Insectivore diet and abundance determine the contribution of bird species to services and disservices in an agricultural ecosystem

Abstract Insectivorous birds provide an essential ecosystem service in agriculture by feeding on arthropods considered pests but can also consume arthropods considered to be natural enemies of such pests. Therefore, depending on the dietary composition of the birds, intraguild predation could outweigh pest control services. This study addressed the agronomic quality, defined as the trade-off between ecosystem services (pest control) and disservices (intraguild predation), of 26 insectivorous bird species in culturally and economically important cider apple orchards in northern Spain. We used DNA-metabarcoding techniques to analyze bird’ diets. First, we examined whether the trophic position of bird species can be inferred from functional traits related to foraging and movement behavior and from the degree of insectivory in their diets. Then, we tested whether bird abundance and trophic position influenced agronomic quality, based on the proportion of arthropod crop pests and natural enemies in their diet. Finally, we combined bird abundance, insectivory and agronomic quality to infer the potential contribution of each bird species to pest control. Bird trophic position was positively related to the degree of insectivory, with this effect being modulated by traits related to body size. The trophic position of birds was inversely related to their agronomic quality. Nevertheless, the agronomic quality only slightly affected the differential contribution of bird species to the whole assemblage effect. Overall, we found the potential of insectivorous bird species to control pests can be estimated based on their trophic position. Yet, in agroecosystems with uneven avian species abundance, the potential contribution of bird species to pest-control services may be driven by their quantitative contribution rather than by the bird’s per capita, qualitative effects. Finally, our results suggest that rare insectivorous birds may have a redundant role in pest control, due to the overwhelming functional dominance of common species.