Factors shaping insectivorous farmland bird abundance in intensively cultivated arable fields: Insights through the former Iron Curtain

The decline of farmland bird populations is a well-documented phenomenon that has primarily been attributed to agricultural intensification. However, the specific mechanisms that make intensively used farmland unsuitable for breeding birds have not yet been satisfactorily elucidated, particularly for insectivorous ground-nesting species. To bridge this knowledge gap, we studied the breeding abundance, invertebrate food supply and habitat characteristics of the Yellow Wagtail (Motacilla flava) in arable fields in the lowlands of south-western Slovakia. This region is characterized by the largest field sizes in the European Union, with conditions that are typical for high-intensity farmland areas in parts of Central and Eastern Europe where farmland underwent collectivization. Since Central and Eastern European farmland is considered one of the strongholds of the Yellow Wagtail in Europe, we tested the capacity of winter wheat, rapeseed and maize to support its local population. The Yellow Wagtail abundance was best predicted by habitat characteristics: birds chose fields without woody plants in their surroundings and with suitable crop height and coverage, while the effect of invertebrate food supply was insignificant. Focussing on individual crops, we found that the birds did not prefer the maize and rapeseed fields despite these having a richer food supply, likely because these crops had an unsuitable vegetation structure. The low overall abundance of the studied species indicates that farmland consisting of large fields creates unsuitable conditions for breeding. Therefore, agricultural policy instruments leading to a reduction of field size or the creation of herbaceous field edges should be supported.