Assortative pairing and divergent evolution in Darwin’s Small Tree Finch, Camarhynchus parvulus

Size assortative mating has received increasing attention due to its potential to drive divergence and perhaps speciation. In this study, we examined assortative pairing at 17 nests of Darwin’s Small Tree Finches, Camarhynchus parvulus. We found positive assortative pairing for two traits: bill length and tarsus length, and these traits showed a significant positive correlation to each other. Assortative pairing could be driven by female choice for similar phenotypes because male–male competition has rarely been observed in the Small Tree Finch, nor have males been observed to reject potential mates. Given the high heritability of bill morphology in Darwin’s finches, it is possible that female preference for male bill length, a trait that is known to be important for foraging, will influence offspring bill size to maximise efficient exploitation of resources. The finding of size assortative pairing on the basis of tarsus length requires more research, but suggests different trait utilities for different foraging niches. Interestingly, the highland distribution of tarsus length across the population showed a unimodal distribution, but a bimodal distribution after pairing. While not significant, we found comparatively large differences across study plots in tarsus length, which suggests the possibility of phenotype–habitat matching at a small spatial scale in this species. Our findings are significant in the context of the adaptive radiation of Darwin’s finches as they are consistent with the allopatric model of speciation but also show potential for adaptive divergence in sympatry in Darwin’s tree finches.     

Seasonal variation in the diet of citril finches <Emphasis Type="Italic">Carduelis citrinella</Emphasis>: are they specialists or generalists?

The seasonal variation of food selection in mountainous citril finches was studied in the Black Forest. citril finches show a strong seasonal variation in quantity and quality of food plants. From November to March, only a few plants have been observed as diet, namely seeds of the germander and the mountain pine (period of food restriction). From April to June, the seeds of a few key food plants such as the dandelion, common sorrel, vernal grass, and mountain pine gain special importance during breeding and the rearing period of the young (period of food specialization). Finally, from June/July to October, citril finches enter into a generalist period with a wide variety of seeds of herbs and grasses (period of food generalization). The changes in food selection are well adapted to the changing resource availabilities and climatic conditions of the Black Forest mountains. In direct comparison to other finches, pure mountain-dwelling species such as citril finches and Syrian serins show a clearly higher seasonal variability in food selection than do lowland species such as goldfinches, linnets, and greenfinches.     

Speed of exploration and risk-taking behavior are linked to corticosterone titres in zebra finches

The existence of consistent individual differences in behavioral strategies ("personalities" or coping styles) has been reported in several animal species. Recent work in great tits has shown that such traits are heritable and exhibit significant genetic variation. Free-living birds respond to environmental stresses by up-regulating corticosterone production. Behavior during mild stress can occur in accordance to two types of coping styles, i.e. active and passive. Using artificially selected lines of zebra finches that vary in the amount of corticosterone produced in response to a manual restraint stressor we ran three "personality" experiments. We show that birds in the different corticosterone lines differ in their exploratory and risk-taking behaviors. There was an increase in exploratory behavior as corticosterone titre increased but only in the low corticosterone line. Birds in high corticosterone line showed greater risk-taking behavior than birds in the other lines. Thus, in general, higher levels of circulating corticosterone following a mild stress result in greater exploratory behavior and greater risk taking. This study shows that lines of animals selected for endocrine hormonal responses differ in their "coping" styles or "personalities".     

Evolution of sexual dichromatism: contribution of carotenoid- versus melanin-based coloration

In birds, carotenoid-based plumage coloration is more dependent on physical condition and foraging abilities and less constrained developmentally than is melanin-based coloration. Thus, female mate choice for honest signals should result in more intense sexual selection on carotenoid- than on melanin-based plumage coloration. Using variation in sexual dimorphism as an indirect measure of the intensity of sexual selection, we tested the prediction mat variation in sexual dimorphism is driven more by change in carotenoid-based coloration between males and females dian by change in melanin-based coloration. Examination of historical changes in carotenoid- versus melanin-based pigmentation in 126 extant species of Cardueline finches supported this prediction. We found that carotenoid-derived coloration changed more frequendy among congeners dian melanin-based coloration. In both sexes, increase in carotenoid-based coloration score, but not in melanin-based coloration score, was strongly associated with increase in sexual dichromatism. In addition, sexual dimorphism in carotenoid-based coloration contributed more to overall dichromatism than dimorphism in melanin-based plumage. Our results supported die hypothesis that melanin-based and carotenoid-based coloration have fundamentally different signal content and suggest that combining melanin-based and carotenoid-based coloration in comparative analyses is not appropriate.     

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

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.     

Acoustic discrimination of sympatric morphs in Darwin's finches: a behavioural mechanism for assortative mating?

