Plumage Coloration Research Articles

The draft genome of the Temminck’s tragopan (Tragopan temminckii) with evolutionary implications

BackgroundHigh-quality genome data of birds play a significant role in the systematic study of their origin and adaptive evolution. The Temminck’s tragopan (Tragopan temminckii) (Galliformes, Phasianidae), a larger pheasant, is one of the most abundant and widely distributed species of the genus Tragopan, and was defined as class II of the list of national key protected wild animals in China. The absence of a sequenced genome has restricted previous evolutionary trait studies of this taxa.ResultsThe whole genome of the Temminck’s tragopan was sequenced using Illumina and PacBio platform, and then de novo assembled and annotated. The genome size was 1.06 Gb, with a contig N50 of 4.17 Mb. A total of 117.22 Mb (11.00%) repeat sequences were identified. 16,414 genes were predicted using three methods, with 16,099 (98.08%) annotated as functional genes based on five databases. In addition, comparative genome analyses were conducted across 12 Galliformes species. The results indicated that T. temminckii was the first species to branch off from the clade containing Lophura nycthemera, Phasianus colchicus, Chrysolophus pictus, Syrmaticus mikado, Perdix hodgsoniae, and Meleagris gallopavo, with a corresponding divergence time of 31.43 million years ago (MYA). Expanded gene families associated with immune response and energy metabolism were identified. Genes and pathways associated with plumage color and feather development, immune response, and energy metabolism were found in the list of positively selected genes (PSGs).ConclusionsA genome draft of the Temminck’s tragopan was reported, genome feature and comparative genome analysis were described, and genes and pathways related to plumage color and feather development, immune response, and energy metabolism were identified. The genomic data of the Temminck’s tragopan considerably contribute to the genome evolution and phylogeny of the genus Tragopan and the whole Galliformes species underlying ecological adaptation strategies.

Back in black: melanin-rich skin colour associated with increased net diversification rates in birds.

Evolutionary biologists have long been interested in understanding the factors that promote diversification in organisms, often focussing on distinct and/or conspicuous phenotypes with direct effects on natural or sexual selection such as body size and plumage coloration. However, multiple traits that potentially influence net diversification are not conspicuous and/or might be concealed. One such trait, the dark, melanin-rich skin concealed beneath the feathers, evolved more than 100 times during avian evolution, frequently in association with white feathers on the crown and UV-rich environments, suggesting that it is a UV-photoprotective adaptation. Furthermore, multiple species are polymorphic, having both light and dark skin potentially aiding occupation in different UV radiation environments. As such these polymorphisms are predicted to occur in species with large latitudinal variation in their distribution. Furthermore, by alleviating evolutionary constraints on feather colour, the evolution of dark skin may promote net diversification. Here, using an expanded dataset on bird skin coloration of 3033 species we found that more than 19% of species had dark skin. In contrast to our prediction, dark skinned birds have smaller distribution ranges. Furthermore, both dark skin and polymorphism in skin coloration promote net diversification. These results suggest that even concealed traits can influence large scale evolutionary events such as diversification in birds.

Two High-Quality Cygnus Genome Assemblies Reveal Genomic Variations Associated with Plumage Color

As an exemplary model for examining molecular mechanisms responsible for extreme phenotypic variations, plumage color has garnered significant interest. The Cygnus genus features two species, Cygnus olor and Cygnus atratus, that exhibit striking disparities in plumage color. However, the molecular foundation for this differentiation has remained elusive. Herein, we present two high-quality genomes for C. olor and C. atratus, procured using the Illumina and Nanopore technologies. The assembled genome of C. olor was 1.12 Gb in size with a contig N50 of 26.82 Mb, while its counterpart was 1.13 Gb in size with a contig N50 of 21.91 Mb. A comparative analysis unveiled three genes (TYR, SLC45A2, and SLC7A11) with structural variants in the melanogenic pathway. Notably, we also identified a novel gene, PWWP domain containing 2A (PWWP2A), that is related to plumage color, for the first time. Using targeted gene modification analysis, we demonstrated the potential genetic effect of the PWWP2A variant on pigment gene expression and melanin production. Finally, our findings offer insight into the intricate pattern of pigmentation and the role of polygenes in birds. Furthermore, these two high-quality genome references provide a comprehensive resource and perspective for comparative functional and genetic studies of evolution within the Cygnus genus.

