“The coolest thing about this paper...is that it shows that the effects of [pesticides] aren’t always obvious,” says Jessica Hua , an environmental toxicologist at Binghamton University who was not involved in the study. “This isn’t direct mortality—the effects are more subtle. And no one would have seen that without tracking these birds.”

A team of researchers has found that a widely used insecticide called imidacloprid causes sparrows to lose drastic amounts of weight and delay their migratory schedules, potentially stripping the birds of their best shot at a successful breeding season. The study , published today in the journal Science, is the first to track the fates of wild birds exposed to pesticides. It also adds to a growing body of evidence that shows the toxic chemicals that abound in agriculture harm far more than the species they’re intended for—often in subtle and insidious ways.

The skies are growing increasingly silent. For the past several decades, bird populations around the globe have been plummeting —and it appears one of the world’s most popular pesticides may be partly to blame.

Two years ago, a team led by Eng found that to be exactly the case . After being fed a neonic called imidacloprid, captive white-crowned sparrows ( Zonotrichia leucophrys ) stopped eating, dropping up to a quarter of their body weight within three days. They also temporarily lost their ability to orient northward in a lab-based experiment—a handicap that could prove disastrous for migrating birds.

That’s bad enough news for bugs. But these chemicals could also take a toll on animals that access the pesticide where it’s most concentrated: on treated seeds themselves, spilled during the planting process, or lurking near the soil surface.

Neonics are the most commonly used insecticide in the world, and can be found slathered on the seeds of many commercial crops, including corn, canola, and soybeans. When the seeds sprout, a fraction of the water-soluble drug gets ferried into growing tissues, turning the entire plant toxic to any insect feeding on it.

But a few years ago, Margaret Eng , an environmental toxicologist at the University of Saskatchewan in Canada, began to wonder if the birds were suffering more directly from neonics’ effects.

In recent years, it’s become clear that upticks in neonics are also linked to downswings in certain bird species—a trend that some researchers have attributed to insect-eating species suddenly finding themselves without a consistent supply of food.

The fact that pesticides hurt more than pests isn’t news. In particular, neonicotinoids (neonics for short)—the class of insecticides that includes imidacloprid, and act by overstimulating and destroying nerve cells—have long been under fire for the bevy of off-target effects it has on local ecosystems, including declines in bee populations in several countries .

Eng was struck by the birds’ rapid deterioration—but it remained unclear whether the results would hold true in the real world. So she and her colleagues repeated their experiment in the field, capturing a new cohort of sparrows making a pitstop in Ontario en route to their spring breeding grounds. As with their captive study, the researchers mimicked natural exposure by carefully dosing the birds with one of two small quantities of imidacloprid dissolved in sunflower oil. Each dose comprised less than the amount of pesticide that can be found on a single corn seed, or less than 1.5 percent of a sparrow’s daily diet, based on current application rates as defined by the United States Environmental Protection Agency, as well as an analogous organization in Canada.

After monitoring the sparrows for several hours, the team found that, even in tiny doses, imidacloprid was a powerful appetite suppressant, confirming the results of their previous experiment. In the six hours after they ingested the pesticide, sparrows on the higher dose ate 70 percent less than those that had been fed only sunflower oil, and lost 6 percent of their body mass.

Shortly after this period of surveillance, the researchers attached lightweight trackers to the sparrows and sent them on their way.

Unlike before, the sparrows didn’t seem to have trouble staying on course—once they left, that is. While untreated birds hung around the stopover site for about half a day to rest and refuel before resuming their journey, the intoxicated sparrows lingered for an extra two or three days before taking flight. Eng thinks the birds may have delayed their departure to give themselves time to regain their appetites and recover from the pesticide’s discombobulating effects, like those observed in the team’s previous study.

The white-crowned sparrow is one of many migratory species that's vulnerable to pesticides at stopover sites. Image Credit: BirdImages, iStock

A prolonged pitstop might seem like no big deal. But for a bird on the go, timing is everything: A prompt arrival at spring breeding grounds boosts its chances of claiming a good nesting site, hooking up with a choice mate, and producing healthy offspring. That leaves little time for dilly-dallying along the way—and even brief delays in this strict schedule can prove disastrous, says study author Christy Morrissey, who supervises Eng’s work.

The researchers didn’t follow the sparrows’ fates throughout the rest of the breeding season. But it’s not a stretch to imagine how bouts of extreme weight loss or nausea could waylay a bird on a harrowing odyssey, Morrissey says. “These minuscule doses [are] enough to disrupt the birds’ whole migration timeline,” she says. “This has clear knock-on effects for population stability.”

Ironically, when neonics first came onto the scene in the 1980s, they were billed as “safer” alternative to an older generation of more toxic pesticides. On a cell-by-cell basis, the chemical exerts less potent effects in vertebrates like birds and mammals than in insects. But “less potent” doesn’t mean “harmless.”

“This is a pesticide that’s been considered safe because it’s fine-tuned to target specific species,” says Corina Newsome, a wildlife biologist and avian conservationist at Georgia Southern University who was not involved in the study. “This study helps blow the lid off [that idea].”

Cereal grain seeds coated with pesticides, strewn in the soil. Image Credit: Margaret Eng, University of Saskatchewan

And there are more than bird lives at stake. After they’re applied, the chemicals also tend to linger in soil, where they can then get flushed into streams and rivers, potentially affecting the residents of more distant habitats, including amphibians and aquatic invertebrates. Even mammals like deer, raccoons, and rabbits have been spotted dipping into contaminated seed stocks.

“These [pesticides] exist throughout the whole agricultural landscape,” Eng says. “They’ve become ubiquitous.”

Utz Klages, a spokesperson for Bayer, a major manufacturer of imidacloprid, disputes the study’s connection between the use of neonics and declines in songbird health, arguing that real-world exposure levels are far below those that disrupt migratory behavior. “Bayer remains convinced that evidence shows that neonicotinoids are safe when applied responsibly and properly in accordance with the label instructions,” he says in an email.

Morrissey, however, stresses that the study’s doses are realistic. The time when pesticides are applied to seeds coincides perfectly with spring migrations, she says, and previous research has shown that several bird species will snack on seeds spilled during planting.

Last year, the European Union banned three neonics, including imidacloprid, for all outdoor uses, citing damages to bees as the primary motivation. But Eng and Morrissey don’t think single chemical bans will be a panacea for biodiversity around agricultural areas. Where one pesticide disappears, another will quickly take its place, Morrissey says. And many of the new ones being rolled out by companies have toxic properties similar to their predecessors, she adds.

A receiving station in the Motus Wildlife Tracking System, an automated radiotelemetry network that tracks the movements of tagged animals, like the study's white-crowned sparrows. Image Credit: Margaret Eng, University of Saskatchewan

Getting farmers “off the pesticide treadmill,” she says, won’t be easy. Where possible, non-chemical alternatives for pest control could be considered. One possibility, she says, involves investing in regenerative agriculture—a practice that leverages the natural resilience that comes with more ecologically diverse plots of land.

These transformations won’t happen overnight. But Newsome and Hua both advocate for further research to identify the critical stopover sites that support the most species of migrating birds. Here, Newsome says, researchers and farmers can cooperate to minimize the impacts of agriculture on wildlife, or provide safe havens to shelter weary travelers.

In the meantime, pesticides could stand to be used more judiciously, Hua says. It’s also worth considering the ways in which their design and manufacture could be modified to avoid these negative consequences in the first place.

“Is it possible for our society to be proactive instead of reactive, and prevent something harmful from getting out to the market?” she says. “It’s something to think about.”