, particularly their height structure. We used a high-resolution lidar model of the complete network of linear woody features in southwest England (9424 km(2)), summarising linear feature lengths by height class. Associations were tested between heights of linear woody features and the abundance of 22 farmland birds, using bird survey data summarised for 1446 near-contiguous tetrads, and a weighted version of the phi coefficient of association. Land cover mapping defined tetrads as grassland, mixed or arable farmland. Results showed that the linear woody network was dominated by features corresponding to managed hedgerows (1.5-2.9 m tall, 42-47% of the network by land cover type), followed by tree lines (>= 6.0 m, 28-35 %). All songbird species had statistically significant, but weak, associations with combinations of land cover and height class of linear woody features, although land cover appeared to be the dominant factor. Many species showed more positive associations with linear woody features on arable farmland than on grassland, particularly for taller hedgerows and tree lines. The results suggest that land-use diversification may benefit some farmland songbirds, such as introducing pockets of arable farming in landscapes dominated by intensively managed grassland. Diverse heights in the linear woody network, incorporating tall hedgerows and trees, would also likely benefit a range of songbird species. The study demonstrates the significant potential of lidar in characterising the structure of linear woody features at the landscape scale, facilitating detailed analyses of wildlife habitat associations and landscape ecology.