Convergent evolution of kidney sizes and supraorbital salt glands for birds living in saline habitats.

Only a small number of avian species inhabit salty environments. To understand how they adapted, we examined the evolution of kidney sizes, supraorbital salt glands (SSGs), and the utilization of salty habitats across 230 species spanning 25 avian orders. Phylogenetic analysis indicates that SSGs, large kidneys, and thriving in salty habitats emerged convergently in birds. Transition rate analysis reveals that species possessing SSGs and large kidneys tended to move from low-to high-salinity environments, while others moved in the opposite direction. However, habitat salinity also influenced kidney evolution; lineages residing in high-salinity environments tended to develop larger kidneys than those in low-salinity environments. Our findings suggest that SSGs and large kidneys may have evolved through adaptation to high salinity. Overall, habitat conditions and physiological traits influenced avian adaptation to salty environments in a reciprocal manner. These results shed the new light on the evolutionary mechanisms underlying functional diversity in birds.

Weak gene-gene interaction facilitates the evolution of gene expression plasticity.

BACKGROUND: Individual organisms may exhibit phenotypic plasticity when they acclimate to different conditions. Such plastic responses may facilitate or constrain the adaptation of their descendant populations to new environments, complicating their evolutionary trajectories beyond the genetic blueprint. Intriguingly, phenotypic plasticity itself can evolve in terms of its direction and magnitude during adaptation. However, we know little about what determines the evolution of phenotypic plasticity, including gene expression plasticity. Recent laboratory-based studies suggest dominance of reversing gene expression plasticity-plastic responses that move the levels of gene expression away from the new optima. Nevertheless, evidence from natural populations is still limited. RESULTS: Here, we studied gene expression plasticity and its evolution in the montane and lowland populations of an elevationally widespread songbird-the Rufous-capped Babbler (Cyanoderma ruficeps)-with reciprocal transplant experiments and transcriptomic analyses; we set common gardens at altitudes close to these populations' native ranges. We confirmed the prevalence of reversing plasticity in genes associated with altitudinal adaptation. Interestingly, we found a positive relationship between magnitude and degree of evolution in gene expression plasticity, which was pertinent to not only adaptation-associated genes but also the whole transcriptomes from multiple tissues. Furthermore, we revealed that genes with weaker expressional interactions with other genes tended to exhibit stronger plasticity and higher degree of plasticity evolution, which explains the positive magnitude-evolution relationship. CONCLUSIONS: Our experimental evidence demonstrates that species may initiate their adaptation to new habitats with genes exhibiting strong expression plasticity. We also highlight the role of expression interdependence among genes in regulating the magnitude and evolution of expression plasticity. This study illuminates how the evolution of phenotypic plasticity in gene expression facilitates the adaptation of species to challenging environments in nature.

Genomic architecture underlying morphological and physiological adaptation to high elevation in a songbird.

Organisms often acquire physiological and morphological modifications to conquer ecological challenges when colonizing new environments which lead to their adaptive evolution. However, deciphering the genomic mechanism of ecological adaptation is difficult because ecological environments are often too complex for straightforward interpretation. Thus, we examined the adaptation of a widespread songbird-the rufous-capped babbler (Cyanoderma ruficeps)-to a relatively simple system: distinct environments across elevational gradients on the mountainous island of Taiwan. We focused on the genomic sequences of 43 birds from five populations to show that the Taiwan group split from its sister group in mainland China around 1-2 million years ago (Ma) and colonized the montane habitats of Taiwan at least twice around 0.03-0.22 Ma. The montane and lowland Taiwan populations diverged with gene flow between them, suggesting strong selection associated with different elevations. We found that the montane babblers had smaller beaks than the lowland ones, consistent with Allen's rule, and identified candidate genes-COL9A1 and SOX11-underlying the beak size changes. We also found that altitudinally divergent mutations were mostly located in noncoding regions and tended to accumulate in chromosomal inversions and autosomes. The altitudinally divergent mutations might regulate genes related to haematopoietic, metabolic, immune, auditory and vision functions, as well as cerebrum morphology and plumage development. The results reveal the genomic bases of morphological and physiological adaptation in this species to the low temperature, hypoxia and high UV light environment at high elevation. These findings improve our understanding of how ecological adaptation drives population divergence from the perspective of genomic architecture.

