Taxonomy, habitat and conservation status explain the diet composition of insectivorous birds in Europe

Populations of European birds and insects are declining, with consequences for trophic interactions. Insectivorous birds are particularly vulnerable to losses of arthropods, yet the factors shaping their diet composition remain insufficiently understood. We conducted a quantitative review of the arthropod diets of 94 European insectivorous bird species and assessed how diet richness and composition relate to taxonomy, developmental stage, climate, conservation status, habitat, migration status, sampling intensity, and methodological approach. Taxonomy, habitat and conservation status (both population trend over time and current status) were the primary predictors of diet richness and composition. Species of population concern in Europe relied more on Coleoptera and Orthoptera prey, while species with declining population trends and species in agricultural, artificial and semi-natural habitats showed strong dependence on Coleoptera prey. Species in forests and of global population concern had a higher percentage of Arachnida prey, while birds in inland water habitats relied on Diptera prey. Anthropogenic activities such as the use of insecticides and changes in land use in agricultural and forest habitats negatively impact the abundance and diversity of arthropods, including Coleoptera and Arachnida. These declines will cascade up the food chain, ultimately diminishing both common and threatened bird species by limiting their prey. Species of global conservation concern may be particularly at risk, as they exhibit the lowest prey taxonomic richness. It is therefore crucial that future legislation and policy interventions consider these indirect impacts on bird diets, as these vulnerable species are at high risk of further losses in arthropod prey.