Risk assessment of pesticide seed treatment for farmland birds using refined field data

Summary Due to reductions in winter food resources, newly sown cereal seeds have become a key component of many bird species' diets, but these seeds are often treated with pesticides that may cause toxic effects. To complete an appropriate risk assessment, data on treated seed toxicity need to be combined with information about the risk of exposure of birds in the field and the factors that modulate such exposure. We studied the abundance of pesticide‐treated seeds available for birds in the field, the pesticides and their concentrations in treated seeds, and the bird species observed in the field that were feeding on these pesticide‐treated seeds. The exposure of red‐legged partridge to treated winter cereal seeds was characterized through the analysis of crop and gizzard contents of hunted individuals ( n = 189). Moreover, we measured the contribution of cereal seeds in the autumn–winter diet of partridges in order to assess the potential risk of exposure to pesticide‐treated seeds. Density of treated seeds on the soil surface after sowing (11·3 ± 1·2 seeds m −2 in the centre of field and 43·4 ± 5·5 seeds m −2 in the headlands) was enough to provide, in an area between 6 and 50 m 2 , doses of six active ingredients above those indicating acute (i.e. a dose capable of killing 50% of individuals of a sensitive species) and / or chronic (no observed effect level) toxicity. Up to 30 bird species were observed consuming treated cereal seeds in recently sown fields. Corn bunting was identified as an appropriate focal passerine species for the risk assessment of pesticide‐treated seeds. We found that treated seeds were an important route of pesticide ingestion for red‐legged partridge; pesticide residues (six fungicides and two insecticides) were found in 32·3% of crops and gizzards. Cereal seeds represented more than half (53·4 ± 4·3%) of total biomass consumed by partridges from October to February. Synthesis and applications . The field exposure data combined with previous studies about the toxicity to partridges of using pesticide‐treated seeds point to an unacceptable risk of this practice to farmland birds. Our results suggest that the prophylactic use of pesticide‐coated seeds should be avoided, with the approval of this treatment considered on a case‐by‐case basis and accompanied with specific measures to minimize risks of adverse effects on avian communities.