Cryptic species in a colorful genus: Integrative taxonomy of the bush robins (Aves, Muscicapidae, Tarsiger) suggests two overlooked species.

Several cryptic avian species have been validated by recent integrative taxonomic efforts in the Sino-Himalayan mountains, indicating that avian diversity in this global biodiversity hotspot may be underestimated. In the present study, we investigated species limits in the genus Tarsiger, the bush robins, a group of montane forest specialists with high species richness in the Sino-Himalayan region. Based on comprehensive sampling of all 11 subspecies of the six currently recognized species, we applied an integrative taxonomic approach by combining multilocus, acoustic, plumage and morphometric analyses. Our results reveal that the isolated north-central Chinese populations of Tarsiger cyanurus, described as the subspecies albocoeruleus but usually considered invalid, is distinctive in genetics and vocalisation, but only marginally differentiated in morphology. We also found the Taiwan endemic T. indicus formosanus to be distinctive in genetics, song and morphology from T. i. indicus and T. i. yunnanensis of the Sino-Himalayan mountains. Moreover, Bayesian species delimitation using BPP suggests that both albocoeruleus and formosanus merit full species status. We propose their treatment as 'Qilian Bluetail' T. albocoeruleus and 'Taiwan Bush Robin' T. formosanus, respectively.

Oxytocin variation and brain region-specific gene expression in a domesticated avian species.

The Bengalese finch was domesticated more than 250 years ago from the wild white-rumped munia (WRM). Similar to other domesticated species, Bengalese finches show a reduced fear response and have lower corticosterone levels, compared to WRMs. Bengalese finches and munias also have different song types. Since oxytocin (OT) has been found to be involved in stress coping and auditory processing, we tested whether the OT sequence and brain expression pattern and content differ in wild munias and domesticated Bengalese finches. We sequenced the OT from 10 wild munias and 11 Bengalese finches and identified intra-strain variability in both the untranslated and protein-coding regions of the sequence, with all the latter giving rise to synonymous mutations. Several of these changes fall in specific transcription factor-binding sites, and show either a conserved or a relaxed evolutionary trend in the avian lineage, and in vertebrates in general. Although in situ hybridization in several hypothalamic nuclei did not reveal significant differences in the number of cells expressing OT between the two strains, real-time quantitative PCR showed a significantly higher OT mRNA expression in the cerebrum of the Bengalese finches relative to munias, but a significantly lower expression in their diencephalon. Our study thus points to a brain region-specific pattern of neurochemical expression in domesticated and wild avian strains, which could be linked to domestication and the behavioral changes associated with it.

Domestication obscures genomic estimates of population history.

Domesticated species are valuable models to examine phenotypic evolution, and knowledge on domestication history is critical for understanding the trajectories of evolutionary changes. Sequentially Markov Coalescent models are often used to infer domestication history. However, domestication practices may obscure the signal left by population history, affecting demographic inference. Here we assembled the genomes of a recently domesticated species-the society finch-and its parent species-the white-rumped munia-to examine its domestication history. We applied genomic analyses to two society finch breeds and white-rumped munias to test whether domestication of the former resulted from inbreeding or hybridization. The society finch showed longer and more runs of homozygosity and lower genomic heterozygosity than the white-rumped munia, supporting an inbreeding origin in the former. Blocks of white-rumped munia and other ancestry in society finch genomes showed similar genetic distance between the two taxa, inconsistent with the hybridization origin hypothesis. We then applied two Sequentially Markov Coalescent models-psmc and smc++-to infer the demographic histories of both. Surprisingly, the two models did not reveal a recent population bottleneck, but instead the psmc model showed a specious, dramatic population increase in the society finch. Subsequently, we used simulated genomes based on an array of demographic scenarios to demonstrate that recent inbreeding, not hybridization, caused the distorted psmc population trajectory. Such analyses could have misled our understanding of the domestication process. Our findings stress caution when interpreting the histories of recently domesticated species inferred by psmc, arguing that these histories require multiple analyses to validate.

