On Floreana, the smallest inhabited island in the Galápagos, populations of several species of birds have either been extirpated or, based on anecdotal evidence and small‐scale surveys, are declining. Our objective, therefore, was to conduct a comprehensive survey of landbirds encompassing the entire island during three breeding seasons (2014–2016). We conducted surveys at 59 points in 2014, 257 in 2015, and 295 in 2016. Each survey point was sampled once. We detected 12 species during our surveys. Galápagos Flycatchers, Yellow Warblers, Small and Medium ground‐finches, and Small Tree‐Finches were widely distributed over the entire island. Common Cactus‐Finches and Medium Tree‐Finches had more restricted distributions in the lower or higher parts of the island. Few Dark‐billed Cuckoos (Coccyzus melacoryphus), Paint‐billed Crakes (Neocrex erythrops), Galápagos Doves (Zenaida galapagoensis), and Galápagos Short‐eared Owls (Asio flammeus galapagoensis) were recorded. Small Ground‐Finches and Small Tree‐Finches were found at densities comparable to those on other Galápagos Islands, whereas densities of Galápagos Flycatchers and Yellow Warblers were higher on Floreana than on other islands. Endemic Medium Tree‐Finches were confined to an area of 24 km², mainly in the highlands, but were still widespread and common in their restricted habitat, with the number of territories estimated to be between 3900 and 4700. Of 22 originally occurring landbirds on Floreana, no fewer than 10 species have either been extirpated or are likely to have been extirpated since the arrival of the first human inhabitants. The combined effects of introduced mammals, large‐scale habitat destruction, and direct human persecution were responsible for the extirpation of six species during the 19th century. Three additional species have been extirpated since 1960, likely due to the introduction of the parasitic fly Philornis downsi, and this fly remains a major threat for the remaining bird species. Developing strategies for reducing the impact of these flies on the birds of the Galapagos Islands must be a high priority. In addition, habitat management and restoration, including the control of invasive plants and promotion of native tree species, will be critically important in conserving landbird populations on Floreana.

In the Galápagos Islands many endemic bird species, including the emblematic Darwin's finches, show significant declines in population size. The endemic little vermilion flycatcher Pyrocephalus nanus, classified as vulnerable with high extinction risk, is strongly declining and broods regularly fail. We investigated multiple causes for breeding failure by comparing breeding success, infestation intensity by the hematophagous larvae of the introduced parasitic fly Philornis downsi, predation, parental food provisioning rate and prey attack rates as indicators of food availability at three study sites differing in anthropogenic habitat alterations: Alcedo on Isabela Island with its pristine habitat remote from human settlements, El Cura, also on Isabela Island but dominated by farmland, and Mina Roja on Santa Cruz Island, a site highly altered by introduced invasive plant species, mainly the blackberry. To test for the causal role of parasitism, we reduced the number of P. downsi larvae in half the nests at each site on Isabela Island, and used the other nests as control. When infestation intensity was experimentally reduced, both breeding success and food provisioning rates increased significantly in El Cura, but not in Alcedo, where breeding success and food provisioning rates were overall higher and infestation intensity lower than in El Cura. In the very small population of the little vermilion flycatcher in Mina Roja most nests were abandoned during the incubation phase before nests were infested by P. downsi. Mammalian predation played a minor role in brood loss at all three study sites. Our experimental study demonstrates that the recently introduced parasitic fly significantly affects breeding success of an endangered endemic bird species, and suggests that the effects are modulated by natural levels of P. downsi infestation and habitat‐related rates of food provisioning. Conservation measures should include P. downsi control combined with creating and maintaining open foraging areas.

