Birds fed a common pesticide lost weight rapidly and had migration delays

The world’s most widely used insecticides may delay the migrations of songbirds and hurt their chances of mating. 

In the first experiment to track the effects of a neonicotinoid on birds in the wild, scientists captured 24 white-crowned sparrows as they migrated north from Mexico and the southern United States to Canada and Alaska. The team fed half of those birds with a low dose of the commonly used agricultural insecticide imidacloprid and the other half with a slightly higher dose. An additional 12 birds were captured and dosed with sunflower oil, but no pesticide.  

Within hours, the dosed birds began to lose weight and ate less food, researchers report in the Sept. 13 Science. Birds given the higher amount of imidacloprid (3.9 milligrams per kilogram of body mass) lost 6 percent of their body mass within six hours. That’s about 1.6 grams for an average bird weighing 27 grams. Tracking the birds (Zonotrichia leucophrys) revealed that the pesticide-treated sparrows also lagged behind the others when continuing their migration to their summer mating grounds.

The findings suggest that neonicotinoid insecticides, already implicated in dropping bee populations, could also have a hand in the decline of songbird populations across North America. From 1966 to 2013, the populations of nearly three-quarters of farmland bird species across the continent have precipitously dropped.

The researchers dosed the birds in the lab with carefully measured amounts of pesticide mixed with sunflower oil. In the wild, birds might feed on seeds coated with imidacloprid. The highest dose that “we gave each bird is the equivalent of if they ate one-tenth of [a single] pesticide-coated corn seed,” says Christy Morrissey, a biologist at the University of Saskatchewan in Saskatoon, Canada. “Frankly, these were minuscule doses we gave the birds.”