Naris deformation in Darwin’s finches: Experimental and historical evidence for a post-1960s arrival of the parasite Philornis downsi

An annotated checklist of the Diptera of the Galápagos Archipelago (Ecuador)

The Diptera fauna of the Galápagos Archipelago is updated and an annotated checklist is presented. Currently 50 families, 207 genera, and a minimum of 324 species are recorded from the islands. Approximately 107 species are considered to have arrived on the Galápagos Islands through human introductions, an estimated 101 species are considered endemic, 42 species have naturally colonized the islands from mainland Americas, 21 species are either introduced or arrived naturally and 53 species remain unidentified. The following new combination is proposed: Chrysanthrax primitivus (Walker) is moved to Hemipenthes Loew as H. primitivus (Walker) comb. nov. All references to the Galápagos taxonomic literature are included, known island species distributions listed and general remarks on the biology of many species are provided.

A comparative study of RNA yields from museum specimens, including an optimized protocol for extracting RNA from formalin-fixed specimens

Animal specimens in natural history collections are invaluable resources in examining the historical context of pathogen dynamics in wildlife and spillovers to humans. For example, natural history specimens may reveal new associations between bat species and coronaviruses. However, RNA viruses are difficult to study in historical specimens because protocols for extracting RNA from these specimens have not been optimized. Advances have been made in our ability to recover nucleic acids from formalin-fixed paraffin-embedded samples (FFPE) commonly used in human clinical studies, yet other types of formalin preserved samples have received less attention. Here, we optimize the recovery of RNA from formalin-fixed ethanol-preserved museum specimens in order to improve the usability of these specimens in surveys for zoonotic diseases. We provide RNA quality and quantity measures for replicate tissues subsamples of 22 bat specimens from five bat genera (Rhinolophus, Hipposideros, Megareops, Cynopterus, and Nyctalus) collected in China and Myanmar from 1886 to 2003. As tissues from a single bat specimen were preserved in a variety of ways, including formalin-fixed (8 bats), ethanol-preserved and frozen (13 bats), and flash frozen (2 bats), we were able to compare RNA quality and yield across different preservation methods. RNA extracted from historical museum specimens is highly fragmented, but usable for short-read sequencing and targeted amplification. Incubation of formalin-fixed samples with Proteinase-K following thorough homogenization improves RNA yield. This optimized protocol extends the types of data that can be derived from existing museum specimens and facilitates future examinations of host and pathogen RNA from specimens.

Female in-nest attendance predicts the number of ectoparasites in Darwin's finch species

Selection should act on parental care and favour parental investment decisions that optimize the number of offspring produced. Such predictions have been robustly tested in predation risk contexts, but less is known about alternative functions of parental care under conditions of parasitism. The avian vampire fly (Philornis downsi) is a myasis-causing ectoparasite accidentally introduced to the Galápagos Islands, and one of the major mortality causes in Darwin's finch nests. With an 11-year dataset spanning 21 years, we examine the relationship between parental care behaviours and number of fly larvae and pupae in Darwin's finch nests. We do so across three host species (Camarhynchus parvulus, C. pauper, Geospiza fuliginosa) and one hybrid Camarhynchus group. Nests with longer female brooding duration (minutes per hour spent sitting on hatchlings to provide warmth) had fewer parasites, and this effect depended on male food delivery to chicks. Neither male age nor number of nest provisioning visits were directly associated with number of parasites. While the causal mechanisms remain unknown, we provide the first empirical study showing that female brooding duration is negatively related to the number of ectoparasites in nests. We predict selection for coordinated host male and female behaviour to reduce gaps in nest attendance, especially under conditions of novel and introduced ectoparasites.

Temporal and spatial variation in sex-specific abundance of the avian vampire fly (Philornis downsi)

