Effect of habitat structure and crop diversity on common and threatened birds breeding in semi-natural field margins

Field margins are inherent elements of European agricultural landscapes, thought to be crucial for maintaining a high biodiversity level. The interactions between the structure of field margins and biota have been recognized, yet the understanding of the importance of various components of the margins and adjacent areas is still incomplete. The aim of our study was therefore to determine the relative importance of structural features of the margins and diversity of adjacent crops for birds breeding in field margins. The study was carried out in 70 field margins covered with semi-natural vegetation, situated in SW Poland. Both the number of species and bird density were most strongly positively related to the development of tree and shrub layers, while presence of ditches and percentage of reed cover (positive effects) had much less importance. Analyses conducted at the species level revealed the complexity of bird-habitat interactions resulting from various requirements of individual species. Overall, for 22 abundant species and for the threatened species, the development of the shrub layer turned out to be a particularly important feature of field margins positively associated with occurrence of e.g. Emberiza citrinella, Linaria cannabina and Streptopelia turtur. Other significant positive effects had the development of the tree layer (e.g. Turdus merula and Turdus philomelos), presence of ditch (e.g. Acrocephalus palustris, Sylvia communis), and number of gaps in woody vegetation (e.g. Lanius collurio). The diversity of adjacent crops had positive effects only to some threatened species (e.g. L. collurio and E. calandra). Abundances of common and threatened birds were not correlated, which reflects different habitat demands of these two groups. Because of diverse bird-habitat relationships, maintaining a variety of field margins (in particular the shrubby ones), should be accepted as a rule in biodiversity-oriented management of agricultural landscapes.