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Iris colour in passerine birds: Why be bright-eyed?
Abstract and Figures
An initial survey of iris coloration in passerine birds (Aves: Passeriformes) showed that a brightly pigmented iris is much more common in southern African and Australian birds than in those from Europe, temperate North America, and Venezuela. However, the only statistical correlation reflected the distribution of particular bird families in these regions. Ten family-level groups considered to represent monophyletic taxa were then selected for a more detailed analysis, comparing iris coloration with distribution, status, taxonomy, plumage patterns, and some biological and behavioural characters for 1143 species. No pattern associating iris colour with particular traits was common to all families, but within families there were statistically significant associations with both plumage and biology. Our expectation that social behaviour would be an important predictor of iris colour was not supported, but critical information is still lacking for many species. Future studies of avian behavioural ecology should examine critically the role of iris coloration in individual species.
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... Sexual selection is one predominant force that contributes to avian dichromatism (Andersson 1994, Price 2008. Sex-based variation in avian eye color is reported by several authors such as Bortolotti et al. (2003), Craig and Hulley (2004), and Polakowski et al. (2020). Worthy (1978) showed that the dark-eyed birds tend to be quicker in feeding, flight, and escape when disturbed than the light-eyed birds. ...
... Another idea is that the iris coloration affects the amount of light that reaches the eye (Craig and Hulley 2004). The iris color could contribute to visual clarity (Savalli 1995). ...
Sexual dimorphism may appear in various forms such as anatomical, plumage, or vocal features. Production of such variation can be costly. However, sexual dimorphism may provide long-term fitness benefits. The Sri Lanka Bush Warbler (Elaphrornis palliseri) is a sexually monomorphic and monotypic passerine endemic to the montane cloud forests of the island of Sri Lanka. The iris of this bird shows a marked variation and individuals have either red or ivory-cream colored irises. We studied the iris dichromatism of Sri Lanka Bush Warbler with respect to morphometric and sexual differences. We used PCR-based molecular sexing (n = 8 individuals) and multivariate statistics to analyze morphometric data (n = 20). We found that the iris dichromatism of Sri Lanka Bush Warbler is sex-specific: all females examined (n = 6) had ivory-cream irises and both males examined had bright-red irises, whereas none of the phenotypic traits examined were sexually dimorphic. Our results suggest that the sexual dichromatism observed in this dense understorydwelling, cloud forest bird might play a central role as a short-distanced optical signal.
... 100% of the males expressed brown coloration in iris (n = 12 of this as something associated to the biochemical compound Pteridine, which he posited as the most common of the pigments responsible for eye coloration; later on confirmed by Oliphant (1987) (Dias, Goedert, and Macedo 2009). This prominent phenotypic trait, while varying interspecifically, can also express intraspecific variation whereas it could be attributed to maturation with age (Crook 1964, Pearson 1966, Ervin 1975, Picozzi 1981, Stutterheim 1981, Newton and Marquiss 1982, Wilkinson 1982, Craig 1984, Wilkinson 1988, Peterson 1991, Sweijd and Craig 1991, Craig and Hulley 2004, subspecies differentiation (Negro, Blázquez, and Galván 2017), and sexual dimorphism ( Hardy 1973, Hudon and Muir 1996, Pyle 1997, Craig and Hulley 2004. As such, there have been previous studies that used iris coloration as a determinant of age class, as well as a rapid and practical indicator of sex in sexually dimorphic species (Wood and Wood 1972, Trauger 1974, Rosenfield and Bielefeldt 1997, Smith et al. 2005, Nogueira and Alves 2008. ...
