The conservation benefits of fallows for farmland birds depend on local farmland structure in Central Europe

Fallows are a cornerstone of farmland biodiversity conservation in Europe, yet their effectiveness varies regionally. In this study, we examined how local structural features of farmland, specifically field size and land-use heterogeneity, influence bird responses to fallows in Central Europe. We surveyed 408 study sites in agricultural landscapes of Slovakia and Austria, recording 2947 individuals of 51 bird species. We then quantified overall bird abundance, species richness, and the abundances of functional guilds defined by nesting strategy, trophic level, and habitat preference. Both bird abundance and diversity increased with increasing fallow area and land-use heterogeneity and decreased with increasing mean field size. However, the benefits of fallows were context dependent: the positive interaction between fallow area and mean field size indicated that the negative effect of large fields weakened as fallow area increased. Among bird guilds, ground-nesting species, open-habitat birds, and insectivores showed the same pattern, with fallows partly offsetting the negative effects of large fields. Fallows also had positive effects on the abundance of arboreal and hole-nesting birds, granivores, and insectivores, while land-use heterogeneity positively affected several guilds, including open-habitat species and granivores. These findings highlight the importance of tailoring fallow-based conservation to the local farmland context. In post-socialist regions dominated by large fields, fallows can partly compensate for the negative effects of field consolidation on bird communities and thus provide a pragmatic pathway for biodiversity recovery without requiring fundamental changes to land ownership or parcel structure. At the same time, land-use heterogeneity remained beneficial, supporting higher bird abundance and diversity in farmland.