Common insectides have lowered songbird populations – they eat a couple seeds and lose weight.

National Geographic reports on a study that has found nicotine-based insecticides – the world’s most widely used pesticides – act like appetite suppressants for songbirds. Which means they’re not able to migrate to save their lives, which is why populations have plummeted:

“We show a clear link between neonicotinoid exposure at real-world levels and an impact on birds,” says lead author Margaret Eng, a post-doctoral fellow at the University of Saskatchewan Toxicology Center.

Spring bird migration occurs when farmers are planting, and most crops in the United States and Canada are grown with neonicotinoid-treated seeds. Birds may suffer repeated exposure at successive stopover sites where they rest and feed. That may extend migration delays and their consequences, the study concludes.

Neonicotinoids, introduced in the late 1980s, were supposed to be a safer alternative to previous insecticides. But study after study has found that they play a key role in insect decline, especially bees. The EU banned the use of the chemicals in 2018 because they were killing pollinators.

The populations of more than 75 percent of songbirds and other birds that rely on agricultural habitat in North America have significantly declined since 1966. The new study reveals how neonicotinoids, also known as neonics, could be directly contributing to the die-offs. Just last month a comprehensive study concluded that the widespread use of neonicotinoids had made America’s agricultural landscape 48 times more toxic to honeybees, and likely other insects, than it was 25 years ago.

[R]esearchers captured white-crowned sparrows during a stopover on their spring migratory route from the U.S. to the boreal region of Canada, which spans the top of the country. Individual sparrows were fed either one very small dose of the most commonly used neonicotinoid, called imidacloprid, or a slightly higher dose, or one with no insecticide.

Each bird was weighed and its body composition measured before and after exposure. Birds given a higher dose of the pesticide had lost 6 percent of their body mass when weighed again six hours later.

The high dose given is comparable to a bird eating one-tenth of a single sunflower seed or corn seed treated with imidacloprid, or three or more wheat seeds, says co-author Christy Morrissey, an ecotoxicologist at the University of Saskatchewan. “It’s a minuscule amount, a tiny fraction of what these birds would eat daily,” Morrissey said in an interview.

The captured sparrows were released shortly after their second weigh in—and after a tiny tag transmitter was glued between their wings. The tag allowed tracking of their movements in the wild. The dosed sparrows didn’t immediately continue their migration like the undosed ones. The high-dose sparrows hung around the stopover site for an extra 3.5 days recovering from their intoxication and regaining their lost weight, the study concluded.

Fortunately, imidacloprid metabolizes fairly quickly in birds. But an extra 3.5-day delay in migration can mean the sparrows might miss their chance to breed, says Morrissey. “Small birds may only breed once or twice in their lifetimes and missing out could lead to population declines.”

The new study showed that sparrows lost crucial body fat amounting to an average of 9 percent for low-dosed birds and 17 percent for the birds that received higher doses.