Sexual dimorphism and dichromatism in Steere's Liocichla (Liocichla steerii)

ABSTRACT.   Although sexual differences in birds can be extreme, differences between males and females in body size and plumage color are more subtle in many species. We used a genetic-based approach to determine the sex of male and female Steere's Liocichla ( Liocichla steerii ) and examine the degree of size dimorphism and plumage dichromatism in this apparently monomorphic species. We found that males were significantly larger than females. In addition, Steere's Liocichla have a prominent yellow plumage patch on the lores that was significantly larger in males than females for both live birds and museum specimens. We also used reflectance spectrometry to quantify the color of the yellow-green breast feathers of Steere's Liocichla and found no significant differences between males and females in brightness, intensity, saturation, or hue. However, females tended to have brighter breast plumage, particularly at long wavelengths. Collectively, these color variables were useful in discriminating birds according to sex when used in a discriminant function analysis. Our study suggests that sexual selection may be more widespread than once assumed, even among birds considered monomorphic, and emphasizes the need for additional data from tropical and subtropical species.     

Effects of nestling condition on UV plumage traits in blue tits: an experimental approach

Intraspecific sexual and social communications are among themost important factors shaping costly color traits in birds.Condition capture models assume that only animals in superiorcondition can develop and maintain a colorful plumage. Althoughthere is good evidence that carotenoid-based components of plumagecolors show condition dependence, the situation is more controversialwith the underlying UV-reflecting structural component. We conducteda brood size manipulation in blue tits (Parus caeruleus) toinvestigate condition-dependent effects on plumage colorationin male and female offspring. Carotenoid chroma and UV reflectanceof the yellow breast plumage showed condition-dependent expressionin male and female fledglings. However, only males that wereraised in reduced broods had higher UV reflectance in the UV/bluetail feathers, whereas female tail coloration did not differbetween treatments. Our data suggest that there is a sex-specificeffect on the blue but not the yellow plumage and that thisis related to differences in the signaling function of bothplumage traits. Although sexual selection may already act onmale nestlings to develop colorful tail feathers for the nextbreeding season, the UV/yellow breast feathers are molted duringthe postjuvenile molt, and their signaling value is likely tobe important for both sexes during the extended postfledglingphase.     

Brightness variability in the white badge of the eagle owl Bubo bubo

The application of modern spectrometry to the study of avian colour variability has revealed ignored patterns of colour variation such as male‐biased sexual dichromatism and seasonal variability in the plumage. However, the variation in the achromatic property of such traits, that is in the total light reflectance of the spectrum (i.e., brightness), has commonly been overlooked. The evolution of signals based on brightness should be favoured in those species that are active when light is scarce, i.e. at dawn and dusk. The eagle owl Bubo bubo is monogamous and apparently monomorphic in plumage‐coloration. In this species, sexual and territorial call behaviour is mainly performed at dawn and dusk, during which a white patch on the throat is repeatedly exposed at each call. We measured the total light reflectance of the feathers of this badge in 39 eagle owl specimens from museum collections. We found seasonal variability and sexual dichromatism in the brightness of the plumage badge. The total reflectance of this trait peaked during the territorial‐mating period. Moreover, females showed higher values of brightness than males, in agreement with the reversed body size dimorphism present in this and many other raptor species. Finally, female but not male body size was positively correlated with white badge reflectance.     

Males in seemingly female‐like plumage do not mimic females: UV reflectance reveals temporal cryptic dimorphism in a manakin species exhibiting delayed plumage maturation

Manakins (Pipridae) are neotropical birds that usually exhibit delayed plumage maturation (DPM). Thus, while plumage of most adult male manakins is brightly conspicuous, subadult males and females are basically dull‐olive green. Although sexual dichromatism in some bird species may be evident only through UV reflectance, this phenomenon, known as hidden sexual dichromatism, has not been previously studied in manakins to compare subadult males and females. Within this framework, we carried out spectrophotometric analyses in searching for hidden sexual dichromatism in the white‐bearded manakin Manacus manacus, through comparison of UV spectra in females and subadult males in green plumage. Our results revealed UV reflectance in both sexes in green plumage. Moreover, we found UV spectral differences in homologous color patches between sexes, particularly at belly. Since the observed differences may allow intraspecific sex recognition of individuals in green plumage, our results do not support the female‐mimicry hypothesis to explain delayed plumage maturation in the white‐bearded manakin. Although our findings dismiss the female mimicry hypothesis, we cannot state whether these results support the non‐mutually exclusive cryptic and status signaling hypotheses. We propose then, that dull coloration of subadult males may serve both as a cryptic trait and to limit the energetic costs of acquiring the adult plumage before sexual maturity. Meanwhile, differential UV color traits between sexes in green plumage may allow adult males to avoid unnecessary energy expenditures in courtship displays in the presence of males near leks, and to selectively focus their the courtship displays on females. In accordance with the status signaling hypothesis, subadult males can be recognized both as males and subordinates and consequently may practice courtship displays without suffering aggressions by adult males. Our results highlight the importance to include a wider range of spectrophotometric information analyses for testing hypotheses regarding delayed plumage maturation.     

