Birds on the Galápagos Islands known as Darwin’s finches have been threatened since the 1990s by parastic, blood-sucking flies (Philornis downsi) that were accidentally introduced to the islands via ships and boats. The flies now infest all land birds there, including most of the 14 species of Darwin’s finches, two of which are endangered. Fewer than 100 mangrove finches remain on Isabela Island, and only about 1,620 medium tree finches exist, all on Floreana Island.
The nest flies lay their eggs in finch nests, and when the eggs hatch they become larvae or maggots, which feed on the blood of baby birds and mother finches.
However, scientists have found a way to allow the birds to protect themselves by “self-fumigating” their nests. According to research published in the journal Current Biology, this “self-fumigation” method may help endangered birds and even some mammals.
Biologists set out cotton balls treated with permethrin, a mild pesticide, and then the finches used the cotton to help build their nests, killing the parasitic fly maggots that threaten the baby birds.
Sarah Knutie, a doctoral student at the University of Utah and the study’s first author, got the idea four years ago at her dorm in the Galápagos when she noticed Darwin’s finches “were coming to my laundry line, grabbing frayed fibers from the line and taking it away, presumably back to their nests,” she recalled.
“If the birds can be encouraged to incorporate fumigated cotton into their nests, then they may be able to lessen the effects of the parasites,” she reasoned.
According to her colleague Dale Clayton, permethrin is safe for the birds.
“This is the same stuff in head-lice shampoo you put on your kid, he said. “Permethrin is safe. No toxicologist is going to argue with that.”
During the experiment, Knutie and colleagues set up two lines of 15 cotton dispensers and they found cotton balls were collected by at least four species of Darwin’s finches.
After birds in a given nest finished breeding and left the nest, the scientists collected the nest, dissected it, counted the number of parasitic fly maggots and then weighed and separated all of the nest materials, including cotton. Nests with treated cotton contained half the number of maggots as untreated nests, and nests with the highest levels of permethrin contained the fewest number of maggots.
“If the birds insert a gram or more of treated cotton -– about a thimbleful –- it kills 100 percent of the fly larvae,” Clayton said.
Similar experiments have been done with mice, as researchers provided nesting materials that were treated with insecticide to see if they might help control the ticks that spread Lyme disease. The same method may help protect other birds and mammals in the future.
“Novel parasites and diseases are an increasing threat to our environment,” Knutie said. “Many animals cannot defend themselves against such parasites. Therefore, the animals need our help in developing effective ways to protect them. Our method of self-fumigation is a simple and immediate solution that can help Darwin’s finches combat this devastating parasite.”
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