Quantitative food web analyses can provide insights into the specificity of consumers such as herbivores, parasites and parasitoids. Understanding such patterns can be useful in forecasting the potential benefits and risks of biological control agents being considered for introduction against invasive species. The avian vampire fly, Philornis downsi (Diptera: Muscidae), is a neotropical bird parasite that is invasive in the Galapagos Islands, where it is causing substantial mortality of endemic bird species. We used a novel in‐field experimental food web approach within the native range of P. downsi in Ecuador to test the hypotheses that pupal parasitoids known to attack P. downsi specialize on members of the genus Philornis, which occur only in bird nests. We deployed pupae of non‐Philornis fly species adjacent to bird nests to assess specificity of the parasitoids and used two indices to assess specificity: Resource Range (RR), which evaluates the breadth of host use, and Pair Difference Index (PDI), which evaluates interaction strength. The results revealed very strong compartmentalization within the guild of pupal fly parasitoids, with four species attacking only Philornis spp. Both specificity indices indicated significant levels of specificity towards the genus Philornis for two of these species: Conura annulifera and Trichopria sp. novus. We also assessed the specificity of the two dominant Philornis species attacking 11 bird species and preference of the two dominant parasitoid species for bird species. Although there was some significant preference for particular bird species by the Philornis spp., there was no indication that this drove specificity patterns by the parasitoids. Policy implications. Our results confirm previous laboratory studies indicating specificity by C. annulifera and support the hypothesis that this species would produce few, if any, nontarget impacts if released into Galapagos to suppress populations of the P. downsi. These results can inform an environmental risk assessment framework to guide governmental agencies in deliberation on potential field releases of parasitoids in the Galapagos Islands.

Populations of several species of birds endemic to the Galápagos Islands have declined during recent decades, including endemic Little Vermilion Flycatchers (Pyrocephalus nanus). Understanding the reasons for the low breeding success of this species is a prerequisite for developing efficient conservation strategies. Studies of sympatric Darwin's finches suggest two potential reasons: parasitism by the invasive parasitic fly Philornis downsi and extreme climatic events. We investigated the role of each in the breeding success of Little Vermilion Flycatchers during three breeding seasons in the agricultural zone of Isabela Island. We found that Little Vermilion Flycatchers were severely affected by P. downsi, depending on the time of breeding. Nest success was high early in the breeding season (60% were successful) when rates of P. downsi prevalence and intensity were low, but nest success was zero and all nests were infested later in the breeding season. Philornis downsi prevalence and intensity increased with increasing temperature. Both low and high levels of rainfall had a negative effect on nest survival. A parasite removal experiment using insecticide confirmed the detrimental effect of the invasive parasite; nests infested with P. downsi had significantly lower nest success than treated nests. Injection of insecticide into nest bases can be an efficient short-term way to increase the nesting success of Little Vermilion Flycatchers, but finding long-term measures to control the P. downsi population is of utmost importance. La mouche vampire aviaire Philornis downsi menace la moucherolle des GalápagosRÉSUMÉ. Les populations de plusieurs espèces d'oiseaux endémiques des îles Galápagos ont décliné depuis quelques décennies, y compris celle des moucherolles des Galápagos (Pyrocephalus nanus). Il est essentiel de comprendre les raisons du faible taux de reproduction de cette espèce pour développer des stratégies de conservation efficaces. Des études menées sur les pinsons de Darwin sympatriques suggèrent deux raisons possibles : le parasitisme lié à la mouche vampire aviaire Philornis downsi et les événements climatiques extrêmes. Nous avons enquêté sur le rôle de chaque succès de reproduction des moucherolles des Galápagos au cours de trois saisons de reproduction dans la zone agricole de l'île d'Isabela. Nous avons constaté que les moucherolles des Galápagos étaient très affectées par la P. downsi, selon le moment de la période de reproduction. Le succès des nids était élevé au début de la saison de reproduction (60 % d'entre eux réussissaient) lorsque la prévalence et l'intensité des taux de P. downsi étaient faibles, alors que le succès des nids était nul et que tous les nids étaient infestés plus tard au cours de la saison de reproduction. La prévalence et l'intensité des populations de Philornis downsi augmentait avec la hausse des températures. Les niveaux de pluviosité faibles ou élevés entraînaient des effets négatifs sur la survie des nids. Une expérience d'élimination des parasites à l'aide d'insecticid...

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