Farmland bird diversity in contrasting agricultural landscapes of southwestern Poland

Breeding bird communities were investigated in three contrasting agricultural landscapes of SW Poland to assess differences in abundance and determine the importance of field margins for bird populations. Counts were conducted in twelve 50-ha plots differing in landscape structure (density of all, permanent and shrubby field margins, and occurrence of high vegetation). Maps of bird distributions were used to analyze the associations of birds with four habitats: cropped fields, permanent fallows, field margins, mid-field woodlots. Ordination and classification techniques were applied to check between-plot differences in community composition. All landscapes supported high densities of total communities and farmland specialists. Species richness and bird densities were significantly related to field margin aggregations and arranged along a decreasing gradient of landscape heterogeneity: shrubby mosaic, open mosaic, open plain plots. Between-plot differences suggest that the occurrence of non-cropped habitats is more important for bird abundance than diversification of land-use. PCA and cluster analysis identified differences in species composition between the landscape types; and RDA revealed the significance of the shrubby and permanent margins for the community structure. The aggregation of shrubby margins seems to be a particularly useful predictor of bird abundance and overall biodiversity. Most species and breeding pairs were preferentially associated with margins, despite the habitat covering merely 4% of the area. The data confirmed that landscape heterogeneity with mosaic seminatural field margins is responsible for the persistence of internationally important bird populations in Poland. In such complex arable systems, preventing landscape simplification is the most effective method of biodiversity protection.