Ground-breeding farmland birds are disproportionately negatively affected by the impact of agricultural intensification and mostly still dramatically declining in Europe. One main reason for this decline is the lack of suitable nesting sites. While some off-field measures showed positive effects on bird productivity, we are currently not achieving sufficient conservation with these, so that supplementing them with in-field measures seems inevitable. Over the past years, several in-field measures have been developed aiming at providing suitable nesting sites in crop fields by manipulating the crop architecture, i.e., the density and/or height of the crops. However, we currently lack an overview on what has been tested and resulted in stabilizing population of ground-breeding farmland birds. In this systematic review, we provide a qualitative assessment of current knowledge and knowledge gaps on such in-field measures and their effects on European crop-breeding farmland birds. In doing so, we accounted for specific birds’ requirements on their breeding habitat. We found only very few studies on the effectiveness of crop architecture-related in-field measures on ground-breeding farmland birds. Knowledge gaps exist for effects on individual species in general, their reproduction (rather than population density), the influence of landscape and local contexts on the effectiveness, and the optimal spatial arrangement of measures to maximize their efficiency. This shows an urgent need for more research on a holistic scale. However, the few existing studies suggest, that there is a high potential for crop architecture-related in-field measures to promote ground-breeding farmland birds, and thus bring them back as ‘agricultural by-products’.