Understanding the range and behaviour of an invasive species is critical to identify key habitat areas to focus control efforts. Patterns of range use in parasites can differ temporally, across life stages and between sexes. The invasive avian vampire fly, Philornis downsi, spends the larval stage of its life within bird nests, feeding on developing nestlings and causing high levels of mortality and deformation. However, little is known of the ecology and behaviour of the non-parasitic adult fly life stage. Here, we document sex-specific temporal and spatial patterns of abundance of adult avian vampire flies during a single Darwin's finch breeding season. We analyse fly trapping data collected across 7 weeks in the highlands (N = 405 flies) and lowlands (N = 12 flies) of Floreana Island (Galápagos). Lowland catches occurred later in the season, which supports the hypothesis that flies may migrate from the food-rich highlands to the food-poor lowlands once host breeding has commenced. Fly abundance was not correlated with host nesting density (oviposition site) but was correlated with distance to the agricultural zone (feeding site). We consistently caught more males closer to the agricultural zone and more females further away from the agricultural zone. These sex differences suggest that males may be defending or lekking at feeding sites in the agricultural zone for mating. This temporal and sex-specific habitat use of the avian vampire fly is relevant for developing targeted control methods and provides insight into the behavioural ecology of this introduced parasite on the Galápagos Archipelago.

Is the grass always greener on the other side? Weak relationships between vegetation cover and parasitic fly infestations

Understanding parasite-host ecology is increasingly important for conservation efforts in a changing world. Parasitic nest flies in the genus Philornis (Diptera: Muscidae) have been implicated in the decline of endemic island species and are also known to negatively impact breeding success of the critically endangered Ridgway's hawk (B. ridgwayi) on the island of Hispaniola. Despite the importance of these effects on hosts, and extensive research of Philornis downsi in the Galápagos, the ecology of most species of philornid nest flies is poorly understood. We examined biotic factors related to Philornis pici infestations of nestling Ridgway's hawks in the Dominican Republic, where both fly and hawk are native. We found grass-cover was negatively associated with P. pici infestations, while coverage and height of other vegetation classes (tree, shrub, herbaceous, and bare ground) had no association, which is interesting considering recent landscape-level changes to Ridgway's hawk habitat. Anthropogenic activities in Los Haitises National Park, the last strong-hold of Ridgway's hawk, have shifted the landscape from primary forest to a fragmented secondary forest with smallholder or subsistence farms and grassy patches. New information on the ecology of nest flies in their native habitat can inform conservation efforts and allow us to make recommendations for future research.

Avian vampire fly (Philornis downsi) mortality differs across Darwin's finch host species

In invasive parasites, generalism is considered advantageous during the initial phase of introduction. Thereafter, fitness costs to parasites, such as host-specific mortality, can drive parasites towards specialism to avoid costly hosts. It is important to determine changes in host specificity of invasive populations to understand host-parasite dynamics and their effects on vulnerable host populations. We examined changes in mortality in the introduced avian vampire fly (Philornis downsi) (Diptera: Muscidae), a generalist myasis-causing ectoparasite, between 2004 and 2020 on Floreana Island (Galápagos). Mortality was measured as the proportion of immature larvae found upon host nest termination. Over the time period, the avian vampire fly was most abundant and had low mortality in nests of the critically endangered medium tree finch (Camarhynchus pauper) and had the highest mortality in nests of hybrid tree finches (Camarhynchus spp.). Low larval mortality was also found in small tree (Camarhynchus parvulus) and small ground finch (Geospiza fuliginosa) nests. Selection could favour avian vampire flies that select medium tree finch nests and/or avoid hybrid nests. Overall, the finding of differences in avian vampire fly survival across host species is parsimonious with the idea that the introduced fly may be evolving towards host specialisation.

Nestling behaviour predicts naris deformation in Darwin’s finches parasitized by the avian vampire fly

Although in-nest parasitism can reduce the fitness of avian hosts, the severity of these effects may vary with host physiology and behaviour. If certain nestling behaviours are beneficial for resisting parasitism, then selection may favour some behavioural phenotypes over others. Here, we tested whether differences in nestling behaviour mediate the negative effects of parasitism, using small ground finches (Geospiza fuliginosa), on Floreana Island, that had been parasitized by the invasive avian vampire fly (Philornis downsi). We first established, using 4 years of breeding data (2005, 2006, 2010 and 2020), that nestlings exposed to more parasites had larger nares and, among older nestlings only, lower body mass. We then examined, using data from the 2020 season, whether each nestling’s behaviour (specifically, its response to human handling) predicted the severity of its naris deformation. When faced with high-intensity parasitism, more responsive nestlings (i.e. those that struggled more during handling) had larger nares compared to more docile nestlings. This suggests that more responsive nestlings suffer greater fitness costs due to parasitism, although we also discuss alternative explanations. Future work should consider the stability and heritability of these nestling behavioural differences and whether parasite-induced selection shapes behavioural variation at the population level.

