Medium Tree Finch Research Articles

Aggressive behavior as a predictor of home range size: findings from both range-restricted and widespread Darwin’s finch species

AbstractInformation about an animal’s home range (the geographical area in which it accesses resources for survival and reproduction) is vital broadly for understanding animal behavior and specifically for developing conservation management plans. Although personality traits—consistent individual differences in behavior across time and contexts—may affect an animal’s home range size, we still lack a breadth of empirical studies across systems to fully appreciate this influence. Here, we tested the relationship between behavioral responses and home range size in two Darwin’s finch species on Floreana Island: the critically endangered and range-restricted Medium Tree Finch (Camarhynchus pauper) and the common and widespread Small Ground Finch (Geospiza fuliginosa). Using a combination of rapid-assessment assays during both short-term captivity and in the field, we measured exploration and aggressiveness in males from both species. We then used radio telemetry to measure each bird’s home range size over a week-long period. We predicted that (1) fast-exploring individuals would have larger home ranges, and (2) more aggressive individuals would have smaller home ranges. We found that Medium Tree Finches had smaller home ranges than Small Ground Finches, that exploratory behavior was positively correlated with home range size only in Small Ground Finches, and that, in both species, individuals’ aggressiveness was negatively correlated with home range size, whereby the more aggressive individuals occupied smaller home ranges. We conclude that behavioral responses that align with major personality traits can predict home range size, which may provide an important tool for the conservation and management of endangered species when, for example, selecting individuals with different personality profiles for reintroduction.

Exploration behavior differs between Darwin’s finch species and predicts territory defense and hatching success

Darwin’s finch species in the Galápagos Islands are famously distinguished by their morphology but less attention has been given to behavioral differences between species. In this study, we compared behavior between four Darwin’s finch species on Floreana Island: small ground finch (Geospiza fuliginosa), medium ground finch (G. fortis), small tree finch (Camarhynchus parvulus), and medium tree finch (C. pauper). After capturing birds using mist-nets, we measured three behavioral traits: (1) boldness during human handling, (2) exploration in a novel environment, and (3) aggressiveness towards their mirror image. First, we found that ground finches were bolder and more exploratory than tree finches, consistent with their distinct ecological niches on Floreana Island and with the theoretical prediction that diet generalists should be less wary of novelty. Second, we tested the ecological validity of these behavioral variables at the individual level by relating them to territory defense behavior and breeding success. We found that males that were more exploratory in the novel environment also reacted more aggressively to a simulated territory intruder and showed lower offspring hatching success during the breeding season. Hence, our findings support previous work showing behavioral differences between Darwin’s finch species and also suggest pathways by which behavioral differences among individuals might influence fitness.Significance statementClosely-related species that overlap in their geographical range may differ in their morphology and/or behavior, allowing them to occupy different ecological niches. In this study, we explored behavioral differences between four Darwin’s finch species on Floreana Island in the Galápagos Archipelago. We found clear interspecies differences in behavior, with the ground finches struggling more often during handling (boldness) and visiting more sectors in a novel environment (exploration) compared to the tree finches. After birds were released, we continued to observe a subset of male finches in the wild. An individual’s exploration behavior significantly predicted both its aggressive response to a territory intruder (simulated using song playback) and offspring hatching success during the breeding season. This suggests that individual differences in exploration behavior can potentially be used as a proxy for territorial behavior in the wild and may also predict fitness outcomes.

Buzzes are used as signals of aggressive intent in Darwin’s finches

Abstract Signals of aggression may potentially reduce the fitness costs of conflict during agonistic interactions if they are honest. Here we examined whether the ‘buzz’ vocalization in two species of Darwin’s finches, the small tree finch, Camarhynchus parvulus, and the critically endangered medium tree finch, C. pauper, found in Floreana Island, Galápagos Archipelago, is a signal of aggression. Specifically, we assessed three criteria for aggressive signalling (context, predictive, and response criteria) in an observational study and a playback experiment. In the observational study, buzzes by the resident male were more common when an intruder was present on the territory in medium tree finches but not small tree finches (context criterion). In the playback experiment, buzzes increased during and after a simulated intrusion for both species (context criterion). Buzzes before the playback period predicted aggressive responses by males (predictive criterion) but buzzes during playback did not. Finally, both species responded more strongly to playbacks of conspecific buzzes compared to conspecific songs and heterospecific buzzes (response criterion). Together the results support the aggressive signal hypothesis for buzz vocalizations, although future studies are needed to understand the evolution and development of this interesting signal.