Avian nuptial plumage coloration changes differently when exposed to urban and rural areas

Avian plumage coloration could be altered by external factors like dust accumulation, but the effect of environmental conditions on such post-moult colour changes are poorly studied. Here, we investigated how exposure to the atmosphere in habitats of differing anthropogenic pressures modifies UV/blue structural plumage coloration — a sexually selected trait in eastern bluebirds Sialia sialis. We collected feather samples from live birds and distributed them in three types of habitats: (i) urban roads with low traffic, (ii) urban roads with high traffic, and (iii) rural roads with low traffic. After 4 weeks, the brightness of feathers decreased in all types of habitats-resulting in less-ornamented coloration. Changes in UV and blue chroma, however, varied with habitat type; chroma decreased in urban areas (becoming less ornamented), but increased in rural locations (becoming more ornamented). According to physiological models of avian colour vision, however, these changes in coloration tended to be below perceptual thresholds. Our results underscore the importance of how human activity can influence plumage colour of individual birds during the period between moult cycles which could have an impact on avian visual signalling. The study opens venues for further studies focusing on how the optical properties of specific airborne particles can influence coloration of wild-living animals.Significance statementPlumage coloration plays an important role in avian visual communication. In addition to moult-based change, coloration may change between moult cycles due to exposure to external factors like, for example, dust accumulation. Ecological factors modifying a passive plumage change, however, remains poorly studied. We collected samples of structurally coloured feather from a species wherein plumage colour is sexually selected and exposed them to outdoor conditions. We demonstrate that UV/blue chroma of coloration changes differently in urban and rural habitats. It increased in rural and decreased in urban areas. On the other hand, brightness decreased in both habitat types. Using physiological models of avian colour vision, however, we found that these colour changes were likely not perceptible to birds. Our study provides the first evidence that changes in coloration of fully grown feathers may be habitat dependent.

Distribution of phoretic mites and lice in Pseudolynchia canariensis living on pigeons and the relationship with seasonality, carrier sex, plumage coloration and age of definitive hosts

Among the parasites, some groups that have a limited capacity for locomotion, such as mites and lice, the transmission is challenging to win. These ectoparasites disperse through direct contact between hosts or, in some cases, through phoresy. However, these processes are not well-documented in detail because they are difficult to observe and quantify. In the present study, the patterns of distribution of skin mites and phoretic lice on hippoboscid louse fly Pseudolynchia canariensis sampled from Columba livia were evaluated. The analyzed pigeons were juveniles and adults, with three distinct plumage colors: blue checker, spread, or wild type, and were caught over 24 months. A total of 1,381 hippoboscid flies were collected on 377 hosts. The plumage color did not influence the infestation patterns of louse flies on juvenile and adult pigeons, nor did it influence the infestation patterns of skin mites and phoretic lice on the hippoboscid flies. However, the environmental temperature was directly related to higher prevalence, mean infestation intensity, and phoretic species richness on P. canariensis during the hottest seasons. Furthermore, a higher abundance of phoretic mite eggs, including embryonated eggs, was observed in females of P. canariensis in all seasons.

Losses of sexual dichromatism involve rapid changes in female plumage colors to match males in New World blackbirds.

Differences in coloration between the sexes (sexual dichromatism) can increase or decrease in a species through evolutionary changes in either or both sexes diverging or converging in their colors. Few previous studies, however, have examined the relative rates of such changes, particularly when dichromatism is lost. Using reflectance data from 37 species of the New World blackbird family (Icteridae), we compared evolutionary rates of plumage color change in males and females when dichromatism was either increasing (colors diverging) or decreasing (colors converging). Increases in dichromatism involved divergent changes in both sexes at approximately equal rates. Decreases in dichromatism, in contrast, involved changes in females to match male plumage colors that were significantly more rapid than any changes in males. Such dramatic changes in females show how selection can differ between the sexes. Moreover, these evolutionary patterns support the idea that losses of dimorphism involve genetic mechanisms that are already largely present in both sexes, whereas increases in dimorphism tend to involve the appearance of novel sex-specific traits, which evolve more slowly. Our results have broad implications for how sexual dimorphisms evolve.

Mutations in SLC45A2 lead to loss of melanin in parrot feathers.