Timing of infestation influences virulence and parasite success in a dynamic multi-host-parasite interaction between the invasive parasite, Philornis downsi, and Darwin's finches

Recently commenced host–parasite interactions provide an excellent opportunity to study co-evolutionary processes. Multi-host systems are especially informative because variation in virulence between hosts and temporal changes provides insight into evolutionary dynamics. However, empirical data under natural conditions are scarce. In the present study, we investigated the interaction between Darwin’s finches and the invasive fly Philornis downsi whose larvae feed on the blood of nestlings. Recently, however, the fly has changed its behavior and now also attacks incubating females. Two sympatric hosts are affected differently by the parasite and parasite load has changed over time. Our study observed a reversal of trends described two decades ago: while, currently, small tree finches (Camarhynchus parvulus) experience significantly higher parasite load than warbler finches (Certhidea olivacea), this was the opposite two decades ago. Currently, fledging success is higher in warbler finches compared to small tree finches. Our data indicate that not only intensity but also timing of infestation influences hosts’ reproductive success and parasite fitness. During incubation, prevalence was higher in warbler finches, but once chicks had hatched, prevalence was 100% in both species and parasite load was higher in small tree finches. Furthermore, our results suggest faster development and higher reproductive success of P. downsi in small tree finch nests. A change in host preference driven by larvae competition could have led to the reversal in parasite load.

Evidence for rapid downward fecundity selection in an ectoparasite (Philornis downsi) with earlier host mortality in Darwin's finches

Fecundity selection is a critical component of fitness and a major driver of adaptive evolution. Trade‐offs between parasite mortality and host resources are likely to impose a selection pressure on parasite fecundity, but this is little studied in natural systems. The ‘fecundity advantage hypothesis’ predicts female‐biased sexual size dimorphism whereby larger females produce more offspring. Parasitic insects are useful for exploring the interplay between host resource availability and parasite fecundity, because female body size is a reliable proxy for fecundity in insects. Here we explore temporal changes in body size in the myiasis‐causing parasite Philornis downsi (Diptera: Muscidae) on the Galápagos Islands under conditions of earlier in‐nest host mortality. We aim to investigate the effects of decreasing host resources on parasite body size and fecundity. Across a 12‐year period, we observed a mean of c. 17% P. downsi mortality in host nests with 55 ± 6.2% host mortality and a trend of c. 66% higher host mortality throughout the study period. Using specimens from 116 Darwin's finch nests (Passeriformes: Thraupidae) and 114 traps, we found that over time, P. downsi pupae mass decreased by c. 32%, and male (c. 6%) and female adult size (c. 11%) decreased. Notably, females had c. 26% smaller abdomens in later years, and female abdomen size was correlated with number of eggs. Our findings imply natural selection for faster P. downsi pupation and consequently smaller body size and lower parasite fecundity in this newly evolving host–parasite system.

Behavioral Responses of the Invasive Fly Philornis downsi to Stimuli from Bacteria and Yeast in the Laboratory and the Field in the Galapagos Islands

Philornis downsi Dodge and Aitken (Diptera: Muscidae) is an avian parasitic fly that has invaded the Galapagos archipelago and exerts an onerous burden on populations of endemic land birds. As part of an ongoing effort to develop tools for the integrated management of this fly, our objective was to determine its long- and short-range responses to bacterial and fungal cues associated with adult P. downsi. We hypothesized that the bacterial and fungal communities would elicit attraction at distance through volatiles, and appetitive responses upon contact. Accordingly, we amplified bacteria from guts of adult field-caught flies and from bird feces, and yeasts from fermenting papaya juice (a known attractant of P. downsi), on selective growth media, and assayed the response of flies to these microbes or their exudates. In the field, we baited traps with bacteria or yeast and monitored adult fly attraction. In the laboratory, we used the proboscis extension response (PER) to determine the sensitivity of males and females to tarsal contact with bacteria or yeast. Long range trapping efforts yielded two female flies over 112 trap-nights (attracted by bacteria from bird feces and from the gut of adult flies). In the laboratory, tarsal contact with stimuli from gut bacteria elicited significantly more responses than did yeast stimuli. We discuss the significance of these findings in context with other studies in the field and identify targets for future work.