Not out of the woods yet: Signatures of the prolonged negative genetic consequences of a population bottleneck in a rapidly re-expanding wader, the black-faced spoonbill Platalea minor.

The long-term persistence of a population which has suffered a bottleneck partly depends on how historical demographic dynamics impacted its genetic diversity and the accumulation of deleterious mutations. Here we provide genomic evidence for the genetic effect of a recent population bottleneck in the endangered black-faced spoonbill (Platalea minor) after its rapid population recovery. Our data suggest that the bird's effective population size, Ne , had been relatively stable (7500-9000) since 22,000 years ago; however, a recent brief yet severe bottleneck (Ne  = 20) which we here estimated to occur around the 1940s wiped out >99% of its historical Ne in roughly three generations. Despite a >15-fold population recovery since 1988, we found that black-faced spoonbill population has higher levels of inbreeding (7.4 times more runs of homozygosity) than its sister species, the royal spoonbill (P. regia), which is not thought to have undergone a marked population contraction. Although the two spoonbills have similar levels of genome-wide genetic diversity, our results suggest that selection on more genes was relaxed in the black-faced spoonbill; moreover individual black-faced spoonbills carry more putatively deleterious mutations (Grantham's score > 50), and may therefore express more deleterious phenotypic effects than royal spoonbills. Here we demonstrate the value of using genomic indices to monitor levels of genetic erosion, inbreeding and mutation load in species with conservation concerns. To mitigate the prolonged negative genetic effect of a population bottleneck, we recommend that all possible measures should be employed to maintain population growth of a threatened species.

Egg recognition abilities of tit species in the Paridae family: do Indomalayan tits exhibit higher recognition than Palearctic tits?

Recent studies have shown that the closely related cinereous tit ( Parus cinereus) and green-backed tit ( P. monticolus) in China display strong egg recognition ability in contrast to tit species in Europe, which lack such ability. However, egg recognition in other populations of cinereous and green-backed tits and additional Paridae species still requires further research. Here, we compared the egg recognition abilities of cinereous tits across China, green-backed tits ( P. m. insperatus) in Taiwan, China, and five other species from the Paridae family, including the marsh tit ( Poecile palustris), varied tit ( Sittiparus varius), willow tit ( Poecile montanus), coal tit ( Periparus ater), and ground tit ( Pseudopodoces humilis). Results showed that the Hebei (58.8% egg rejection, n=17) and Liaoning populations (53.3%, n=15) of cinereous tits, and the Guizhou (100%, n=12) and Taiwan populations (75%, n=12) of green-backed tits all exhibited high egg recognition ability. The egg recognition ability of these tits was significantly greater than that of the other five species in the Paridae family. The varied tit (5.4%, n=37), marsh tit (8.3%, n=12), willow tit (Hebei: 25%, n=20; Beijing: 9.5%, n=21), coal tit (16.7%, n=18), and ground tit (0, n=5) species all showed low egg recognition abilities, with no significant differences found among them. Egg recognition was not associated with a single phylogenetic group but occurred in several groups of tits. In particular, those species widely distributed in the Indomalayan realm, thus overlapping with small cuckoo species, displayed strong egg recognition ability, whereas tit species in the Palearctic realm exhibited low or no egg recognition ability.

Strict allopatric speciation of sky island Pyrrhula erythaca species complex.

