White-crowned sparrow (Credit: Pete Myers)

Common insecticide threatens survival of wild, migrating birds

Neonicotinoids are harming more than bees.

Migrating songbirds exposed to small amounts of a neonicotinoid pesticide suffered weight loss and migration delays, both of which could reduce their chances of survival, according to a new study.


The study is the first to examine the impacts of neonicotinoids—in this case, one called imidacloprid—on wild birds and suggests the pesticides are putting migrating birds at risk during their migration, which hampers their ability to survive and, ultimately, reproduce.

"The sublethal effects of imidacloprid on food consumption, body condition, and stopover duration have clear links with survival and reproduction and are predicted to negatively affect populations of migratory birds that commonly use agricultural habitats for refueling," the authors wrote in a study published today in Science.

Neonicotinoids—widely used on corn, cotton, sorghum, soybeans and on some other fruits and vegetables—are thought to be at least partially behind bee declines in recent years and also have been linked to widespread impacts on aquatic insects and invertebrates.

Spring migration for birds happens the same time that many farmers are seeding pesticide-treated crops in northern midlatitudes, which is the heart of the Midwest and major U.S. farming regions.

Researchers gave small doses—amounts they'd likely be exposed to in the wild—of imidacloprid to white-crowned sparrows during the birds' spring migration through southern Ontario, Canada. "The [doses] here were extremely low … minute," Christy Morrissey, an ecotoxicologist in the University of Saskatchewan's College of Arts and Science and the School of Environment and Sustainability and senior author on the study, told EHN.

They measured the birds' body before and after exposure and they used radio transmitters to track them. Migratory birds' main fuel source for flying hundreds of miles during migration is fat – to make the strenuous trips they need to put on about 50 to 100 percent of their body mass in a short period of time with fat, Morrissey said.

"They change their whole physiology and become extremely fat in a short period of time and that is burned when they fly," she said.

Birds given the highest dose lost 6 percent of their body mass within six hours. Also, the exposure caused birds to stay an average of 3.5 days longer at the stopover site on their migration route compared to birds that weren't dosed.

"When exposed to these chemicals it caused this anorexic response, they reduced food consumption and essentially rapidly lost weight," Morrissey said.

Morrissey went on: "When we released them with radio tag on, we did not see disorientation, as we expected them to, but quite surprising for us, they didn't fly. They wouldn't leave the stopover site."

She said migration is a critical period for birds and any delays in these travels—which include hundreds of miles of nonstop flight —can "seriously" harm nesting and reproduction.

"The decision for birds to depart a stopover has to do with their internal fuel stores, as well as good weather conditions, or if there's a headwind, but we controlled for those in our study," Morrissey said. "We think they don't have sufficient fuel in the tank."

Such exposures may partially explain why migrant and farmland bird species are declining so dramatically worldwide, Morrissey added. About 74 percent of bird species in North America that rely on farm habitat have suffered population declines since 1966.

Morrissey and colleagues say these impacts are likely due to the pesticide suppressing the birds' appetites—which would mean they would eat less food and not have the fuel and energy to re-start their migrating flights.

Neonicotinoids are neurotoxic, and overstimulate the nervous system.

"[Neonicotonoids] are structurally similar to nicotine — nicotine is an appetite suppressant," Morrissey said. "These low doses could have caused birds to lose their appetite and there could be an additional effect of metabolizing the chemical and its general toxicity causes them to lose weight."

"This is bigger than the bees"

Adult white-crowned sparrow (Credit: Pete Myers)

Morrissey said, while there are differences between species' response to any contaminant, there's "no real reason this would be unique" and only impacting white-crowned sparrows.

Most previous studies have reported the pesticides' health impacts to insects, such as bees, however, more and more studies show problems for birds, including:

  • A 2015 study that found imidacloprid-treated seeds can kill red-legged partridges and reduce their offspring's immunity;
  • A 2013 study that reported three different types of neonicotinoids can reduce egg size and fertilization rate in re-legged partridge;
  • A 2017 study—from the same researchers involved in the new study—that found imidacloprid impacted white-crowned sparrows' orientation during migration.(In the new study, disorientation was not one of the impacts).

"Our study shows that this is bigger than the bees — birds can also be harmed by modern neonicotinoid pesticides which should worry us all," said co-author and biologist Bridget Stutchbury of York University in a statement.

Morrissey said, in seeking solutions to this exposure, banning individual chemicals will not work.

There are "already replacement for neonics—and they're just as toxics as neonics, they're just a different name," she said.

Rather we "need to change the whole system to make it more resilient."

"Monoculture, single crop agriculture is heavily reliant on chemicals for production, unfortunately, that's just not conducive to life and biodiversity," she said.

"We should incentivize farmers to diversify systems rather than substituting one chemical for another."

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