Bird plumage coloration is a complex and multifactorial process that involves both genetic and environmental factors. Diverse pigment groups contribute to plumage variation in different birds. In parrots, the predominant green color results from the combination of 2 different primary colors: yellow and blue. Psittacofulvin, a pigment uniquely found in parrots, is responsible for the yellow coloration, while blue is suggested to be the result of light scattering by feather nanostructures and melanin granules. So far, genetic control of melanin-mediated blue coloration has been elusive. In this study, we demonstrated that feather from the yellow mutant rose-ringed parakeet displays loss of melanosome granules in spongy layer of feather barb. Using whole genome sequencing, we found that mutation in SLC45A2, an important solute carrier protein in melanin synthetic pathway, is responsible for the sex-linked yellow phenotype in rose-ringed parakeet. Intriguingly, one of the mutations, P53L found in yellow Psittacula krameri is already reported as P58A/S in the human albinism database, known to be associated with human OCA4. We further showed that mutations in SLC45A2 gene affect melanin production also in other members of Psittaculidae family such as alexandrine and plum-headed parakeets. Additionally, we demonstrate that the mutations associated with the sex-linked yellow phenotype, localized within the transmembrane domains of the SLC45A2 protein, affect the protein localization pattern. This is the first evidence of plumage color variation involving SLC45A2 in parrots and confirmation of associated mutations in the transmembrane domains of the protein that affects its localization.

Survival of a polymorphic species in seasonally snow‐covered forests

Color polymorphism is an adaptive strategy in which a species exhibits multiple color phenotypes in a population. Often, phenotypes are variably suited to different environmental conditions which may buffer the population against variable conditions. Modern climate change is creating novel selective pressures for many species, especially in winter habitats. Few studies have quantified the benefits of polymorphism for allowing species to cope with climate‐induced environmental change, particularly for species with more cryptic differences between morphs. We investigated how color polymorphism mediates selective pressures in ruffed grouse Bonasa umbellus, a winter‐adapted bird species of North American forests. Ruffed grouse display phenotypic variation in plumage color, ranging from red to gray. Over five winters (2015–2022), we monitored weather conditions, habitat use, and weekly survival for 94 grouse to test whether individuals had lower survival when phenotypically mismatched with snow cover (e.g. a gray bird on a snowless landscape or a red bird in snow). Grouse phenotypically mismatched with snow cover had lower survival, but only when winter survival rates were lowest. During winters of lower overall survival, red grouse exhibited higher survival during snow‐free periods, whereas gray grouse had higher survival when snow was present. We also found that open habitat negatively impacted survival, regardless of color. While the effect of phenotypic mismatch was variable among years, it was a stronger predictor of survival than land cover, suggesting that snow is an important habitat feature mediating overwinter survival. Our work offers an advancement in understanding how environmental variability affects geographic variation in and maintenance of multiple color phenotypes in seasonally‐snow covered environments. Our finding that interactions between color morph and snow cover are important for conferring winter survival provides further evidence that color polymorphism may serve as a buffer against rapidly changing conditions and a pathway for persistence of polymorphic species.Keywords: Bonasa umbellus, color polymorphism, phenotypic mismatch, ruffed grouse, winter ecology, Wisconsin

The Relationship Between Plumage Color, Genetic Group and Their Interaction on Quail’s Performance

The goal of this study is to identified the relationship between plumage color (white and brown), genetic group (Ukrainian and local) and their interaction on quail’s performance during their growth (1-6 weeks) and production (up to 8 weeks) periods. The study was conducted at Nineveh Research Department/Agricultural Research Office. The results for the growth period demonstrated that the Ukrainian quail was superior to the local quail for each of the characteristics of body weight, weight gain, feed consumption and Feed conversion ratio. The FC and FCR both increased significantly for the brown quail over the white quail for all weeks of the experiment, while the weekly weight gain fluctuated between the two colors. The brown quail also had a significant increase in body weight over the white quail. For the majority of the experiment’s weeks, the brown and white Ukrainian quail outperformed the brown and white local birds in most of the qualities being studied. During the production period, the Ukrainian quail’s FCR (g feed / g egg mass) and age at first egg’s lay were significantly lower than those of the local quail, The Ukrainian quail significantly outweighed the local quail in terms of the hen weight at first egg, first egg weight, the number, and weight of eggs produced during 8 weeks, eggs mass, eggs number/ female, hatching %, fertility %, and the hatching from the fertility %. The characteristics above did not appear to be affected by the white or brown plumage color, in comparison to local white and brown quail, the age at first egg and the FCR (g feed/g egg mass) were significantly lower in Ukrainian white and brown quail. Except for egg weight and food consumption /bird, all of the aforementioned variables were significantly higher in the Ukrainian white and brown quails than in the local white and brown quails.

The role of head and body cues in visual individual recognition in grey parrots (Psittacus erithacus).