Functional traits and foraging behaviour: Avian vampire fly larvae change the beak and fitness of their Darwin's finch hosts

AbstractThe avian beak is a key morphological trait used for foraging. If parasites alter beak shape, we may expect changes in host foraging behaviour. Larvae of the avian vampire flyPhilornis downsicause naris enlargement in Darwin's finch nestlings when first and second instar larvae consume keratin, blood and tissue from inside the beak of the developing host. This naris malformation persists into adulthood, where nares that are >15% of total beak length are considered enlarged.We measured effects of parasite‐induced naris enlargement on foraging behaviour, foraging niche overlap and body condition in Darwin's finches on Floreana Island. Foraging behaviour was ranked by the stress per foraging technique exerted on the beak and ranged from least stress for ‘gleaning’ to most stress for ‘chip off bark’.Naris enlargement occurred in 34% of adult birds. The most common foraging technique differed among species: medium tree finches (Camarhynchus pauper) often chipped off bark to extract subsurface prey, small tree finches (C. parvulus) often gleaned surface prey from foliage, hybrids gleaned prey from bark and foliage, and small ground finches (Geospiza fuliginosa) mostly foraged on the ground. InC. pauper, birds with naris enlargement did more gleaning and less subsurface prey excavation. Foraging niche across species was most similar in birds with naris enlargement. Finally, body condition was lower in insectivorous tree finches with malformed beaks.A novel aspect of this study is the idea that parasite‐induced alterations to phenotype affect ecological processes and interspecific interactions at large temporal and spatial scales. The parasitism occurs early in life but the ecological effects of this parasitism, if causative, are happening later.Read the freePlain Language Summaryfor this article on the Journal blog.

Nesting Success and Nesting Height in the Critically Endangered Medium Tree Finch (Camarhynchus pauper)

When different introduced species across trophic levels (parasite, predator) invade island systems, they may pose significant threats to nesting birds. In this study, we measure nesting height and infer causes of offspring mortality in the critically endangered Medium Tree Finch (Camarhynchus pauper), an island endemic restricted to Floreana Island on the Galápagos Archipelago. Considering all nests at which a male built a nest, sang and attempted to attract a female (n = 222 nests), only 10.4% of nests produced fledglings (5% of nests had total fledging success, 5.4% of nests had partial fledging success). Of the 123 nests chosen by a female, 18.7% produced fledglings and of 337 eggs laid, 13.4% produced fledglings. Pairing success was higher for older males, but male age did not predict nesting success. All nests with chicks were infested with avian vampire fly larvae (Philornis downsi). We attributed the cause of death to avian vampire fly if chicks were found dead in the nest with fly larvae or pupae (45%) present. We inferred avian (either Asio flammeus galapagoensis or Crotophaga ani) predation (24%) if the nest was empty but dishevelled; and black rat (Rattus rattus) predation (20%) if the nest was empty but undamaged. According to these criteria, the highest nests were depredated by avian predators, the lowest nests by rats, and intermediate nests failed because of avian vampire fly larvae. In conclusion, there is no safe nesting height on Floreana Island under current conditions of threats from two trophic levels (introduced parasitic dipteran, introduced mammalian/avian predators; with Galápagos Short-Eared Owls being the only native predator in the system).

Avian vampire fly (Philornis downsi) mortality differs across Darwin\u2019s 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.

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.

Minimum longevity and age-related male plumage in Darwin’s finches on Floreana Island

Several Darwin’s finch populations on the Galapagos Islands are declining and/or locally extinct. Acoustic surveys provide useful information about population size, but do not provide information on the age or morphology of birds. This study uses mist netting data collected during 2004–2016 on Floreana Island with the aim of evaluating minimum longevity in Darwin’s finches. The study species are the Small Tree Finch (Camarhynchus parvulus), hybrid tree finches (Camarhynchus spp.), Medium Tree Finch (Camarhynchus pauper), and Small Ground Finch (Geospiza fuliginosa). In total, 1032 Darwin’s finches were mist netted and 86 of 707 males and 14 of 325 females were recaptured across years. We used the proportion of black plumage to age males, and the age at first capture plus the number of years between recapture to estimate minimum longevity. Minimum longevity ranged from 12 to 15 years and was lowest in the critically endangered C. pauper (12 years). The average number of years between first capture and last recapture was significantly lower in females than in males. Because long-term mist netting provides information on age structure, recruitment and longevity in males and females, it should be a key component of effective conservation planning.

Conservation status of landbirds on Floreana: the smallest inhabited Galápagos Island

On Floreana, the smallest inhabited island in the Galapagos, 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. Galapagos 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), Galapagos Doves (Zenaida galapagoensis), and Galapagos Short-eared Owls (Asio flammeus galapagoensis) were recorded. Small Ground-Finches and Small Tree-Finches were found at densities comparable to those on other Galapagos Islands, whereas densities of Galapagos 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.