Increasing evidence of post-divergence gene flow between taxa is shifting our understanding on the mode of speciation. A fundamental question arises concerning the circumstances under which strict allopatric speciation still holds true. Sky island populations might undergo reduced gene flow by niche conservatism to highland habitats and follow divergence in an allopatric manner. In this study, we tested this hypothesis in the sky island Grey-headed Bullfinch (Pyrrhula erythaca) species complex via statistical analyses of both genetic and ecological data. Results of coalescent-based analysis of multiple nuclear loci suggested that P. e. owstoni likely colonized Taiwan island during the severe mid-Pleistocene glacial climate followed by strictly allopatric divergence from P. e. erythaca distributed in Himalayas-Hengduan mountains and central North China. Results of ecological niche modeling suggested that their speciation may be attributed to the niche conservatism of these birds and the lack of a suitable ecological corridor during subsequent milder glacial episodes. In addition, we delimited the traditionally defined P. erythaca into two full species, P. erythaca in the Asian mainland and P. owstoni on the island of Taiwan, based on both genetic and behavioural evidences. These results suggest that ecology can have a dynamic role in allowing highland populations to expand their ranges and isolated by habitat barriers to diversify in a strictly allopatric manner.

The Making of a Flight Feather: Bio-architectural Principles and Adaptation.

The evolution of flight in feathered dinosaurs and early birds over millions of years required flight feathers whose architecture features hierarchical branches. While barb-based feather forms were investigated, feather shafts and vanes are understudied. Here, we take a multi-disciplinary approach to study their molecular control and bio-architectural organizations. In rachidial ridges, epidermal progenitors generate cortex and medullary keratinocytes, guided by Bmp and transforming growth factor ß (TGF-ß) signaling that convert rachides into adaptable bilayer composite beams. In barb ridges, epidermal progenitors generate cylindrical, plate-, or hooklet-shaped barbule cells that form fluffy branches or pennaceous vanes, mediated by asymmetric cell junction and keratin expression. Transcriptome analyses and functional studies show anterior-posterior Wnt2b signaling within the dermal papilla controls barbule cell fates with spatiotemporal collinearity. Quantitative bio-physical analyses of feathers from birds with different flight characteristics and feathers in Burmese amber reveal how multi-dimensional functionality can be achieved and may inspire future composite material designs. VIDEO ABSTRACT.

To Trim or Not to Trim: Effects of Read Trimming on the De Novo Genome Assembly of a Widespread East Asian Passerine, the Rufous-Capped Babbler (Cyanoderma ruficeps Blyth).

Trimming low quality bases from sequencing reads is considered as routine procedure for genome assembly; however, we know little about its pros and cons. Here, we used empirical data to examine how read trimming affects assembled genome quality and computational time for a widespread East Asian passerine, the rufous-capped babbler (Cyanoderma ruficeps Blyth). We found that scaffolds assembled from raw reads were always longer than those from trimmed ones, whereas computational times for the former were sometimes much longer than the latter. Nevertheless, assembly completeness showed little difference among the trimming strategies. One should determine the optimal trimming strategy based on what the assembled genome will be used for. For example, to identify single nucleotide polymorphisms (SNPs) associated with phenotypic evolution, applying PLATANUS to gently trim reads would yield a reference genome with a slightly shorter scaffold length (N50 = 15.64 vs. 16.89 Mb) than the raw reads, but would save 75% of computational time. We also found that chromosomes Z, W, and 4A of the rufous-capped babbler were poorly assembled, likely due to a recently fused, neo-sex chromosome. The rufous-capped babbler genome with long scaffolds and quality gene annotation can provide a good system to study avian ecological adaptation in East Asia.

Frequent detection of anticoagulant rodenticides in raptors sampled in Taiwan reflects government rodent control policy.

