Experimental Demonstration of the Fitness Consequences of an Introduced Parasite of Darwin's Finches

Abstract

BackgroundIntroduced 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. An experimental approach in which parasite abundance is manipulated in the field can be the most informative means of assessing a parasite's impact on the host. The parasitic fly Philornis downsi, recently introduced to the Galápagos Islands, feeds on nestling Darwin's finches and other land birds. Several correlational studies, and one experimental study of mixed species over several years, reported that the flies reduce host fitness. Here we report the results of a larger scale experimental study of a single species at a single site over a single breeding season.Methodology/Principal FindingsWe manipulated the abundance of flies in the nests of medium ground finches (Geospiza fortis) and quantified the impact of the parasites on nestling growth and fledging success. We used nylon nest liners to reduce the number of parasites in 24 nests, leaving another 24 nests as controls. A significant reduction in mean parasite abundance led to a significant increase in the number of nests that successfully fledged young. Nestlings in parasite-reduced nests also tended to be larger prior to fledging.Conclusions/SignificanceOur results confirm that P. downsi has significant negative effects on the fitness of medium ground finches, and they may pose a serious threat to other species of Darwin's finches. These data can help in the design of management plans for controlling P. downsi in Darwin's finch breeding populations.

Experimental test of the effect of introduced hematophagous flies on corticosterone levels of breeding Darwin’s finches

“…Nests were dissected within 8 h of collection and P. downsi larvae, pupae, and eclosed pupal cases were counted. First instar larvae were not included in tallies of parasite abundance because they are too small to see reliably in the nest material (Koop et al, 2011). Total parasite abundance was calculated as the sum of all second and third instar larvae, pupae, and eclosed pupal cases found in a nest.…”

“…Adult females incubate the eggs and brood the offspring. Adult males do not sit on the nest and, therefore, do not appear to come into contact with the parasites (Huber et al, 2010; Koop et al, 2011). P. downsi was first documented in the Galapagos Islands in 1964 (Causton et al, 2006); however, it was not until 1997 that the fly was observed in large numbers in nests (Fessl et al, 2001; Fessl and Tebbich, 2002).…”

“…P. downsi was first documented in the Galapagos Islands in 1964 (Causton et al, 2006); however, it was not until 1997 that the fly was observed in large numbers in nests (Fessl et al, 2001; Fessl and Tebbich, 2002). P. downsi has a significant negative effect on the growth rates and fledging success of medium ground finches ( Geospiza fortis ) (Koop et al, 2011). However, little is known regarding the effect of P. downsi on adult birds, or whether such effects contribute to observed decreases in reproductive success.…”

Galápagos mockingbirds tolerate introduced parasites that affect Darwin's finches

“…A notorious example of an introduced parasite is the nest fly Philornis downsi, which has been implicated in the decline of critically endangered species of Darwin's finches, such as the mangrove finch (Camarhynchus heliobates) (O'Connor et al 2009, Fessl et al 2010. Several studies have shown that P. downsi reduces the reproductive success of the medium ground finch (Geospiza fortis) and other species of Darwin's finches (reviewed in Koop et al 2011). Once the eggs hatch, fly larvae feed on the blood of nestlings and adult females as they brood the nestlings.…”

“…Adult flies, which are not parasitic, lay their eggs in the nests of finches and other land birds in the Galápagos. In some years, 100% of finch nests fail to produce fledglings due to P. downsi (Koop et al 2011, 2013a, O'Connor et al 2013. Several studies have shown that P. downsi reduces the reproductive success of the medium ground finch (Geospiza fortis) and other species of Darwin's finches (reviewed in Koop et al 2011).…”