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

The rate of evolution depends on the strength of selection, which may be particularly strong for introduced parasites and their naive hosts. Because natural selection acts on phenotypes and because parasites can alter host phenotype, one fruitful starting point to measure the impact of novel pathogens is to quantify parasite-induced changes to host phenotype. Our study system is Darwin’s finches on Floreana Island, Galápagos Archipelago, and the virulent fly larvae of Philornis downsi that were first discovered in Darwin’s finch nests in 1997. We use an experimental approach and measure host phenotype in parasitized and parasite-free chicks in Darwin’s small ground finch (Geospiza fuliginosa). Beak size did not differ between the two treatment groups, but naris size was 106% larger in parasitized chicks (∼3.3 mm) versus parasite-free chicks (∼1.6 mm). To test if P. downsi was present prior to the 1960s, we compared naris size in historical (1899–1962) and contemporary birds (2004–2014) on Floreana Island in small ground finches (G. fuliginosa) and medium tree finches (Camarhynchus pauper). Contemporary Darwin’s finches had significantly larger naris size (including extreme deformation), whereas historical naris size was both smaller and less variable. These findings provide the first longitudinal analysis for the extent of P. downsi-induced change to host naris size and show that Darwin’s finches, prior to the 1960s, were not malformed. Thus natural selection on altered host phenotype as a consequence of P. downsi parasitism appears to be contemporary and novel.

Linked loci and Galapagos finch size

Evolutionary Genetics![Figure][1] Medium and large ground finches diverged in beak size after a severe drought. PHOTO: © FRANS LANTING STUDIO/ALAMY STOCK PHOTO Observations of parallel evolution in the finches of the Galapagos, including body and beak size, contributed to Darwin's theories. Lamichhaney et al. carried out whole-genome sequencing of 60 Darwin's finches. These included small, medium, and large ground finches as well as small, medium, and large tree finches. A genomic region containing the HMGA2 gene correlated strongly with beak size across different species. This locus appears to have played a role in beak diversification throughout the radiation of Darwin's finches. Science , this issue p. [470][2] [1]: pending:yes [2]: /lookup/doi/10.1126/science.aad8786

Divergent foraging behavior in a hybrid zone: Darwin’s tree finches (Camarhynchus spp.) on Floreana Island

Abstract Hybrid speciation is increasingly recognized as a mechanism for novel evolutionary trajectories. However, we know very little about the ecology of a contact zone that has arisen in sympatry. This study examines the foraging behavior and fitness of two species of Darwin’s tree finches (Camarhynchus parvulus, C. pauper) and hybrid offspring on Floreana Island. Previous study showed that the percentage of hybrids in the tree finch population increased from 19% in 2005 to 41% in 2010, and their body and beak size increased by ~5% (parental phenotype did not change). In 2005–2006, all three tree finch groups (two parental species and hybrid birds) used the same foraging substrate, technique, and height. By 2010–2013, the small tree finch C. par-vulus had changed its foraging technique and the medium tree finch C. pauper had changed its foraging height. Both parental species had higher body condition when foraging at (divergent) mean foraging heights per species but hybrid birds did not. We discuss the implications of conserving forest to facilitate vertical niche expansion and the role of hybridization for genetic persistence.

Species Collapse via Hybridization in Darwin’s Tree Finches

Species hybridization can lead to fitness costs, species collapse, and novel evolutionary trajectories in changing environments. Hybridization is predicted to be more common when environmental conditions change rapidly. Here, we test patterns of hybridization in three sympatric tree finch species (small tree finch Camarhynchus parvulus, medium tree finch Camarhynchus pauper, and large tree finch: Camarhynchus psittacula) that are currently recognized on Floreana Island, Galápagos Archipelago. Genetic analysis of microsatellite data from contemporary samples showed two genetic populations and one hybrid cluster in both 2005 and 2010; hybrid individuals were derived from genetic population 1 (small morph) and genetic population 2 (large morph). Females of the large and rare species were more likely to pair with males of the small common species. Finch populations differed in morphology in 1852-1906 compared with 2005/2010. An unsupervised clustering method showed (a) support for three morphological clusters in the historical tree finch sample (1852-1906), which is consistent with current species recognition; (b) support for two or three morphological clusters in 2005 with some (19%) hybridization; and (c) support for just two morphological clusters in 2010 with frequent (41%) hybridization. We discuss these findings in relation to species demarcations of Camarhynchus tree finches on Floreana Island.

Avian population survey in the Floreana highlands: is Darwin’s Medium Tree Finch declining in remnant patches ofScalesiaforest?