Plumage colour in nestling blue tits: sexual dichromatism,condition dependence and genetic effects

Sexual-selection theory assumes that there are costs associated with ornamental plumage coloration. While pigment-based ornaments have repeatedly been shown to be condition dependent, this has been more difficult to demonstrate for structural colours. We present evidence for condition dependence of both types of plumage colour in nestling blue tits (Parus caeruleus). Using reflectance spectrometry, we show that blue tit nestlings are sexually dichromatic, with males having more chromatic (more 'saturated') and ultraviolet (UV)-shifted tail coloration and more chromatic yellow breast coloration. The sexual dimorphism in nestling tail coloration is qualitatively similar to that of chick-feeding adults from the same population. By contrast, the breast plumage of adult birds is not sexually dichromatic in terms of chroma. In nestlings, the chroma of both tail and breast feathers is positively associated with condition (body mass on day 14). The UV/blue hue of the tail feathers is influenced by paternally inherited genes, as indicated by a maternal half-sibling comparison. We conclude that the expression of both carotenoid-based and structural coloration seems to be condition dependent in blue tit nestlings, and that there are additional genetic effects on the hue of the UV/blue tail feathers. The signalling or other functions of sexual dichromatism in nestlings remain obscure. Our study shows that nestling blue tits are suitable model organisms for the study of ontogenetic costs and heritability of both carotenoid-based and structural colour in birds.     

Brightness variability in the white badge of the eagle owl Bubo bubo

The application of modern spectrometry to the study of avian colour variability has revealed ignored patterns of colour variation such as male-biased sexual dichromatism and seasonal variability in the plumage. However, the variation in the achromatic property of such traits, that is in the total light reflectance of the spectrum (i.e., brightness), has commonly been overlooked. The evolution of signals based on brightness should be favoured in those species that are active when light is scarce, i.e. at dawn and dusk. The eagle owl Bubo bubo is monogamous and apparently monomorphic in plumage-coloration. In this species, sexual and territorial call behaviour is mainly performed at dawn and dusk, during which a white patch on the throat is repeatedly exposed at each call. We measured the total light reflectance of the feathers of this badge in 39 eagle owl specimens from museum collections. We found seasonal variability and sexual dichromatism in the brightness of the plumage badge. The total reflectance of this trait peaked during the territorial-mating period. Moreover, females showed higher values of brightness than males, in agreement with the reversed body size dimorphism present in this and many other raptor species. Finally, female but not male body size was positively correlated with white badge reflectance.     

Juvenile coloration of Florida Scrub-Jays ( Aphelocoma coerulescens ) is sexually dichromatic and correlated with condition

The Florida Scrub-Jay is a monogamous cooperative breeder in which both males and females display extensive structurally based blue plumage. Juveniles of this species exhibit blue tail and wing feathers that they begin growing as nestlings, and some of these feathers are retained throughout their first year. Although the birds appear to be sexually monochromatic, we assessed whether cryptic dichromatism exists in both the magnitude and pattern of coloration in tail feathers of juvenile Florida Scrub-Jays. We then determined whether variation in plumage coloration is associated with nutritional condition during molt. Tails of juvenile male Florida Scrub-Jays exhibit a greater proportion of UV reflectance than those of females. Mass at age 11 days and ptilochronology of the juvenile tail feathers were used as measures of individual nutritional condition during feather growth, and the latter was found to be positively associated with UV chroma. These data demonstrate that Florida Scrub-Jays are sexually dichromatic and suggest that variation in plumage color may be condition dependent, although we cannot rule out alternative explanations. Juvenile plumage coloration, therefore, has the potential to function as a signal of individual quality in both males and females.     

The reliability of achromatic displays is island-dependent in nocturnal Storm Petrels

Nocturnal birds rely on achromatic visual signals to assess rivals and potential mates, but variation in the expression of these displays has been understudied. Here we use UV-visible reflectance spectrometry to study colour variation and the potential signalling function of the dark brown chest and white rump plumage – a colour pattern conspicuously exhibited during twilight courtship displays – in nocturnal Mediterranean Storm Petrels Hydrobates pelagicus melitensis from three islands with different exposures to predation and human disturbance. Human activity may increase perceived predation risk and thus affect activity budgets and the physiological stress response, with possible consequences on the relationship between individual body condition and plumage coloration. We found that chest and especially rump feathers reflected in the ultraviolet (UV) but there was no evidence of sexual dichromatism. However, we found support for a male-specific signalling role of plumage coloration. Indeed, chest yellow–red chroma and rump UV chroma were positively related to body condition in males, but only in the two islands representing the poorer environmental conditions. Therefore, environmental variability can potentially modify the reliability of achromatic displays, with possible consequences on sexual selection patterns in a nocturnal bird.     