... 100% of the males expressed brown coloration in iris (n = 12 of this as something associated to the biochemical compound Pteridine, which he posited as the most common of the pigments responsible for eye coloration; later on confirmed by Oliphant (1987) (Dias, Goedert, and Macedo 2009). This prominent phenotypic trait, while varying interspecifically, can also express intraspecific variation whereas it could be attributed to maturation with age (Crook 1964, Pearson 1966, Ervin 1975, Picozzi 1981, Stutterheim 1981, Newton and Marquiss 1982, Wilkinson 1982, Craig 1984, Wilkinson 1988, Peterson 1991, Sweijd and Craig 1991, Craig and Hulley 2004, subspecies differentiation (Negro, Blázquez, and Galván 2017), and sexual dimorphism ( Hardy 1973, Hudon and Muir 1996, Pyle 1997, Craig and Hulley 2004. As such, there have been previous studies that used iris coloration as a determinant of age class, as well as a rapid and practical indicator of sex in sexually dimorphic species (Wood and Wood 1972, Trauger 1974, Rosenfield and Bielefeldt 1997, Smith et al. 2005, Nogueira and Alves 2008. ...
Eye color, particularly iris, plays a role in social signaling such as in mate recognition and sexual selection. In the Asiatic lineage of large frugivorous
hornbills, genus Buceros are known to exhibit sexual dichromatism in iris coloration that manifests
upon reaching definitive adult stage; except for
the Philippine endemic Buceros hydrocorax and
Buceros mindanensis, which is yet to be confirmed.
Institutions (n = 8) known to have Rufous Hornbills
(Buceros hydrocorax) in captivity were surveyed
for the subspecies, age, sex, iris coloration. Supplementary materials such as digital images of
Rufous Hornbills uploaded on the Internet Bird
Collection (https://www.hbw.com/ibc) were also
sampled. Combined, a total of 23 individuals were
sampled and identified as Buceros hydrocorax (n =
13), Buceros mindanensis mindanensis (n = 8), and
Buceros mindanensis semigaleatus (n = 2). Of the
23, 10 (n = 5:4:1) were males while 13 (n = 8:3:2)
were females. Images of Rufous Hornbills with
complete information from IBC (n = 6); Buceros
hydrocorax (n = 4), B. m. mindanensis (n = 1), and
B. m. semigaleatus (n = 1). 100% of the males expressed brown coloration in iris (n = 12). Likewise,
all females had pale blue iris (n = 17) regardless
of the subspecies. Fisher’s exact test results (p <
0.0001) suggest an association between sex and
iris color even with confidence level set at 95%, indicative that the iris colors are mutually exclusive.
By selecting images per subspecies, RGB values
plotted in Euclidean color space indicate subspecies differentiation between male species.
... Having a dull pigmented iris may have facilitated the accommodation of the smartphone device. Most species in Europe have a dull iris [28], as seen in our cohort. It is of note that fundus assessment with the Peek Retina ® device was acceptable or satisfactory in all eyes with a yellow iris, but in none of the eyes with an orange iris. ...
Ophthalmic examination is essential in the avian triage process in order to apply prompt therapeutic plans and evaluate rehabilitation potential. Fundoscopy is traditionally performed by direct or indirect ophthalmoscopy. Recent technological developments have enabled the design of a small-sized and affordable retinal imaging system to examine the fundus. We investigate the use of a smartphone-based device to realize fundus examination through a prospective cross- sectional observational study. Seventy-seven eyes of 39 birds of 15 different species were evaluated using the smartphone-based device in a rescue wildlife center. Pupil dilation was achieved prior to examination via rocuronium topical application. Assessment of fundus by the smartphone was classified as satisfactory, moderately satisfactory, and unsatisfactory. Fundus examination was also performed with a 20D, 30D, or 78D lens for comparison. Pupillary dilation was satisfactory, moderately satisfactory, or absent in 17, 32, and 28 eyes, respectively. Fundus examination with the smartphone-based device was satisfactory, moderately satisfactory, or unsatisfactory in 44, 15, and 18 eyes, respectively. The feasibility of the fundus examination was affected by the form of the globe; by the quality of pupil dilation; by the color of the iris (images could not be obtained from species with an orange, bright iris); and by the species, with owls (Strigiformes) being the easiest to observe.
Based on these findings, fundus examination was feasible in most bird species examined in this study.