Individual recognition underlies social behaviors in many species and is essential for complex social interactions commonly occurring between conspecifics. Focusing on visual perception, we explored this process in African grey parrots (Psittacus erithacus) using the matching-to-sample (MTS) method commonly used in primate research. We used cards made from photographs of familiar conspecific in four consecutive experiments, first testing the ability of our subjects (two male and one female adult) to match the photographs of familiar individuals and then creating modified stimuli cards to determine which visual aspects and features were crucial for successful recognition of a familiar conspecific. All three subjects were able to successfully match different photographs of familiar conspecifics in Experiment 1. Experiments 2-4 showed that modification of the facial area in the photograph had only a weak effect on subjects' success rates in MTS tasks. On the other hand, changes in the plumage color or obscuring of abdominal cues impaired their ability to successfully match conspecifics' photographs in some tasks. This study implies that African grey parrots process visual information holistically. Moreover, the process of individual recognition in this species differs from what we find in primates, including humans, where faces play a crucial role. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

Morphological descriptors of indigenous chickens: as a selection guide

ABSTRACT The visual morphologies in animals are associated positively or negatively with production, adaptation, and behavioral characteristics. For rural farmers with limited records, these features are considered reliable phenotypic markers. Thus, this study was intended to explore the unique morphological features of indigenous chickens as a farmer's selection guide. A total of 1,060 adult chickens from four rural districts were investigated for nine morphological traits. The single-comb type, supposed to be of good fertility, was the most prevalent. Yellow and white shanks were predominant, followed by gray, greenish-yellow, green, and black pigmentations. Crested and silky chickens were more frequent in highland areas, whereas naked-neck chickens were observed in lowlands. Chickens of the lowlands have brighter, more uniform plumage colors and yellow shank, beak, and earlobe colors. On the other hand, the highland and mid-altitude chickens are characterized by dark, colorful combinations of various feather colors: laced plumage, white shank, rose, and deformed rose combs. Moreover, multiple correspondence analyses revealed that morphological features have special associations with districts. Thus, the observed morphological heterogeneity of the indigenous chickens can be used as a good indicator of selective breeding. Therefore, the associations of these characteristics with productivity qualities could be confirmed by quantitative and molecular tools.

Population genomics reveals that a missense mutation in EDNRB2 contributes to white plumage color in pigeons

Plumage color is an important economic trait for breed feature identification and consumer's requirement in pigeons. The domestic pigeon has multiple type of plumage color, thereby providing a unique opportunity for identify the genetic basis of plumage coloration. White feather color is common for meat and medicinal use. To investigate the genetic variation associated with white plumage color in pigeons, we use genome resequencing and population genomics to identify the genomic regions with strong selective signature between pigeons with brown and white plumage color. Meanwhile, we obtained some candidate genes with melanin or melanosome biosynthesis in selected region. Finally, we identified a missense mutation p.E256K in the EDNRB2 completely associated with white plumage color. These findings provide a basis for genetic variation in pigeons with plumage color phenotype.

Plumage color degradation indicates reproductive effort: an experiment

Plumage color has traditionally been regarded as a static ornamental trait, but evidence is accumulating for significant color changes without molt that typically reduce the conspicuousness of ornamentation. In some species, the social partner seems to increase its reproductive investment if the color trait is experimentally enhanced, suggesting that color change could act as a signal. However, the information content of this signal is so far unclear. For example, birds in poor condition or making greater effort may deteriorate more severely. We used brood size manipulations to alter the reproductive effort of male and female collared flycatchers Ficedula albicollis. Both sexes showed less severe decline in some reflectance attribute of their white breast when their brood was experimentally reduced. In each sex, greater deterioration of the reflectance trait affected by the manipulation was accompanied by increased feeding rate by the partner. These feeding patterns do not prove, but are consistent with, a compensatory response by the partner to induced degradation. The manipulation effects on color change we detected confirm for the first time that plumage color deterioration can indicate current reproductive effort, thereby providing a potential fitness advantage to social partners that react to such deterioration.