Elevational plumage divergence in the Rufous-capped Babbler (Cyanoderma ruficeps) on a mountainous island

Environment plays an important role in the evolution of plumage coloration in birds and may also lead to sexual dichromatism if males and females face different selection pressures. Mountains exhibit varying ecological conditions along their elevation gradient that may impose divergent selection on elevationally widespread species, causing intraspecific plumage divergence. For example, UV light environments often vary between montane and lowland habitats, which could potentially cause differences in plumage UV reflection between birds occurring in the two types of habitats. However, few studies have examined the effects of elevation on plumage evolution. In this study, we quantified the plumage coloration of the Rufous-capped Babbler Cyanoderma ruficeps from montane and lowland habitats on a mountainous island, Taiwan. We aimed to examine whether their plumage showed differences associated with changing ecological environments across the elevational gradient. The results supported that the plumage of babblers occupying montane habitats had higher UV-reflectance and brightness than that of lowland birds, corresponding to the higher UV intensity in montane than lowland background light environments. The elevational differences were mainly found across the ventral parts of babblers that had relatively higher levels of UV reflectance compared with their dorsal parts. Alternatively, the brighter plumage, with higher UV-reflectance in montane than lowland birds, might be mediated by physiological adaptation to other ecological factors, such as parasite pressures. The elevational differences in plumage UV-reflectance and brightness were more dramatic in males than in females. However, we found significant sexual dichromatism in different body parts between montane and lowland babblers in which females had brighter or stronger UV-associated coloration than males, suggesting that sexual selection has little impact on babbler plumage. Our study suggests the importance of elevational divergent selection associated with UV light or other ecological environments on avian plumage evolution.     

Melanin, carotenoid and structural plumage ornaments: information content and role in great tits Parus major

The importance of plumage colour as an indicator of individual quality and the basis of sexual selection has long been recognized. Of the three generally distinguished classes of plumage colours, melanin-based ornaments are traditionally considered to provide less reliable information than carotenoid-based traits. However, the role of structural ornaments in multiple signalling systems has rarely been examined, and no study has compared the information content and role of the three ornament types simultaneously. Here we investigated three plumage ornaments in great tits Parus major : the size of the melanin-based breast stripe, the carotenoid-based colour of the yellow breast and the structurally based reflectance properties of the black crown. We worked on both the mechanistic and the functional levels. First, we assessed the dependence of ornaments on body condition during moult using ptilochronology. Second, we estimated assortative mating for these traits, as a measure of mutual sexual selection. Only the spectral attributes of crown feathers correlated with body condition during moult. However, breast stripe size was related to age, while the brightness of the yellow breast indicated body size. Relative crown ultraviolet reflectance was much higher in males than in females. Assortative mating was strongest for crown ultraviolet reflectance, but composite measures suggest that a system of multiple sexually selected traits with different information content may work in this population. These data support the accumulating evidence that the condition-dependence of melanin and carotenoid coloration is not qualitatively different. They also suggest that more research should target the reflectance properties of dark plumage areas in general, and ultraviolet crown ornamentation in tits in particular.     

Egg Colour Covaries with Female Expression of a Male Ornament in the Spotless Starling (Sturnus unicolor)

The sexually selected egg colour hypothesis (SSECH) proposes that egg colouration is as a post-mating sexually selected signal of female phenotypic quality, maintained by a higher allocation of paternal care. Similarly, some female traits can reflect genetic quality or condition and males could use this information in mate choice or in modulating parental investment. In our study, we examined the correlation of individual variation in egg colouration with female expression of a male ornament and how male feeding covaried with these two female traits in the spotless starling, in which egg colour varies widely between clutches and where both sexes possess showy throat feathers that are age dependent and that may signal individual quality. According to the SSECH, high-quality females (females with longer throat feathers) are expected to lay more colourful eggs than low-quality females and males should modify their feeding behaviour accordingly. By means of a principal component analysis, we found that most of the variation in egg colouration was due to brightness differences, and in a lower proportion to chromatic variation. Chromatic variation reflected a ultraviolet (UV) vs. greenness trade-off and was positively associated with throat feather length: females with larger throat feathers laid eggs with higher UV and lower green reflectance. However, egg brightness was not related to female feather length, as the SSECH would predict. Male feedings were positively related to female throat feather length and negatively related to chromatic variation, meaning that males contributed more to nests of females with long throat feathers who laid eggs with higher UV and lower green reflectance. In conclusion, our data provide mixed support for the SSECH: although egg chromatic variation was related to female expression of a male ornament and male parental care, we found no evidence that egg brightness was involved in these processes.     