Anticoagulant rodenticides (ARs) are known to cause extensive secondary exposure in top predators in Europe and North America, but there remains a paucity of data in Asia. In this study, we collected 221 liver samples from 21 raptor species in Taiwan between 2010 and 2018. Most birds were recovered from rescue organizations, but some free-ranging individuals were obtained from bird-strike prevention measures at airports. ARs were detected in 10 species and more than half of the total samples. Common rodent-eating Black-winged Kites (Elanus caeruleus) had the highest prevalence (89.2%) and highest average sum concentration (0.211 ±â€¯0.219 mg/kg), which was similar between free-ranging birds at airports and injured birds from rescue organizations. Scavenging Black Kites (Milvus migrans) and snake-eating Crested Serpent-eagles (Spilornis cheela) had the second highest prevalence or sum concentration, respectively. Seven different AR compounds were detected, of which brodifacoum was the most common and had the highest average concentration, followed by flocoumafen and bromadiolone. The frequency of occurrence in the three most numerous species (Black-winged Kite, Crested Goshawk [Accipiter trivirgatus], and Collared Scops-owl [Otus lettia]) was significantly higher in autumn than summer, which is consistent with the timing of the Taiwanese government's supply of free ARs to farmers. Regional differences in the detection of individual compounds also tended to reflect differences in human population density and use patterns (in agriculture or urban-dominated environments). Clinical poisoning was confirmed in Black Kites with sum concentrations as low as 0.026 mg/kg; however, further study of interspecific differences in AR sensitivity and potential population effects are needed. In addition, continued monitoring remains important given the Taiwanese government has modified their farmland rodent control policy to gradually reduce free AR supplies since 2015.

Population divergence and gene flow in two East Asian shorebirds on the verge of speciation.

Genetic isolation of populations over evolutionary time leads to the formation of independent species. We examined a pair of shorebirds - the Kentish Plover Charadrius alexandrinus and the enigmatic White-faced Plover C. dealbatus - which display strong plumage differentiation, yet show minimal genetic divergence based on previous mitochondrial and microsatellite work. Two scenarios may lead to this situation: (1) they represent clinal or poorly diverged populations with limited genomic differentiation despite substantial plumage variation, or (2) they are diverging taxa at the cusp of speciation, with ongoing limited gene flow obliterating signals of differentiation in traditional genetic markers. We compared the genotypes of 98 plovers (59 Kentish Plovers, 35 White-faced Plovers and 4 genomic hybrids) sampled in eastern Asia and Europe using ddRADSeq to harvest over 8000 genome-wide SNPs. In contrast to previous studies, our analyses revealed two well defined genomic clusters, with limited hybridization and a narrow contact zone. We also uncovered significant differences in bill length and further sex-specific differences in size, which may signal differences in mate choice between Kentish and White-faced Plovers. Our results support the hypothesis that this shorebird duo is on the verge of speciation.

Standing genetic variation as the predominant source for adaptation of a songbird.

What kind of genetic variation contributes the most to adaptation is a fundamental question in evolutionary biology. By resequencing genomes of 80 individuals, we inferred the origin of genomic variants associated with a complex adaptive syndrome involving multiple quantitative traits, namely, adaptation between high and low altitudes, in the vinous-throated parrotbill (Sinosuthora webbiana) in Taiwan. By comparing these variants with those in the Asian mainland population, we revealed standing variation in 24 noncoding genomic regions to be the predominant genetic source of adaptation. Parrotbills at both high and low altitudes exhibited signatures of recent selection, suggesting that not only the front but also the trailing edges of postglacial expanding populations could be subjected to environmental stresses. This study verifies and quantifies the importance of standing variation in adaptation in a cohort of genes, illustrating that the evolutionary potential of a population depends significantly on its preexisting genetic diversity. These findings provide important context for understanding adaptation and conservation of species in the Anthropocene.

From the Himalayas to a continental Island: Integrative species delimitation in the Brownish-flanked Bush Warbler Horornis fortipes complex.

