By Dr. Sarah Knutie
The fate of many bird species is uncertain. Those the authorities classify as “critically endangered” especially so. Only exceptional conservation measures can save them. While habitat destruction is a major cause of extinction, introduced species are a most serious threat—and one that we are usually completely helpless to control.
One of those desperately rare species is the mangrove finch of the Galapagos—one of the group of “Darwin’s finches” that so inspired the young Charles Darwin. Estimates are fewer than 80 individuals and it’s declining. Worse still, is the threat of an introduced parasitic fly that kills the young in the nest. How can one control that?
The parasitic nest fly Philornis downsi has spread to most islands in the archipelago and is reaping havoc on Darwin’s finches and other land birds. Fly larvae live in the nest where they feed on the blood of nestling finches; in some years, 100 percent of nestlings die when they are parasitized.
The fly might be the final nail in the coffin of some critically endangered species of Darwin’s finches, such as the mangrove finch and medium tree finch. Thus, establishing effective control methods for P. downsi is a top priority for the Galapagos National Park and Charles Darwin Research Station.
We think we have found an immediate stopgap solution.
In 2010, I began my PhD work by studying the effects of P. downsi on Darwin’s finches. One afternoon, while lounging in my hammock at the Research Station dorms, I noticed a female finch land on my laundry line. I snapped a few photos of her as she tugged cotton fibers from a frayed knot.
As she flew off with the collected fibers in her beak, I wondered if finches, such as this one, would incorporate cotton treated with an insecticide—permethrin—into their nests. If so, then perhaps this method could kill P. downsi. (Permethrin is sold over-the-counter to combat head lice and scabies.)
Later that day, I pinned cotton balls sprayed with permethrin to the laundry line. Within a few days, all of the cotton balls had disappeared from the rope.
Inspired by these observations, my colleagues and I designed an experiment in 2013 to test whether Darwin’s finches could be encouraged to “self-fumigate” their nests with cotton balls treated with permethrin. We also directly sprayed other nests with permethrin to determine if killing the parasites would increase finch nestling survival.
Our research team built cotton dispensers, which I can only describe as resembling giant suet birdfeeders. These dispensers were placed at our main field site, El Garrapatero, on the island of Santa Cruz. Over several months, we battled uneven lava rock and 40°C heat in search of active finch nests. During an especially difficult day, I had my first observation of a finch taking cotton from a dispenser and bringing it back to her nest. This particular nest contained nearly two grams of cotton! My hands trembled with excitement as I snapped a blurry picture of the bird with her new nest material.
In the end, we found that permethrin-treated cotton woven into finch nests does reduce the number of parasites. In fact, nests with more than one-gram of permethrin-treated cotton (62 percent of nests) had virtually no parasites.
We presented our findings to researchers at the Station, who are now testing the effectiveness of our method for mangrove finches. I am thrilled that our method may actually help protect one of the most endangered species of bird in the world.