Why do open-farmland specialist birds prefer small fields? The evaluation of mechanisms using a cross-border study

Abstract

European farmland serves as a crucial habitat for many organisms, but the transition from extensive to intensive agriculture has led to a loss of biodiversity that also concerned farmland birds. Intensification has various aspects including landscape homogenisation, whereby large field blocks are created in once heterogeneous landscapes that traditionally consisted of small fields. However, some farmland specialists may indeed benefit from homogeneous landscapes, as they contain a greater share of preferred open habitats. To elucidate this paradox, we focused on a farmland specialist ground-nesting insectivorous bird, the Eurasian Skylark (Alauda arvensis), unravelling its abundance in different landscape structures. The contrasting landscape structures are found in a lowland region along the border between two countries: Slovakia with large fields and Austria with small fields. In this "natural experiment", bird censuses, food supply assessments, and vegetation structure analyses were carried out during the Skylark breeding season. Austria showed significantly higher local abundance of Skylark compared to Slovakia, which persisted whole season. The Skylark numbers were positively correlated with an increasing number of crop types. This positive effect of crop number was likely associated with higher food availability at sites with more crops, highlighting the role of diverse crop compositions in fostering a stable food supply for farmland birds. These results indicate that even open-habitat farmland specialists benefit from the landscape heterogeneity provided by small fields composed of different crops. Slovakia-Austria differences highlight the need for context-specific conservation that should also be a concern for agri-environmental measures within the EU’s Common Agricultural Policy.