Introduced parasite changes host phenotype, mating signal and hybridization risk: Philornis downsi effects on Darwin's finch song

Introduced parasites that alter their host's mating signal can change the evolutionary trajectory of a species through sexual selection. Darwin's Camarhynchus finches are threatened by the introduced fly Philornis downsi that is thought to have accidentally arrived on the Galapagos Islands during the 1960s. The P. downsi larvae feed on the blood and tissue of developing finches, causing on average approximately 55% in-nest mortality and enlarged naris size in survivors. Here we test if enlarged naris size is associated with song characteristics and vocal deviation in the small tree finch ( Camarhynchus parvulus), the critically endangered medium tree finch ( C. pauper) and the recently observed hybrid tree finch group ( Camarhynchus hybrids). Male C. parvulus and C. pauper with enlarged naris size produced song with lower maximum frequency and greater vocal deviation, but there was no significant association in hybrids. Less vocal deviation predicted faster pairing success in both parental species. Finally, C. pauper males with normal naris size produced species-specific song, but male C. pauper with enlarged naris size had song that was indistinguishable from other tree finches. When parasites disrupt host mating signal, they may also facilitate hybridization. Here we show how parasite-induced naris enlargement affects vocal quality, resulting in blurred species mating signals.

Genetic admixture predicts parasite intensity: evidence for increased hybrid performance in Darwin's tree finches

Hybridization can increase adaptive potential when enhanced genetic diversity or novel genetic combinations confer a fitness advantage, such as in the evolution of anti-parasitic mechanisms. Island systems are especially susceptible to invasive parasites due to the lack of defence mechanisms that usually coevolve in long-standing host-parasite relationships. We test if host genetic admixture affects parasite numbers in a novel host-parasite association on the Galápagos Islands. Specifically, we compare the number of Philornis downsi in nests with offspring sired by Darwin's small tree finch (Camarhynchus parvulus), Darwin's medium tree finch (C. pauper) and hybrids of these two species. The number of P. downsi decreased with an increasing genetic admixture of the attending male, and nests of hybrid males had approximately 50% fewer parasites than C. parvulus nests, and approximately 60% fewer parasites than C. pauper nests. This finding indicates that hybridization in this system could be favoured by selection and reveal a mechanism to combat an invasive parasite.

Alien Invasion: Biology of Philornis Flies Highlighting Philornis downsi, an Introduced Parasite of Galápagos Birds

The muscid genus Philornis comprises approximately 50 described species of flies, nearly all of which are obligate parasites of nestling birds. Philornis species are native to the Neotropics and widely distributed from Florida to Argentina. Most research on this group has focused on P. downsi, which was introduced to the Galápagos Islands in the late twentieth century. Although Philornis parasitism kills nestlings in several native host species, nowhere do the effects seem more severe than in P. downsi in the Galápagos. Here, we review studies of native and introduced Philornis in an attempt to identify factors that may influence virulence and consider implications for the conservation of hosts in the Galápagos.

Females drive asymmetrical introgression from rare to common species in Darwin's tree finches

The consequences of hybridization for biodiversity depend on the specific ecological and evolutionary context in which it occurs. Understanding patterns of gene flow among hybridizing species is crucial for determining the evolutionary trajectories of species assemblages. The recently discovered hybridization between two species of Darwin's tree finches (Camarhynchus parvulus and C. pauper) on Floreana Island, Galápagos, presents an exciting opportunity to investigate the mechanisms causing hybridization and its potential evolutionary consequences under conditions of recent habitat disturbance and the introduction of invasive pathogens. In this study, we combine morphological and genetic analysis with pairing observations to explore the extent, direction and drivers of hybridization and to test whether hybridization patterns are a result of asymmetrical pairing preference driven by females of the rarer species (C. pauper). We found asymmetrical introgression from the critically endangered, larger-bodied C. pauper to the common, smaller-bodied C. parvulus, which was associated with a lack of selection against heterospecific males by C. pauper females. Examination of pairing data showed that C. parvulus females paired assortatively, whereas C. pauper females showed no such pattern. This study shows how sex-specific drivers can determine the direction of gene flow in hybridizing species. Furthermore, our results suggest the existence of a hybrid swarm comprised of C. parvulus and hybrid birds. We discuss the influence of interspecific abundance differences and susceptibility to the invasive parasite Philornis downsi on the observed hybridization and recommend that the conservation of this iconic species group should be managed jointly rather than species-specific.