... Intraspecific eye color variation is also common and often associated with age and sex in wild birds [8,[10][11][12]. Although the evolutionary drive shaping avian eye color remains largely unknown, recent studies have shed light on the possible coevolution of eye color and behavior or activity rhythm in birds [13][14][15][16]. Iris color variation may reflect unique evolutionary histories and ecological adaptions and thus provides a unique angle for understanding avian radiation, as well as pigmentation evolution across vertebrates. ...
The eye color of birds, generally referring to the color of the iris, results from both pigmentation and structural coloration. Avian iris colors exhibit striking interspecific and intraspecific variations that correspond to unique evolutionary and ecological histories. Here, we identified the genetic basis of pearl (white) iris color in domestic pigeons ( Columba livia ) to explore the largely unknown genetic mechanism underlying the evolution of avian iris coloration. Using a genome-wide association study (GWAS) in 92 pigeons, we mapped the pearl iris trait to a 9 kb region containing the facilitative glucose transporter gene SLC2A11B . A nonsense mutation (W49X) leading to a premature stop codon in SLC2A11B was identified as the causal variant. Transcriptome analysis suggested that SLC2A11B loss of function may downregulate the xanthophore-differentiation gene CSF1R and the key pteridine biosynthesis gene GCH1 , thus resulting in the pearl iris phenotype. Coalescence and phylogenetic analyses indicated that the mutation originated approximately 5,400 years ago, coinciding with the onset of pigeon domestication, while positive selection was likely associated with artificial breeding. Within Aves, potentially impaired SLC2A11B was found in six species from six distinct lineages, four of which associated with their signature brown or blue eyes. Analysis of vertebrate SLC2A11B orthologs revealed relaxed selection in the avian clade, consistent with the scenario that during and after avian divergence from the reptilian ancestor, the SLC2A11B-involved development of dermal chromatophores likely degenerated in the presence of feather coverage. Our findings provide new insight into the mechanism of avian iris color variations and the evolution of pigmentation in vertebrates.
... In birds, light-colored irides are proximately caused by an increase in the number of reflecting platelets and guanine, and a correlated reduction in the number of melanosomes, which tend to mask reflecting platelets in the iris stoma (Oehme, 1969;Oliphant, 1988;Hudon and Muir, 1996). The ultimate causes (e.g., adaptive function, if any) of interspecific variation in iris color remain poorly known (Zahavi and Zahavi, 1997;Craig and Hulley, 2004;Davidson et al., 2016). We compiled information about iris color from specimen labels and digital photographs uploaded to eBird.org ...
The genus Catharus is a monophyletic group of forest-dwelling passerine birds
known colloquially as nightingale-thrushes, or simply thrushes. This dissertation
integrates multiple sources of information including phylogenetic analyses of nuclear
and mitochondrial DNA sequences, morphometric analyses of study skins and eggs,
spectrographic analyses of vocalizations, and intensive research of primary sources in
libraries and archives, to achieve several interrelated objectives. Chapters 1–5 contain
a systematic review of historical literature to identify potential errors in Catharus
taxonomy and take corrective actions (neotypification) to stabilize nomenclature.
Chapters 6–7 contain phylogenetic reconstructions and analysis of geographic
variation in genetic and phenotypic characters in C. dryas and C. fuscater, two poorly
known Neotropical complexes, to evaluate species limits and test predictions of
competing mechanisms to explain sky island distributions. Chapter 8 contains an
investigation of the evolutionary causes and consequences of seasonal migration with
a phylogeny reconstructed with ultraconserved elements (UCEs), sequenced from a
nearly comprehensive sample of named taxa and dense geographic sampling, and a
similarly comprehensive original dataset of morphometric measurements.
Phylogenetic comparative analysis and maximum likelihood modeling are used to
characterize the migratory phenotype, reconstruct the ancestral state of migrationrelated
characters at relevant nodes of the Catharus phylogeny, and test predictions of
multiple interrelated hypotheses about the evolution of migration and the relationship
between migration and sexual selection.
