Bees can be harmed by low levels of neonicotinoid(新烟碱类农药) pesticides, and now it seems birds can too. Migrating white－crowned sparrows have been found to lose weight after eating seeds treated with one of these chemicals, imidacloprid (吡虫啉〈一种杀虫剂〉), delaying their onward migration by several days.
Such a delay could hamper their chances of successfully breeding. However, the main manufacturer of the pesticide disputes the findings.
The latest twist in the debate over neonicotinoids is the result of work by Christy Morrissey at the University of Saskatchewan in Canada and her team.They caught migrating sparrows, tagged them with tiny radio transmitters and gave them feed containing imidacloprid or an alternative without the chemical. The birds given the pesticide lost up to 6 per cent of their body weight in the 6 hours before release, whereas the other birds hardly lost any. Scans also showed a decline in body fat among the first group.
When released, the birds not fed imidacloprid continued their migration after half a day. Those given the pesticide took four days, on average, to do the same. Morrissey says she also has unpublished evidence that two other neonicotinoids have similar effects.
Birds that arrive late at breeding grounds are less likely to raise the young successfully and may not breed at all, says Morrissey. "This has serious impacts on populations."
The study shows sublethal(亚致死的) 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."
The findings are disputed by Bayer, the main manufacturer of imidacloprid. Real－world neonicotinoid exposure levels are far below those that disrupt migratory behaviour, and the pesticides are safe when applied according to instructions, says a Bayer spokesperson.
Morrissey says the birds were given realistic amounts. They could get the highest dose given in the study by eating just one－tenth of a treated maize seed, a fifth of a soya bean or three canola seeds, for instance. "It's tiny, tiny amounts," she says.
In North America, 57 of the 77 bird species associated with farmland are in decline, with neonicotinoids one possible factor. However, Morrissey says that banning these pesticides isn't the answer because farmers will just use alternatives that may turn out to be as bad. Instead, we need to find ways of farming that don't rely on any chemical fixes, she says.