Multiple ornaments: trade-offs and redundancy in signalling functions in male and female varied tits (Sittiparus varius)

Abstract Signals conveyed by ornaments can be indicators in the process of sexual selection. Specifically, signals may be superimposed on each other to represent an individual’s quality, convey different information, or offer redundant information. As much of the research in this area has focused on relationships between diverse signals in males, those of females have largely been overlooked. From 2022 to 2023, we tested whether three traits, namely acoustic (song display) and visual ornaments (patch size and carotenoid-based plumage coloration), and morphology (wing and tail length), were interrelated in males and whether visual ornaments and morphology were interrelated in females in a wild population of varied tits (Sittiparus varius). We found that males with brighter and more reddish plumage sang at lower maximum and minimum frequencies, with narrower bandwidths and lower frequency peaks. Similarly, a negative correlation between tail length and carotenoid-based plumage coloration was observed in both sexes. Furthermore, a positive relationship was found between signals within traits. These results highlight the trade-offs in signals between different traits, as well as the consistency within the same traits.

Monitoring and Automation System for Bird Feeding and Drinking Based on Internet of Things Using ESP32

Birds are vertebrate animals characterized by feathers and wings. Their melodious sounds and colorful plumage make them a popular choice for bird breeders, which presents a promising business opportunity. Manual feeding and drinking requires farmers to feed and drink every day, which often results in delayed feeding and drinking or inconsistent dosing, causing birds to become susceptible to disease and death. Therefore, this study is intended to create an automated system for monitoring feed and drink at predetermined times utilizing the Internet of Things (IoT) concept. The system's schedule can be remotely accessed and monitored through a website, and it operates in real-time without requiring manual intervention. This allows farmers to monitor and provide birds with food and water even from a distance. The Internet of Things (IoT) system utilized in this research employs an ESP32 microcontroller as the primary controller, which connects to a Wi-Fi network. It features a servo motor designed to refill bird feeders and drinkers. Further, a mini pump is used to replenish water in the drinker based on the owner's predetermined schedule set via the application. The research concludes that the tool functions based on the input time on the website. The system sends notifications to the breeder's smartphone regarding successful feeding and drinking, with an error rate of less than 20% in system testing and over 80% accuracy in system functioning. The outcome is in the form of a tool dispensing feed and drink daily, in accordance with the breeder's specified hours

Genetic mapping identifies SNP mutations in MITF-M promoter associated with melanin formation in Putian black duck

The coloration of plumage in poultry species has substantial economic significance. Putian black ducks encompass 2 distinct strains characterized by black and white plumage variations resulting from selective breeding. This study aimed to identify the molecular mechanisms responsible for plumage coloration in these 2 distinct strains. A comprehensive genome-wide association study was conducted using DNA data sourced from a F2 segregating population, consisting of 71 individuals with black plumage and 39 individuals with white plumage, derived from these distinct 2 strains. This analysis revealed that 894 nucleotide polymorphisms and identified 58 candidate genes. Subsequent Gene Ontology and Kyoto Encyclopedia of Genes and Genomes coenrichment analyses identified MITF as a key candidate gene implicated in melanin biosynthesis. Furthermore, extensive screening of significant polymorphic loci within MITF was carried out via mass spectrometry in 3 distinct populations: 100 individuals with black plumage and 100 individuals with white plumage from the F0 generation; and 50 with black plumage form the F1 generation). Eighteen candidate polymorphic loci were identified demonstrating significant associations with variations in black and white plumage. Notably, 8 of these loci were located within the 2,000 bp region upstream of MITF-M. To validate the critical regulatory role of MITF-M in black and white plumage formation, a dual-fluorescence reporter system was constructed, and dual-fluorescence activity was assessed. The results revealed that the fluorescence activity at wild-type sites (corresponding to black plumage) was significantly higher than that at the mutant-type sites (associated with white plumage) (P < 0.01). To corroborate the pivotal role of MITF-M in black and white plumage formation, qPCR was employed to evaluate the expression levels of various MITF variants in black and white feather bulbs. This analysis demonstrated that only MITF-M exhibited specific expression in black feather bulbs. These results elucidate the central role of polymorphic mutations within the MITF promoter region in the regulation of black and white plumage coloration in Putian black ducks. This study extends our understanding of mechanisms governing duck plumage coloration and provides valuable molecular markers for future research in duck production and breeding based on plumage coloration.

The impact of habitat and migration on plumage colour in Cardinalidae

Abstract Plumage coloration arises from a complex concert of selection pressures incorporating regional ecology, the light environment and genomic architecture. Here, we analyse the evolution of coloration in cardinals and grosbeaks (Cardinalidae) from the avian visual perspective and test the relative roles of life-history traits in shaping plumage evolution. We incorporate life-history data from three separate sources to analyse the correlated evolution of plumage coloration and ecological and habitat classifications for males and females. Our results show that males and females evolve under different axes of selection and that correlations with life-history traits differ between the sexes. We find that preferences for semi-open, fragmented habitat are correlated with male, but not female, plumage complexity. We also find that migration, long heralded as a driver of sexual dichromatism, is correlated with reduced plumage complexity in females, but not significantly correlated with male plumage coloration. Finally, our results showcase that user-defined categorical metrics can influence or, potentially, even mislead the interpretation of results, highlighting the need to measure habitat quantitatively rather than with subjective, categorical metrics.

