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In 2015, U.S. farmers dropped at least of chlorpyrifos and nearly of imidacloprid, two different pesticides, on their fields. Each have been in use for decades, and each does a good job of frying the nervous systems of insects. But the treatments are detrimental to more than just the targeted pest populations—insect populations overall sag in regions where the pesticides are popular. 

There’s less known about the pesticides’ effect on birds, though. Because birds have more complex nervous systems than insects, it’s largely been assumed that these insecticides don't affect birds that much, if at all. But lab research on large seed-eating birds suggests otherwise. Partridges suffer due to imidacloprid, and Japanese Quail from chlorpyrifos; even a single imidacloprid-treated corn seed can kill a Blue Jay.

Margaret Eng, a graduate student at the University of Saskatchewan in Canada, wanted to know how smaller songbirds, like sparrows, might cope with the pesticides. After all, the birds stop on farm fields mid-migration and likely consume the pervasive chemicals—imidacloprid is delivered as a seed coating, and chlorpyrifos granules resemble the grit songbirds intentionally eat to help digest their meals. Her work, this month, shows that even relatively low doses of the pesticides prevent the tiny migrators from orienting themselves for their migrations, and sometimes even cause illness or death. ̽����ѡ Magazine covered her worrisome preliminary results earlier this year, but the final report is even more alarming. 

Since sparrows rely on instincts buried deep in their neurological systems to navigate the skies, Eng wondered how the chemicals might impact migration efforts. She and her research team rounded up wild White-crowned Sparrows in Saskatchewan just before they left on their northbound migrations. Some were fed only seeds, while the rest also drank chlorpyrifos or imidacloprid at one-tenth or one-quarter the level fatal to a House Sparrow. These lab doses are comparable to the imidacloprid levels typically found on four treated canola seeds, or the amount of chlorpyrifos in eight to 12 granules—in each case, a fraction of the hundreds of seeds or grit pieces a sparrow might eat in a day. For two weeks, the team recorded how many migration attempts the birds made from the lab and in which directions, and weighed them periodically.

The birds that received no pesticides managed just fine. They consistently headed north, the correct direction for migration, and kept their weights up. Those on imidacloprid, on the other hand, stopped eating and lost a quarter of their weight—a significant amount for an already light animal—and made aimless migration attempts for three days. They didn’t regain their bearings until 11 days later, though their appetites and weights rebounded by the end of the two weeks. Those that ate chlorpyrifos kept their fat, but had no sense of direction for the entire two-week study.

Shockingly, four of the birds fed imidacloprid died within 24 hours of their last dose: two on their own, and two required euthanization after struggling to breathe and foaming at the crop. Though this was only four birds, “we were surprised by the acute toxicity of the imidacloprid,” says Eng’s boss Christy Morrissey, a wildlife ecotoxicologist at the University of Saskatchewa and subject of ���ܻ�ܲ��Dz�’s previous feature on pesticides and birds. “That is supposed to be the safer chemistry for vertebrates” relative to chlorpyrifos.”

Because of this , chlorpyrifos is more heavily regulated and cannot be used in home gardens or with particular foods, like tomatoes. The U.S. Environmental Protection Agency’s research recommends banning chlorpyrifos on all crops—but in March, Administrator Scott Pruitt . The agency’s of imidacloprid won’t be completed until 2018. Until then, the pesticide is largely unrestricted.  

As for the songbirds stopping to eat in fields laden with these toxic insecticides, Morrissey sighs, “it may be worse to [get sick and] be immobilized, or worse to go off course on migration.” Either way, the study's findings paint a bleak picture for birds as long as both pesticides remain in wide use.  

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