Populations with multiple morphological or behavioural types provide unique opportunities for studying the causes and consequences of evolutionary diversification. A population of the medium ground finch (Geospiza fortis) at El Garrapatero on Santa Cruz Island, Galápagos, features two beak size morphs. These morphs produce acoustically distinctive songs, are subject to disruptive selection and mate assortatively by morph. The main goal of the present study was to assess whether finches from this population are able to use song as a cue for morph discrimination. A secondary goal of this study was to evaluate whether birds from this population discriminate songs of their own locality versus another St Cruz locality, Borrero Bay, approximately 24 km to the NW. I presented territorial males with playback of songs of their own morph, of the other morph, and of males from Borrero Bay. Males responded more strongly to same-morph than to other-morph playbacks, showing significantly shorter latencies to flight, higher flight rates and closer approaches to the playback speaker. By contrast, I found only minor effects of locality on responsiveness. Evidence for morph discrimination via acoustic cues supports the hypothesis that song can serve as a behavioural mechanism for assortative mating and sympatric evolutionary divergence.     

An eye for detail: selective sexual imprinting in zebra finches

To investigate the idea that sexual imprinting creates incipient reproductive isolation between phenotypically diverging populations, I performed experiments to determine whether colony-reared zebra finches would imprint on details of artificial white crests. In the first experiment, adults in one breeding colony wore white crests with a vertical black stripe, while in another colony adults wore crests having a horizontal black stripe; except for their crests, breeders possessed wild-type plumage and conformation. Offspring of both sexes reared in these colonies developed mate preferences for opposite-sexed birds wearing the crest type with which they were reared; neither sex developed a social preference for crested individuals of the same sex. In a second experiment, females reared by crested parents preferred crested males versus males with red leg bands, while control females (reared in a colony of wild-type, uncrested birds) preferred red-banded males in the same test. Results of a third experiment that used sexually dimorphic crest phenotypes indicate that both sexes of offspring imprinted on maternal crest patterns. Results support the hypothesis that sexual imprinting can facilitate isolation both by engendering a preference for population-typical traits and by prioritizing such an imprinting-based preference over species-typical preferences for other traits used in mate choice. Comparison with results of other recent studies indicates that imprinting tendencies of both sexes vary with the characteristics of traits presented as an imprinting stimuli. Tendency to imprint may vary with the perceived information content (e.g., kin, sex, or population indicator) of parental traits, a process dubbed selective sexual imprinting.     

Human activity can influence the gut microbiota of Darwin's finches in the Galapagos Islands

The gut microbiota of animal hosts can be influenced by environmental factors, such as unnatural food items that are introduced by humans. Over the past 30 years, human presence has grown exponentially in the Galapagos Islands, which are home to endemic Darwin's finches. Consequently, humans have changed the environment and diet of Darwin's finches, which in turn, could affect their gut microbiota. In this study, we compared the gut microbiota of two species of Darwin's finches, small ground finches (Geospiza fuliginosa) and medium ground finches (Geospiza fortis), across sites with and without human presence, where finches prefer human‐processed and natural food, respectively. We predicted that: (a) finch microbiota would differ between sites with and without humans due to differences in diet, and (b) gut microbiota of each finch species would be most similar where finches have the highest niche overlap (areas with humans) compared to the lowest niche overlap (areas without humans). We found that gut bacterial community structure differed across sites and host species. Gut bacterial diversity was most distinct between the two species at the site with human presence compared to the site without human presence, which contradicted our predictions. Within host species, medium ground finches had lower bacterial diversity at the site with human presence compared to the site without human presence and bacterial diversity of small ground finches did not differ between sites. Our results show that the gut microbiota of Darwin's finches is affected differently across sites with varying human presence.     

Sisyphean evolution in Darwin's finches

The trajectory of speciation involves geographic isolation of ancestral populations followed by divergence by natural selection, genetic drift or sexual selection. Once started, the process may experience fits and starts, as sometimes diverging populations intermittently reconnect. In theory populations might cycle between stages of differentiation and never attain species status, a process we refer to as Sisyphean evolution. We argue that the six putative ground finch species (genus Geospiza) of the Galápagos Islands represent a dramatic example of Sisyphean evolution that has been confused with the standard model of speciation. The dynamic environment of the Galápagos, closely spaced islands, and frequent dispersal and introgression have prevented the completion of the speciation process. We suggest that morphological clusters represent locally adapted ecomorphs, which might mimic, and have been confused with, species, but these ecomorphs do not form separate gene pools and are ephemeral in space and time. Thus the pattern of morphological, behavioural and genetic variation supports recognition of a single species of Geospiza, which we suggest should be recognized as Darwin's ground finch (Geospiza magnirostris). We argue that instead of providing an icon of insular speciation and adaptive radiation, which is featured in nearly every textbook on evolutionary biology, Darwin's ground finch represents a potentially more interesting phenomenon, one of transient morphs trapped in an unpredictable cycle of Sisyphean evolution. Instead of revealing details of the origin of species, the mechanisms underlying the transient occurrence of ecomorphs provide one of the best illustrations of the antagonistic effects of natural selection and introgression.