An allelic series at the EDNRB2 locus controls diverse piebalding patterns in the domestic pigeon.

Variation in pigment patterns within and among vertebrate species reflects underlying changes in cell migration and function that can impact health, reproductive success, and survival. The domestic pigeon (Columba livia) is an exceptional model for understanding the genetic changes that give rise to diverse pigment patterns, as selective breeding has given rise to hundreds of breeds with extensive variation in plumage color and pattern. Here, we map the genetic architecture of a suite of pigmentation phenotypes known as piebalding. Piebalding is characterized by patches of pigmented and non-pigmented feathers, and these plumage patterns are often breed-specific and stable across generations. Using a combination of quantitative trait locus mapping in F2 laboratory crosses and genome-wide association analysis, we identify a locus associated with piebalding across many pigeon breeds. This shared locus harbors a candidate gene, EDNRB2, that is a known regulator of pigment cell migration, proliferation, and survival. We discover multiple distinct haplotypes at the EDNRB2 locus in piebald pigeons, which include a mix of protein-coding, noncoding, and structural variants that are associated with depigmentation in specific plumage regions. These results identify a role for EDNRB2 in pigment patterning in the domestic pigeon, and highlight how repeated selection at a single locus can generate a diverse array of stable and heritable pigment patterns.

Phenotype predicts interspecific dominance hierarchies in a cloud‐forest hummingbird guild

AbstractCompetition over resources often leads to intra‐ and interspecific interactions, which can be detrimental to the individuals involved. Thus, natural selection should favor communication systems that reliably convey information regarding the relative competitive abilities of an individual, reducing the need for physically damaging confrontation. Body size, sex, age, relatedness, and ornamentation are important factors determining dominance across diverse taxa in intraspecific interactions. These traits, when perceptible, may serve as signals across species in guilds that have frequent interspecific interactions. Hummingbirds provide a tractable system to study such community dynamics due to their high frequency of interactions, variable ornamentation, diverse body sizes, fast metabolism, and large overlap in resource utilization. Even in this system, potential interactions between morphology and coloration are rarely accounted for together when analyzing dominance between species. We take a novel approach to understanding interspecific dominance by assessing behavior, morphology, and coloration across different types of behavioral interactions. Across 11 tropical montane hummingbird species, we find that dominance is predicted by wing size and some metrics of plumage coloration. However, the biological significance of these factors varies between the different dominance behaviors performed. These results inform our understanding of interspecific signaling and its role in the evolution of intraguild communication and resource competition.

The mechanistic, genetic and evolutionary causes of bird eye colour variation

Birds display a rainbow of eye colours, but this trait has been little studied compared with plumage coloration. Avian eye colour variation occurs at all phylogenetic scales: it can be conserved throughout whole families or vary within one species, yet the evolutionary importance of this eye colour variation is under‐studied. Here, we summarize knowledge of the causes of eye colour variation at three primary levels: mechanistic, genetic and evolutionary. Mechanistically, we show that avian iris pigments include melanin and carotenoids, which also play major roles in plumage colour, as well as purines and pteridines, which are often found as pigments in non‐avian taxa. Genetically, we survey classical breeding studies and recent genomic work on domestic birds that have identified potential ‘eye colour genes’, including one associated with pteridine pigmentation in pigeons. Finally, from an evolutionary standpoint, we present and discuss several hypotheses explaining the adaptive significance of eye colour variation. Many of these hypotheses suggest that bird eye colour plays an important role in intraspecific signalling, particularly as an indicator of age or mate quality, although the importance of eye colour may differ between species and few evolutionary hypotheses have been directly tested. We suggest that future studies of avian eye colour should consider all three levels, including broad‐scale iris pigment analyses across bird species, genome sequencing studies to identify loci associated with eye colour variation, and behavioural experiments and comparative phylogenetic analyses to test adaptive hypotheses. By examining these proximate and ultimate causes of eye colour variation in birds, we hope that our review will encourage future research to understand the ecological and evolutionary significance of this striking avian trait.