Effects of Dutch agri-environmental field margins and bird plots on cropland birds

Agri-environmental measures (AEMs) are the main instrument of the Common Agricultural Policy to protect farmland biodiversity, but their effectiveness is questioned. In the Netherlands, AEMs have been implemented by farmers' collectives since 2016, with the goal to improve the spatial targeting and coordination of measures, and thereby increase their effectiveness. The objective of this paper is to assess the effects of two AEMs, bird plots and field margins, on the abundance of seven cropland bird species targeted by the Dutch agri-environmental policy, and on their total abundance and species richness. The study was carried out in four farmers' collectives in the provinces of Groningen, Drenthe and Flevoland in the Netherlands. We assessed how bird abundance and species richness measured on AEM plots and control plots were related to management and landscape factors. Furthermore, we assessed whether bird abundance and species richness measured on AEM plots were related to the spatial concentration of other AEM plots in landscapes surrounding AEM plots. With regard to plot management, we found a significant positive effect of field margins on skylarks, yellowhammers, common linnets, common kestrels, total bird abundance and species richness. Bird plots had a significant positive effect on skylarks and total bird abundance. However, western yellow wagtails, northern lapwings and Eurasian oystercatchers did not benefit from the two studied measures, likely because the vegetation in the bird plots and field margins is too high and dense for these species. Landscape factors, such as landscape openness, had contrasting effects on the analysed species, due to their different habitat requirements. Hence, there are no optimal landscapes for all target species, which underlines the need for a fine scale spatial targeting of measures based on species habitat requirements. Furthermore, the spatial concentration of other AEM plots in the landscape surrounding AEM plots had a significant effect for only two species (yellowhammer and western yellow wagtail), and this effect was negative. Thus, our results suggest that - for the studied species and landscapes - the type of measures and landscape in which they are taken are more important than their spatial concentration.

Automated summary

The effectiveness of agri-environmental measures (AEMs) in protecting farmland biodiversity is questioned. This study assesses the effects of two AEMs, bird plots and field margins, on the abundance of seven cropland bird species in the Netherlands. Field margins have a positive effect on several bird species, while bird plots have a positive effect on skylarks and total bird abundance. However, some bird species do not benefit from these measures possibly due to unsuitable vegetation. Landscape factors have contrasting effects on different species, highlighting the need for fine-scale spatial targeting of measures based on species habitat requirements. The spatial concentration of other AEM plots has a negative effect for only two species, suggesting that the type of measures and landscape are more important than their spatial concentration.