... Unlike other areas of a bird's feather-covered body, irises have continuously been exposed to the elements throughout evolution and may thus have been under sustained selection pressure for color signaling, retaining chromatophores as a result (Oliphant et al. 1992). A few studies show that iris color is likely an important social signal in many species (Craig and Hulley 2004) and may also be under selection for reduced conspicuousness in open-nesting and nocturnal groups (Davidson et al. 2017;Passarotto et al. 2018), but overall the evolution and function of avian iris color has been vastly understudied. ...
The colorful phenotypes of birds have long provided rich source material for evolutionary biologists. Avian plumage, beaks, skin, and eggs—which exhibit a stunning range of cryptic and conspicuous forms—inspired early work on adaptive coloration. More recently, avian color has fueled discoveries on the physiological, developmental, and—increasingly—genetic mechanisms responsible for phenotypic variation. The relative ease with which avian color traits can be quantified has made birds an attractive system for uncovering links between phenotype and genotype. Accordingly, the field of avian coloration genetics is burgeoning. In this review, we highlight recent advances and emerging questions associated with the genetic underpinnings of bird color. We start by describing breakthroughs related to 2 pigment classes: carotenoids that produce red, yellow, and orange in most birds and psittacofulvins that produce similar colors in parrots. We then discuss structural colors, which are produced by the interaction of light with nanoscale materials and greatly extend the plumage palette. Structural color genetics remain understudied—but this paradigm is changing. We next explore how colors that arise from interactions among pigmentary and structural mechanisms may be controlled by genes that are co-expressed, co-expressed or co-regulated. We also identify opportunities to investigate genes mediating within-feather micropatterning and the coloration of bare parts and eggs. We conclude by spotlighting 2 research areas—mechanistic links between color vision and color production, and speciation—that have been invigorated by genetic insights, a trend likely to continue as new genomic approaches are applied to non-model species.
... Due to the inherent conspicuousness of the vertebrate eye [2], color patterns that seem to mimic eyes are often incorporated into signals and traits designed to be maximally salient [7]. However, the function of conspicuous real eyes, found in multiple species across all vertebrate classes (e.g., [16][17][18]), is largely unknown. Indeed, many fish are capable of greatly increasing their eye salience via rapid color change of chromatophores distributed across their irises [14,15,19], making them an ideal system for studying the adaptive function of eye coloration. ...
Understanding the adaptive function of conspicuous coloration has been a major focus of evolutionary biology for much of the last century. Although considerable progress has been made in explaining how conspicuous coloration can be used in functions as diverse as sexual and social signaling, startling predators, and advertising toxicity [1], there remain a multitude of species that display conspicuous coloration that cannot be explained by existing theory. Here we detail a new “matador-like” divertive antipredator strategy based on conspicuous coloration in Trinidadian guppies (Poecilia reticulata). Guppies encountering predatory fish rapidly enhance the conspicuousness of their eyes by blackening their irises. By pitting biomimetic robotic guppies against real predatory fish, we show this conspicuous eye coloration diverts attacks away from the guppies’ center of mass to their head. To determine the function of this seemingly counterintuitive behavior, we developed a method for simulating escape probabilities when live prey interact with ballistic attacking predators, and find this diversion effect significantly benefits black-eyed guppies because they evade capture by rapidly pivoting away from the predator once it has committed to its attack. Remarkably, this antipredator strategy reverses the commonly observed negative scaling relationship between prey size and evasive ability, with larger fish benefiting most from diverting predators. Taken together, our results introduce a new antipredator divertive strategy that may be widely used by conspicuously colored prey that rely on agility to escape their predators.