SummaryIsland species typically exist in pathogen and predator sparse environments before human settlement, and are particularly vulnerable to the impacts of invasive species. In this study, we used the variable circular-plot method to estimate the density of birds in the highlands of Floreana Island, Galápagos Archipelago, where introduced parasites, predators, and habitat degradation are a known threat to endemic species. We recorded the number of birds seen and heard at 15 locations near Cerro Pajas Volcano in 2004 and 2008, an area that harbours the largest expanse of highlandScalesiaforest on Floreana Island. We estimated the change in population density for nine bird species, including five species of Darwin’s finches. We specifically address changes in population density for the locally endemic Medium Tree FinchCamarhynchus pauper, which only occurs on Floreana Island and has a small population size. Comparing 2004 and 2008, our study found lower population density in the Medium Tree Finch, but stable population density in Small Tree FinchC. parvulusand Large Tree FinchC. psittacula. Based on data from three additional highland sites surveyed in 2008, we estimate that the maximum size of the Medium Tree Finch population is 1,620 individuals. In addition to the survey data, we observed breeding males in 2006 and 2008. We found: (1) low nesting success (six out of 63 nests produced fledglings) and highPhilornis downsiparasite intensity, and (2) a biased age structure of the breeding population. No breeding males were one year old in 2006, and no males were five years old in either study year, indicating low reproductive success as well as limited lifespan. This research has contributed to the recent re-evaluation by IUCN, which has changed the Red List status of the Medium Tree Finch from ‘Vulnerable’ to ‘Critically Endangered’.

Philornis downsi parasitism is the primary cause of nestling mortality in the critically endangered Darwin’s medium tree finch (Camarhynchus pauper)

Darwin’s medium tree finch (Camarhynchus pauper) meets the 2009 International Union for Conservation of Nature Red List criteria for a critically endangered species because it has “a very small range on a single island” and is “declining rapidly owing to the effects of the parasite Philornis downsi”, habitat degradation, and introduced predators. The medium tree finch is only found in patches of remnant highland forest on Floreana Island, where it co-exists with breeding populations of small and large tree finches (C. parvulus and C. psittacula). Here, we examine the intensity of P. downsi in nests of small, medium, and large tree finches on Floreana. We expected that parasite intensity would increase with finch body size, and with greater rainfall, and would also correlate with increased nestling mortality. We found a trend in the expected direction for parasite intensity and rainfall. Combined meta-analytically with data from a previous study, the overall trend for the two studies was significant. We also found a significant linear trend in parasite intensity with finch body size. In addition, the medium tree finch exhibited a somewhat higher parasite intensity than would be expected based on body mass alone. Of 63 active medium tree finch nests, 17 nests had nestlings: all of which were infested with P. downsi. Only 25% of medium tree finch nestlings fledged, 28% were depredated, 41% died due to P. downsi parasitism, and 6% died for other reasons.

Jack-of-all-trades or master of one? Variation in foraging specialisation across years in Darwin’s Tree Finches (Camarhynchus spp.)

Theory predicts that species should have wider foraging niches during conditions of resource scarcity. However, empirical evidence to date has shown mixed patterns, including studies of Darwin’s finches that have found narrower foraging niches during conditions of resource scarcity. Here, we compare foraging behaviour in three species of Darwin’s Tree Finches (Camarhynchus spp.) in a dry versus wet year on Floreana Island to examine the change in foraging breadth under conditions of resource scarcity. We provide descriptive data on diet, foraging substrate, technique, height, and foraging time across the Small Tree Finch (C. parvulus), Medium Tree Finch (C. pauper), and Large Tree Finch (C. psittacula). During dry versus wet years, we made the following predictions: (1) lower intraspecific niche breadth (that is, a higher level of specialisation), (2) lower interspecific overlap in all foraging parameters, and (3) longer foraging times due to relative resource scarcity. Our findings showed that the Small and Medium Tree Finches were generalist foragers, while the Large Tree Finch was a specialist. Resource specialisation varied across years of differing rainfall: both generalist species were less specialised (higher Shannon diversity index) during the dry year, while the specialist species was more specialised (lower Shannon diversity index).

Extinction behind our backs: the possible fate of one of the Darwin’s finch species on Isla Floreana, Galápagos

Without regular monitoring a rare species may slip into extinction unnoticed. We report a possible case from the Galápagos archipelago. The warbler finch ( Certhidea fusca) has not been recorded by scientists on Isla Floreana in recent years, and we have been concerned about its status. On a two-day visit to prime habitat in the breeding season of 2004 we used playback of warbler finch song and calls recorded on another island to stimulate an approach of local birds. We failed to find a single warbler finch, whereas we encountered numerous small tree finches ( Camarhynchus parvulus), medium tree finches ( C. pauper) and yellow warblers ( Dendroica petechia). Searches in 1979, 1983, 1997 and 1999 were also unsuccessful. Our continuing failure to find any warbler finches indicates the species must be extremely rare on the island, if not actually extinct. We discuss possible reasons for their demise and draw attention to other species that could be heading in the same direction.