As species serve as basic units of study in many fields of biology, assessments of species limits are fundamental for such studies. Here, we used a multilocus dataset and different coalescent-based methods to analyze species delimitation and phylogenetic relationships in the Brownish-flanked Bush Warbler Horornis fortipes complex, which is widespread in the Sino-Himalayan region. We also examined the vocal and morphometric divergence within this complex. Our genetic results suggested that Horornis fortipes is composed of at least three independently evolving lineages, which diverged 1.1-1.8 million years ago. However, these lineages have hardly diverged in song or morphometrics and only very slightly in plumage. Our result indicate that there are three incipient species in Horonis fortipes complex diverged in central Himalayas and Hengduan Mountains, but not between the continent and Taiwan island.

The niches of nuthatches affect their lineage evolution differently across latitude.

Ecological niche evolution can promote or hinder the differentiation of taxa and determine their distribution. Niche-mediated evolution may differ among climatic regimes, and thus, species that occur across a wide latitudinal range offer a chance to test these heterogeneous evolutionary processes. In this study, we examine (a) how many lineages have evolved across the continent-wide range of the Eurasian nuthatch (Sitta europaea), (b) whether the lineages' niches are significantly divergent or conserved and (c) how their niche evolution explains their geographic distribution. Phylogenetic reconstruction and ecological niche models (ENMs) showed that the Eurasian nuthatch contained six parapatric lineages that diverged within 2 Myr and did not share identical climatic niches. However, the niche discrepancy between these distinct lineages was relatively conserved compared with the environmental differences between their ranges and thus was unlikely to drive lineage divergence. The ENMs of southern lineages tended to cross-predict with their neighbouring lineages whereas those of northern lineages generally matched with their abutting ranges. The coalescence-based analyses revealed more stable populations for the southern lineages than the northern ones during the last glaciation cycle. In contrast to the overlapping ENMs, the smaller parapatric distribution suggests that the southern lineages might have experienced competitive exclusion to prevent them from becoming sympatric. On the other hand, the northern lineages have expanded their ranges and their current abutting distribution might have resulted from lineages adapting to different climatic conditions in allopatry. This study suggests that niche evolution may affect lineage distribution in different ways across latitude.

Tick-borne pathogens in ticks collected from birds in Taiwan.

BACKGROUND: A variety of human diseases transmitted by arthropod vectors, including ticks, are emerging around the globe. Birds are known to be hosts of ticks and can disperse exotic ticks and tick-borne pathogens. In Taiwan, previous studies have focused predominantly on mammals, leaving the role of birds in the maintenance of ticks and dissemination of tick-borne pathogens undetermined. METHODS: Ticks were collected opportunistically when birds were studied from 1995 to 2013. Furthermore, to improve knowledge on the prevalence and mean load of tick infestation on birds in Taiwan, ticks were thoroughly searched for when birds were mist-netted at seven sites between September 2014 and April 2016 in eastern Taiwan. Ticks were identified based on both morphological and molecular information and were screened for potential tick-borne pathogens, including the genera Anaplasma, Babesia, Borrelia, Ehrlichia and Rickettsia. Finally, a list of hard tick species collected from birds in Taiwan was compiled based on past work and the current study. RESULTS: Nineteen ticks (all larvae) were recovered from four of the 3096 unique mist-netted bird individuals, yielding a mean load of 0.006 ticks/individual and an overall prevalence of 0.13%. A total of 139 ticks from birds, comprising 48 larvae, 35 nymphs, 55 adults and one individual of unknown life stage, were collected from 1995 to 2016, and 11 species of four genera were identified, including three newly recorded species (Haemaphysalis wellingtoni, Ixodes columnae and Ixodes turdus). A total of eight tick-borne pathogens were detected, with five species (Borrelia turdi, Anaplasma sp. clone BJ01, Ehrlichia sp. BL157-9, Rickettsia helvetica and Rickettsia monacensis) not previously isolated in Taiwan. Overall, 16 tick species of five genera have been recorded feeding on birds, including nine species first discovered in this study. CONCLUSION: Our study demonstrates the paucity of information on ticks of birds and emphasizes the need for more research on ticks of birds in Taiwan and Southeast Asia. Moreover, some newly recorded ticks and tick-borne pathogens were found only on migratory birds, demonstrating the necessity of further surveillance on these highly mobile species.

