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Bird's-eye view. A Galápagos Islands finch pulls cotton out of a dispenser set by researchers trying to combat parasitic flies.

Bird's-eye view. A Galápagos Islands finch pulls cotton out of a dispenser set by researchers trying to combat parasitic flies.

Sarah Knutie/University of Utah

Cotton Balls Could Save Darwin's Finches

Saving Darwin’s finches could be as simple as arming them with an insecticide. When researchers left pesticide-soaked cotton balls in the birds’ habitats, the finches added bits of the treated cotton to their nests and nearly eliminated one of their chief enemies: parasitic flies.

“This is one of the most incredibly clever bits of practical conservation I’ve seen in my entire career,” says conservation ecologist Stuart Pimm of Duke University in Durham, North Carolina, who was not involved in the new study.

The parasitic nest fly (Philornis downsi), which looks similar to a typical house fly, lays its eggs in the nests of Darwin’s finches—the 15 species of bird on the Galápagos that helped inspire the famed biologist to formulate his ideas about natural selection. When the flies’ eggs hatch, the larvae—each about the size of a grain of rice—suck blood from newborn finch nestlings, often killing them. The parasite has been prevalent on the South American mainland throughout history, where birds have adapted to survive even in its presence. But it wasn’t noticed in nests in the Galápagos until the 1990s, and over the decades since, it has increased in prevalence and has gradually had a larger negative impact on finches.

“Some years, we’ve seen no nestlings at all in an area survive,” says evolutionary biologist Dale Clayton of the University of Utah in Salt Lake City, senior author of the new work. “And some species are getting absolutely hammered by this fly.” The mangrove finch (Camarhynchus heliobates) population, for example, has declined to less than a hundred birds, making it critically endangered. Scientists have tried to fight the parasite by spraying the insecticide permethrin—also used in flea collars for dogs and in lice shampoos—onto nests. But many of the finches’ nests are hard to find, or high in trees and difficult to reach.

While working in the Galápagos, Sarah Knutie—a graduate student in Clayton’s lab—saw finches pulling bits of fiber off a clothing line for their nests. She wondered whether the birds would collect permethrin-treated fibers and carry the pesticide into their nests themselves. To test the idea, Knutie and Clayton set up 30 cotton dispensers, half untreated and half with permethrin-soaked balls, near 26 finch nests. At the end of the nesting and breeding season, 22 of the nests contained cotton, and the birds were just as likely to have collected the permethrin-treated fibers as the untreated ones. Among all nests with the soaked fibers, parasite numbers were lower, and in seven of eight nests that had at least a gram of treated cotton, not a single parasite was found, the team reports today in Current Biology.

“This approach is way easier and just as effective as spraying nests,” Clayton says. “I don’t necessarily think that it’s a long-term permanent solution, but we think of it as a stopgap measure to slow the population declines of finches.”      

The method is creative and could make a crucial difference for the finches, adds Jeff Podos, a biologist at the University of Massachusetts, Amherst, who has studied the bird populations of the Galápagos. “This parasite is a pretty formidable opponent.”

But to protect bird populations throughout the Galápagos, Podos says, someone will have to step up to install and manage the cotton dispensers, which would have to be installed within 20 meters of every nest on the 18 islands of the Galápagos. Getting full coverage of Isabela Island—where the most endangered species of finch lives—would take about 60 cotton dispensers. “These cotton stations have to be spread all over the place,” he says. “What I’m curious about is whether there will be anyone who follows through on the recommendations of this paper.”