It's not just the bees that are harmed by controversial crop pesticides called neonicotinoids – the birds are also disappearing in places where there are high concentrations of the pesticide in the environment, a new study suggests.

The study led by researchers at Radboud University in the Netherlands compared concentrations of the neonicotinoid pesticide imidacloprid measured in lakes and other surface waters around the Netherlands to local changes in 15 farmland bird species from 2003 to 2010.

They found that in areas where concentrations of the pesticide were more than 20 nanograms per litre, populations of birds such as barn swallows, tree sparrow and common starlings fell 3.5 per cent a year, compared to the average population trend for their species. They published their findings in the most recent issue of the journal Nature.

Neonicotinoids were always regarded as selective toxins. But our results suggest that they may affect the entire ecosystem. - Hans de Kroon, Radboud University

"Neonicotinoids were always regarded as selective toxins. But our results suggest that they may affect the entire ecosystem," said Hans de Kroon, a co-author of the paper, in a news release from Radboud University.

Neonicotinoid pesticides have been used since 1995 in the Netherlands and are also commonly used in North America. They are typically coated on agricultural seeds for crops such as corn and canola to protect the plants from insect pests such as aphids. Studies showing harmful effects of the pesticides in bees have prompted the European Commission to introduce a partial, temporary ban on three kinds of neonicotinoids, including imidacloprid, in Europe.

Studies showing harmful effects of neonicotinoid pesticides in bees have prompted the European Commission to introduce a partial, temporary ban on three kinds of neonicotinoids. (Emily Chung/CBC)

Neonicotinoids act as a neurotoxin for insects, but previous studies have shown they're not very toxic to birds. Because of that, the Dutch researchers think the decline in birds is probably due to pesticides unintentionally killing off the insects they rely on to feed their young during the breeding season.

However, they said they can't rule out the possibility of other ways the pesticide may be affecting birds, such as through direct ingestion. According to a study published by Environment Canada researcher Pierre Mineau in 2013, at a single kernel of imidacloprid-treated corn can kill small and "blue jay-sized birds," and sicken larger ones. However, in the Dutch study, all the birds either ate exclusively insects or fed their young exclusively insects during the breeding season.

Other factors tested

In a video posted by Radboud University, de Kroon said his team "looked very thoroughly" for other possible factors besides neonicotinoid pesticides that could explain the results.

"Our analysis shows that based on our data imidacloprid was by far the best explanatory variable for differences in trends between areas," he added.

The researchers discovered the trend by looking at bird count data along with data about imidacloprid concentrations in waterways collected by the local water boards. While many bird species started declining before farmers started using imidacloprid in 1995, local differences in their decline didn't appear until after that time.

In an analysis piece accompanying the paper in Science, University of Sussex biologist Dave Goulson, who studies bees and other insects, noted that only five per cent of imidacloprid applied to crops is actually taken up by the crops themselves. The rest blows away or gets washed into waterways, and may get taken up by other plants.

A number of other researchers have previously suggested that neonicotinoids could be having a negative effect on birds, including Mineau and University of Saskatchewan biologist Christy Morrissey.

On mobile and can't see the video below? Click here.