Explosive radiation and spatial expansion across the cold environments of the Old World in an avian family.

Our objective was to elucidate the biogeography and speciation patterns in an entire avian family, which shows a complex pattern of overlapping and nonoverlapping geographical distributions, and much variation in plumage, but less in size and structure. We estimated the phylogeny and divergence times for all of the world's species of Prunella based on multiple genetic loci, and analyzed morphometric divergence and biogeographical history. The common ancestor of Prunella was present in the Sino-Himalayan Mountains or these mountains and Central Asia-Mongolia more than 9 million years ago (mya), but a burst of speciations took place during the mid-Pliocene to early Pleistocene. The relationships among the six primary lineages resulting from that differentiation are unresolved, probably because of the rapid radiation. A general increase in sympatry with increasing time since divergence is evident. With one exception, species in clades younger than c. 3.7 my are allopatric. Species that are widely sympatric, including the most recently diverged (2.4 mya) sympatric sisters, are generally more divergent in size/structure than allo-/parapatric close relatives. The distributional pattern and inferred ages suggest divergence in allopatry and substantial waiting time until secondary contact, likely due to competitive exclusion. All sympatrically breeding species are ecologically segregated, as suggested by differences in size/structure and habitat. Colonizations of new areas were facilitated during glacial periods, followed by fragmentation during interglacials-contrary to the usual view that glacial periods resulted mainly in fragmentations.

Himalayan black bulbuls (Hypsipetes leucocephalus niggerimus) exhibit sexual dichromatism under ultraviolet light that is invisible to the human eye.

Sexual dichromatism is a key proxy for the intensity of sexual selection. Studies of dichromatism in birds may, however, have underestimated the intensity and complexity of sexual selection because they used museum specimens alone without taking colour-fading into account or only measured conspicuous visual traits in live animals. We investigated whether the Himalayan black bulbul (Hypsipetes leucocephalus nigerrimus), which is sexually monomorphic to the human eye, exhibits sexual dichromatism distinguishable by a spectrometer. We measured the reflectance (within both the human visual perceptive and the ultraviolet ranges) of two carotenoid-based parts and eight dull and melanin-based parts for each individual live bird or museum skin sampled. According to an avian model of colour discrimination thresholds, we found that males exhibited perceptibly redder beaks, brighter tarsi and darker plumage than did females. This suggests the existence of multiple cryptic sexually dichromatic traits within this species. Moreover, we also observed detectable colour fading in the museum skin specimens compared with the live birds, indicating that sexual dichromatism could be underestimated if analysed using skin specimens alone.

Incipient speciation with gene flow on a continental island: Species delimitation of the Hainan Hwamei (Leucodioptron canorum owstoni, Passeriformes, Aves).

Because of their isolation, continental islands (e.g., Madagascar) are often thought of as ideal systems to study allopatric speciation. However, many such islands have been connected intermittently to their neighboring continent during recent periods of glaciation, which may cause frequent contact between the diverging populations on the island and continent. As a result, the speciation processes on continental islands may not meet the prerequisites for strictly allopatric speciation. We used multiple lines of evidence to re-evaluate the taxonomic status of the Hainan Hwamei (Leucodioptron canorum owstoni), which is endemic to Hainan, the largest continental island in the South China Sea. Our analysis of mitochondrial DNA and twelve nuclear loci suggests that the Hainan Hwamei can be regarded as an independent species (L. owstoni); the morphological traits of the Hainan Hwamei also showed significant divergence from those of their mainland sister taxon, the Chinese Hwamei (L. canorum). We also inferred the divergence history of the Hainan and Chinese Hwamei to see whether their divergence was consistent with a strictly allopatric model. Our results suggest that the two Hwameis split only 0.2 million years ago with limited asymmetrical post-divergence gene flow. This implies that the Hainan Hwamei is an incipient species and that speciation occurred through ecologically divergent selection and/or assortative mating rather than a strictly allopatric process.