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Bird numbers declined more in areas where there was a higher concentration of imidacloprid in surface water.

Falling numbers of swallows and starlings due to imidacloprid, study finds

A new study carried out in the Netherlands has shown populations of common insectivorous birds, such as starlings and swallows, are declining in areas where farmland has high levels of the insecticide imidacloprid.

The research, published in the scientific journal Nature this week, analysed detailed data on local bird population trends and environmental factors including the concentration of the neonicotinoid insecticide in surface water.

Imidacloprid is the most widely used insecticide in agricultural systems around the world and is used to treat seeds and bulbs as well as being utilised as a crop spray. It affects insects' central nervous systems causing paralysis and death.

The research by biologists at Radboud University Nijmegen and the Sovon Centre for Field Ornithology is the first that correlates it to possible indirect harmful effects – via the food chain – for vertebrates.

While farmland bird species in Europe have been in decline for many years there are unexplained local differences in the extent of the decline.

Ecologists working at the Institute for Water and Wetland Research at the university combined water quality data from District Water Boards, as many insects that are important for birds spend part of their life cycle in water, with systematic bird counts taken before and after imidacloprid was introduced in 1995. The biologists compared these data sets with a database that records changes in land use.

Dr Ruud Foppen of the Sovon Centre, an organisation that organises and analyses bird counts, said: "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.

"We decided to look at common species of insectivores, such as the starling and the barn swallow, as there is sufficient data available on these birds for us to analyse densities and numbers."

The researchers found a clear trend: the higher the concentrations of imidacloprid in the surface water, the greater the decline in bird numbers. For the fifteen bird species that were included in the study, including warblers, the skylark and the mistle thrush, numbers decreased on average by 3.5 per cent per year in areas with more than 20 nanograms of imidacloprid per litre. This concentration is greatly exceeded in many parts of the Netherlands.

Professor Hans de Kroon, who supervised the study, says it explains the decline better than other factors such as land use. "We looked very thoroughly for other factors that might relate to these birds’ decline. Our analysis shows that, based on our data, imidacloprid was by far the best explanatory factor for differences in the trends between the areas.

"Neonicotinoids were always regarded as selective toxins but our results suggest that they may affect the entire ecosystem. This study shows how important it is to have good sets of field data, and to analyse them rigorously. Thanks to our partnership with organisations such as Sovon, we can discover ecological effects that would otherwise be overlooked."

The researchers do not yet know precisely what causes the decline. Among possible explanations are a lack of food (insects), eating contaminated insects or a combination of both. For a few species, eating seeds coated with insecticide cannot be excluded as an explanation. It is not clear whether breeding success is declining or mortality is increasing, or both.

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