Pesticides can effect white-crowned sparrows Kurt Stricker/Getty

It’s not just bees that are being harmed by the pesticides called neonicotinoids, it’s birds too. A study in Canada has shown that migrating white-crowned sparrows lose weight just hours after eating seeds treated with the neocotinoid imidacloprid, delaying their onward migration by several days. Although the main manufacturer of the pesticide disputes the findings.

Birds that arrive late at breeding grounds are less likely to raise young successfully and sometimes don’t breed at all, says Christy Morrissey at the University of Saskatchewan, Canada, whose team carried out the study. “This has serious impacts on populations.”

In North America, populations of 57 of the 77 bird species associated with farmland are in decline. Morrissey thinks neonicotinoids could be contributing to these declines.


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However, she does not think that banning these pesticides is the answer. Farmers will just use alternative pesticides that may turn out to be just as bad. Instead, we need to find ways of farming that don’t rely on quick chemical fixes, Morrissey says.

Dave Goulson at the University of Sussex agrees. “The regulatory system keeps failing, by allowing new harmful chemicals into use,” he says. “The only long-term solution is to move away from a reliance on pesticides to solve every problem.”

Neonicotinoids are applied to seeds before planting to kill insects that feed on the seedlings. They are much less toxic to birds and mammals than insects, so in theory these animals should not get high enough doses to harm them.

But lab studies by Morrissey have shown that even low doses of imidacloprid make white-crowned sparrows (Zonotrichia leucophrys) lose weight. Now her team has caught wild sparrows on a migratory stopover, tagged them with tiny radio transmitters and fed them either imidacloprid or a harmless control.

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Birds given imidacloprid lost up to 6 per cent of their body weight in the six hours before release, whereas those not given the pesticide lost hardly any. Scans also showed a decline in body fat.

After release the birds not fed imidacloprid continued their migration after half a day. Those given imidacloprid did not leave for four days on average. Morrissey says she has unpublished evidence that two other neonicotinoids have similar effects.

The study shows that sublethal doses of neonicotinoids can have adverse effects on seed-eating birds as well as on beneficial insects such as bees, says Caspar Hallmann of Radboud University in the Netherlands. “Birds – especially small birds – are really dependent on having sufficient body fat during migration.”

Insect eaters

Hallmann thinks neocotinoids could also be affecting insect-eating birds by depriving them of food. In 2014, he found there were fewer insectivorous birds in areas with high neocotinoid levels in water.

In 2017, Hallmann reported a huge decline in flying insects in Germany, sparking worldwide concern. He thinks pesticides like neonicotinoids are partly to blame.

Morrissey’s findings are disputed by Bayer, the main manufacturer of imidacloprid. Real-world neonicotinoid exposure levels are far below levels that disrupt migratory behaviour, and neonicotinoids are safe when applied according to instructions, says company spokesperson Utz Klages.

Morrissey says the birds were given realistic amounts. A bird could get the highest dose given in the study by eating just a tenth of a treated maize seed, a fifth of a soybean or three canola seeds, for instance. “It’s tiny, tiny amounts,” she says.

Planters are designed to place seeds under the soil, but unburied seeds and seed spills are common, says Morrissey. “Seed spills are a huge and increasing problem,” she says. “Farmers don’t have time to go back and clean them up.”

The European Union has imposed limits on neonicotinoid use, but not a complete ban.

Journal reference: Science, DOI: 10.1126/science.aaw9419