Passerines comprise over 50% of the known avian species on Earth, with over 6000 species in greater than 110 families distributed worldwide except for Antarctica (Barker et al. 2004). Classified as the “perching birds,” Passeriformes are able to perch on vertical surfaces (i.e., trees) owed to having an anisodactyl toe arrangement, that is, having three toes directed forward and a fourth directed backward. They are generally small but range from only a few grams (short-tailed pygmy tyrant Myiornis ecaudatus) to nearly 1.5 kg (Raven, Corvus corax). Most species are omnivores, with their diets varying seasonally (i.e., increasing insect consumption during the breeding season), although there are some primarily carnivorous species (e.g., shrikes).
Reintroducing apprehended wild animal in a natural environment is a complex process that involves many steps, including rehabilitating individuals and ensuring viable populations for reproduction; as such, the proportion between males and females to be reintroduced need to be considered carefully. The need of specialized techniques to identify sex on species that do not present discernible sexual dimorphisms can be a hardship to a successful reintroduction. Amazona aestiva, one of the most apprehended species on Brazil, is an example of such case, as sexing techniques employed for it often involves molecular or surgical procedures. Some authors, however, describe potential sexual dimorphisms that could be discernible to the human eye, one of those being an iris color dichromatism between males and females that could present a more conventional way to discern sex on this species. We analyzed the viability of sex identification by direct observation of iris sexual dichromatism, comparing suggestions by professionals familiarized with the species to molecular sexing by Polymerase Chain Reaction (PCR) and measuring color similarity between individuals using digital colorimetry. We found no significant correspondence between sex indication based on direct observation and molecular sexing results, and no relationship between iris color and sex by colorimetric analysis.
A species is here considered to be monogamous is 1) reported rates of polygamy do not regularly exceed c20%, and 2) there is considerable temporal and spatial variation in the rate below that percentage value. Types of variation in mate fidelity in monogamous birds are reviewed: extrapair copulations occurs both when the female is unwilling and where she solicits or tolerates such behaviour. Two lines of circumstances suggest that extrapair copulations may be part of an overall reproductive strategy - mate-guarding and territorial intrusion by males. Opportunistic and facultative polygyny may occur regularly, if in a minority of cases, in some species, as exemplified by consideration of North American passerines, European passerines, and non-passerines. Facultative polyandry is practised by at least 4 species.-P.J.Jarvis
Reviews the mating systems of 122 European passerines. Of all species, 39% are polygynous and 25 of the 47 polygynous species are regular polygynists. Of 46 polygynous species, 63% are monoterritorial. Regular polygyny is more common among polyterritorial passerines. Polygyny is frequently found among species breeding in open habitats (marshes, meadows, pastures and low shrubs). Polyterritorial species tend to have tropical winter quarters more frequently than monoterritorial species. Colonial breeding is absent from the polyterritorial group. Monogamous male passerines participate more frequently in feeding nestlings than their polygynous counterparts. Regular polygynists tend to be sexually dimorphic in plumage more often than infrequent polygynous passerines and polyterritorial passerines similarly tend to be plumage dimorphic more than monoterritorial species. Sexual size dimorphism was more pronounced in monoterritorials. -from Author
Silmmmy.-Two analyses were made relating eye color to size and quickversus-deliberate behavior in families of birds. The areas of behavior studied were flight, feeding and escape. The results indicated that dark-eyed birds tend to be quick and light-eyed birds tend to be deliberate. These behavioral differences are over and above the fact that light-eyed birds tend to be larger than dark-eyed birds. Based on prior studies of eye color and behavior in humans and animals (Worthy, 1974), it was hypothesized that dark eyes are associated with quick behavior and light eyes with deliberate behavior in birds. To test this hypothesis eye-color information was obtained for 3,264 of the species listed in Birds of the Woru: A Checklist (Clements, 1974). In those cases in which eye color differed with age or sex the color for adult males was recorded. For each family the proportion of species with dark (brown or black) eyes was determined. The families ranged in eye darkness from Hummingbirds, all 30 species in the sample dark-eyed, to Herons, all 47 species in the sample light-eyed. Two studies were done relating this variable, eye-darkness, to size and to measures of quick-versus-deliberate2 behavior in the areas of flight, feeding, and escape.
