Common Pesticides Delay Songbird Migration, Trigger Significant Weight Loss

A class of pesticides linked with declining insect numbers across the globe has similarly negative effects on wild songbird populations, new research suggests. As researchers from Canada’s University of Saskatchewan and York University report in the journal Science, white-crowned sparrows exposed to low doses of an insecticide called imidacloprid experienced significant weight loss, leading them to delay seasonal migration and, in turn, lower their chances of successful reproduction and survival.

According to Science News’ Maanvi Singh, the team’s assessment—the first to explore neonicotinoids’ impact on wild birds—centered on 36 sparrows captured while migrating from Mexico to the United States and Canada. The scientists gave low pesticide doses to 12 of these birds and higher doses to another 12. The remaining 12 received sunflower oil. (Per Fast Company’s Adele Peters, the highest dose was on par with levels seen in the wild, amounting to the equivalent of two or three chemical-laced wheat seeds.)

Sparrows fed high doses of imidacloprid lost six percent of their body weight and 17 percent of their fat stores within six hours. Those given lower doses exhibited similar symptoms, including weight loss, lethargy and a lack of interest in food. Overall, Sarah Wells writes for Inverse, high-dose birds ate an average of 70 percent less food than control birds over the course of the six-hour observation period.

When released back into the wild, these same sparrows spent an average of 3.5 days, as opposed to non-dosed birds’ half-day breaks, recovering at migratory stopover sites. As study co-author Margaret Eng, an ecotoxicologist at Saskatchewan, tells Science magazine’s Elizabeth Pennisi, the birds likely needed this time to purge the pesticide from their system, resume normal eating habits and gain back lost fat.

Speaking with Peters, study co-author Bridget Stutchbury, a biologist at York, notes that extended rest stops can leave birds—already disoriented by the toxic chemical—vulnerable to predators. At the same time, she explains, late arrival to a final migration destination may reduce a bird’s chances of finding a mate, particularly if it has a shorter breeding season.

“Small birds may only breed once or twice in their lifetimes and missing out could lead to population declines,” co-author Chrissy Morrissey, also an ecotoxicologist at Saskatchewan, tells National Geographic’s Stephen Leahy.

Neonicotinoids’ harmful effects on honeybees, wild bees and other insect species are well-documented. In August, a paper published in the journal PLoS One found that the pesticide class is almost singlehandedly responsible for the uptick in the United States’ agricultural landscape toxicity. Today, the country’s agriculture is 48 times more toxic to insects than it was 25 years ago; 92 percent of this increase can be attributed solely to the group of chemicals.

It’s worth noting that the European Union, acting in response to a report detailing the pesticides’ harmful effects on honeybees and wild bees, instituted a blanket ban on neonicotinoids at the end of 2018. Canada took similar regulatory steps earlier this year. The U.S.’ Environmental Protection Agency, meanwhile, banned 12 types of neonics in May but has yet to take more decisive action.

David Fischer, chief scientist for pollinator safety at Bayer CropScience—the leading manufacturer of imidacloprid—described the study as solid on a “toxicological basis.” But, he says to the Associated Press’ Patrick Whittle, there is no evidence supporting the idea that pesticide amounts used in the experiment are representative of real-world exposure levels; in an email to National Geographic, Fischer further stated that small songbirds are “incapable of swallowing large seeds such as corn or soybean.”

A separate paper published in Science of the Total Environment contradicts this argument, documenting the presence of neonicotinoid-treated seeds in the soil of 35 percent of recently planted fields. Lead author Charlotte Roy, a wildlife ecologist at Minnesota’s Department of Natural Resources, also points out that smaller birds can crack open large seeds, eating fragments and thereby exposing themselves to toxic chemicals.

“This [new] paper is a signal,” Nicole Michel, a senior quantitative ecologist at the National Audubon Society who was not involved in the study, tells Scientific American’s Jim Daley. “It’s the canary in the coal mine that says neonicotinoids are very bad for bird populations.”

Meilan Solly | | READ MORE

Meilan Solly is Smithsonian magazine's senior associate digital editor, history.