Darwin?s Finches of the Galápagos Islands are one of the most famous examples of evolution by natural selection. Unfortunately, several species of these evolutionary icons have recently come under serious threat from an introduced species of parasitic fly (Philornis downsi). This pest, which was probably accidentally introduced from mainland Central or South America, may have devastating effects on host reproductive success. The endangered Mangrove Finch and other species may be at risk of extinction if a way to combat the fly is not found. The goal of our study is to carefully measure the impact of this invasive parasite on Medium ground finches, and test whether the birds are able to mount immune responses to the flies, or defend themselves in other ways.
This study will provide critical data to inform decisions by management personnel of the Galápagos National Park, which is responsible for the welfare of Galápagos native fauna and flora. The project will provide interdisciplinary training for a postdoc, a Ph.D. student, Ecuadorian volunteers, and several undergrad students in host-parasite evolutionary ecology, behavior, immunology, and conservation biology.
Intellectual Merit Introduced parasites are a particular threat to small populations of hosts living on islands because extinction can occur before hosts have a chance to evolve effective defenses. Recently, the parasitic nest fly Philornis downsi was introduced to the Galapagos Islands. Adult flies, which are not parasitic, lay their eggs in the nests of most land birds. Once the eggs hatch, the larvae live in the nest and feed on the blood of the nestling and adult female birds. Prior to 2008, few rigorous studies had measured the effect of P. downsi on the fitness of Galapagos birds. However, preliminary evidence suggested that P. downsi negatively affected nestling survival in Darwin’s finches, which are endemic to the Galapagos. From 2008-2013, we tested the impact of P. downsi on Darwin’s finch fitness. Each year, we experimentally manipulated the number of flies in nests of the medium ground finch (Geospiza fortis), then measured the effect of the parasites on nestling growth and survival. In most years, a significant reduction in parasite load led to a significant increase in the number of nests with surviving offspring. In 2010 the parasite reduced nestling survival to zero, compared to fumigated nests. Nestlings in fumigated nests also tended to be larger in size prior to leaving the nest, which increased their odds of long term survival. Our next objective was to test whether Darwin’s finches have effective behavioral or immunological defenses against P. downsi. We found that at least eight species of Darwin’s finches have antibody-mediated immune responses to P. downsi. We tested whether the immune response was effective at reducing the number of flies in nests and, whether it would effectively "rescue" nestling survival. Philornis downsi-binding antibody levels were significantly higher among mothers at parasitized nests, compared to mothers at parasitized nests. Mothers with higher antibody levels tended to have fewer parasites in their nests, suggesting that antibodies play a role in defense against parasites. Mothers showed no behavioral changes that would enhance the effectiveness of the immune response. During this year, none of the parasitized nests had any surviving offspring, despite the immune response by mothers. Thus, the study showed that, while mothers had an immune response, it was not sufficient to the survival of nestlings by helping the mothers provide nestlings with more food, for example. Host species that are not impacted by P. downsi may serve as reservoirs for the parasite. Reservoir hosts represent a serious threat to hosts, such as Darwin’s finches, that are relatively defenseless against P. downsi. Anecdotal evidence suggested that Galapagos mockingbirds (Mimus parvulus), another enemic species, are not affected by P. downsi. In 2012-13, we compared the effects of P. downsi on medium ground finch and mockingbird nestling survival. In both years, we found that, although mockingbirds have three times as many parasites as finches, the parasite has no apparent effect on mockingbirds. These results suggest that mockingbirds are reservoir hosts of P. downsi, with major implications for the future population dynamics of the fly and Darwin’s finches. In the final year of the study we developed a technique to help control P. downsi in the short-term. In 2010, we observed Darwin’s finches taking cotton fibers from a cotton laundry line. In 2013, we tested whether they could be encouraged to incorporate cotton treated with a permethrin fumigant into their nests and whether it is effective at reducing Philornis parasites in the nests. Indeed, we found that birds incorporate cotton into their nests and that nests with permethrin-treated cotton have significantly fewer P. downsi parasites than nests with cotton treated with water. The results of this study demonstrate that self-fumigation can be used to mitigate the impact of nest flies on Darwin’s finches. Broader impacts The project has involved 2 post-doctoral researchers, 6 doctoral students (including 2 dissertations), 4 American undergraduate students, 2 high school students and 5 Ecuadorian students. Participants in the project have given lectures at Audubon Society meetings, middle schools, national college and universities, and scientific conferences. Each year, participants in the project also give a lecture at the Charles Darwin Research Station in the Galapagos, which is attended by the public. We have given several lectures to international tourists on the National Geographic Endeavor cruise ship in the Galapagos. The project has also received local and international attention through newspaper and radio press. Participants have contributed to websites, such as the Cornell Lab of Ornithology Neotropical Birds website, and have created YouTube videos describing the project and related field methods. These websites are regularly accessed by the public.
