Species-specific aquatic habitat use predicts pesticide residues in feces of insectivorous birds and bats

Water bodies located in the agricultural landscape may face a substantial contamination by agrochemicals due to modern agricultural practice. Many insect species inhabiting these water bodies have a complex life-cycle with an aquatic phase as larvae and a terrestrial phase as winged adults when they serve as prey for many aerial insectivores, such as bats and birds. Thus, they may act as vectors, transferring pesticides from the water bodies into the terrestrial food webs. This transfer of pesticides from water to land via emerging insects is still poorly studied. This study investigated the contamination of feces from various insectivorous organisms with different foraging mode and aquatic habitat use, quantified by movement analysis. In total, we detected 16 current-use pesticide residues, two legacy compounds and six metabolites in the feces of three species which had a high selection strength towards aquatic habitats: barn swallow (Hirundo rustica), Western house martin (Delichon urbicum), and the common noctule bat (Nyctalus noctula). In contrast, no substances were detected in feces of European starling (Sturnus vulgaris), a species that uses rather terrestrial habitats and is known to feed on terrestrial insects. The fungicide prochloraz, used to combat fungal grain diseases, was the substance with the highest detection rate of 69% of all samples. Five of the substances detected (i.e., bixafen, diflufenican, dinoterb, prochloraz, simazine) are classified as critical in terms of their potential for bioconcentration and ten out of 18 substances are classified to be of concern for mammal short term dietary uptake. While the direct ecotoxicological effects on the organisms are unclear, our study is one of the first ones to infer the contamination pathway via emerging insects and to highlight that species-specific use of aquatic habitats increases pesticide exposure risk in insect-eating birds and bats in farmland.