Neonicotinoid insecticides blamed for the loss of bee colonies are also a key factor in the decline in farmland birds, possibly by killing the insects they feed upon, according to new research published this Wednesday in the peer-reviewed journal “Nature”. Biologists at Radboud University Nijmegen and the Sovon Centre for Field Ornithology in the Netherlands analysed data on local population trends for 15 bird species, nine of them exclusively insectivore. The scientists then compared this datasets to measurements of imidacloprid concentrations in surface water, the most widely used insecticide in agricultural systems. They found that local bird populations decreased with high imidacloprid concentrations. At imidacloprid concentrations of more than 20 nanograms per litre, bird numbers declined on average by 3.5% per year. Additional analyses revealed that this spatial pattern of decline appeared only after the introduction of imidacloprid to the Netherlands, in the mid-1990s. ‘The decline in farmland bird species started before 1995, but the local differences in this decline that we have established after the introduction of imidacloprid are not seen in the counts made before that time,’ says Ruud Foppen of Sovon. According to Professor Hans de Kroon, who supervised the study, this is the first research that correlates imidacloprid to possible indirect harmful effects – via the food chain – for vertebrates. Insecticides explain the decline better than other factors, such as land use, he said. The researchers do not yet know precisely if the declines are caused by a lack of food because insecticides reduce the availability of insects or because birds eat contaminated insects. It is not clear whether breeding success is declining or mortality is increasing, or both, they said. For a few species, eating seeds coated with insecticide could be an explanation.