Environmental factors affecting the densities of owls in Polish farmland during 1980-2005

During the last decades, farmland habitats in central European countries have changed significantly, seriously affecting populations of many farmland bird species. We compiled available published data on densities of three owl species, Athene noctua, Asio otus and Strix aluco collected in the Polish farmland. All results of censuses based on the playback method conducted between 1980-2005 were included in the analysis. The proportions of grassland, fields, built-up land and forest at each studied plot were estimated and used as predictors in additive models. Proportions of main land use types, extracted with the principal component analysis, explained much of the variation found in owl densities, although some of the relationships were nonlinear. In general, owl densities were found to be affected positively by a high percentage of grasslands and built-up land, and negatively by the amount of fields and forests. Little owl densities showed a significant negative trend over the study period. It seems that high prey availability is an important factor accounting for the positive relationship between grassland proportion and owl density. The significant decrease in grassland areas and increase in forest coverage that were recently recorded in Poland may thus negatively affect populations of the three owl species studied here.