The role of sexual and natural selection in shaping patterns of sexual dichromatism in the largest family of songbirds (Aves: Thraupidae)

Males and females can be under different evolutionary pressures if sexual and natural selection is differentially operating in each sex. As a result, many species have evolved sexual dichromatism, or differences in coloration between sexes. Although sexual dichromatism is often used as an index of the magnitude of sexual selection, sexual dichromatism is a composite trait. Here, we examine the evolution of sexual dichromatism in one of the largest and most ecologically diverse families of birds, the tanagers, using the avian visual perspective and a species‐level phylogeny. Our results demonstrate that the evolutionary decreases of sexual dichromatism are more often associated with larger and more frequent changes in male plumage coloration, and evolutionary increases are not more often associated with larger changes in either sex. Furthermore, we show that the crown and ventral plumage regions are correlated with sexual dichromatism in males, and that only male plumage complexity is positively correlated with sexual dichromatism. Finally, we demonstrate that light environment is important in shaping both plumage brilliance and complexity. By conducting a multilevel analysis of plumage evolution in males and females, we show that sexual dichromatism evolves via a mosaic of sexual and natural selection in both sexes.     

Ultraviolet and carotenoid‐based coloration in the viviparous lizard Zootoca vivipara (Squamata: Lacertidae) in relation to age,sex, and morphology

Lizards display structural and pigment‐based colorations, and their visual system is sensitive to wavelengths of 300–700 nm. However, few studies in squamate reptiles have quantified interindividual colour variation that includes the structural ultraviolet (UV) component (300–400 nm). In the present study, we investigated variability of a ventral UV/yellow–red ornamentation in the common lizard Zootoca vivipara, including an analysis of spatial distribution, as well as sex and age differences. We also investigated whether the expression of coloration is related to body size and condition. Our analyses revealed two distinct patches: a gular patch with a strong UV reflectance and a belly patch with a dominant yellow–red reflectance. Males displayed a less saturated throat coloration with higher UV chroma and UV hue, and had a redder but duller belly coloration than females. Yearlings had less elaborate ornaments than adults, although they already displayed a yellow–red sexual dichromatism on the belly. UV sexual dichromatism was only apparent in adults as a result of a weaker UV reflectance in females, suggesting potential fitness costs of a bright UV coloration in that sex. Different colour traits were related to body size in both sexes, as well as to body condition in males. We discuss the potential evolutionary scenarios leading to the maintenance of this ornament in common lizards. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110 , 128–141.     

Ultraviolet plumage ornamentation affects social mate choice and sperm competition in bluethroats (Aves: Luscinia s. svecica): a field experiment

The blue throat feathers of male bluethroats (Luscinia s. svecica) show a reflectance peak in the ultraviolet (UV) waveband (320 to 400 nm). The throat is actively displayed during courtship, suggesting a role for sexual selection on an ultraviolet signal. Indeed, a recent aviary experiment demonstrated that females discriminated against males with artificially reduced UV reflectance (Andersson and Amundsen 1997). Here, we report the results of a similar experimental manipulation applied on free-ranging males. UV-reduced (UVR) males had a lower success in attracting mates, as judged from a significantly later start of egg laying, compared with control (C) males. UVR males also spent significantly less time advertising for additional mates when their own mate was fertile, and they had a lower success in achieving extra-pair fertilizations. Furthermore, UVR males tended to guard their mates more closely and lose more paternity in their own brood than C males did. We conclude that the treatment affected both social and extra-pair mate choice. This is the first experimental evidence that UV signalling influences male mating success in free-ranging birds.     

RECONSTRUCTING THE EVOLUTION OF SEXUAL DICHROMATISM: CURRENT COLOR DIVERSITY DOES NOT REFLECT PAST RATES OF MALE AND FEMALE CHANGE

Males of sexually dimorphic species often appear more divergent among taxa than do females, so it is often assumed that evolutionary changes have occurred primarily in males. Yet, sexual dimorphisms can result from historical changes in either or both of the sexes, and few previous studies have investigated such patterns using phylogenetic methods. Here, we describe the evolution of male and female plumage colors in the grackles and allies (Icteridae), a songbird clade with a broad range in levels of sexual dichromatism. Using a model of avian perceptual color space, we calculated color distances within and among taxa on a molecular phylogeny. Our results show that female plumage colors have changed more dramatically than male colors in the evolutionary past, yet male colors are significantly more divergent among species today. Historical increases in dichromatism have involved changes in both sexes, whereas decreases in dichromatism have nearly always involved females evolving rapidly to look like males. Dichromatism is also associated with mating system in this group, with monogamous taxa tending to exhibit relatively low levels of sexual dichromatism. Our findings suggest that, despite appearances, female plumage evolution plays a more prominent role in sexual dichromatism than is generally assumed.     

Evolution of sexual dichromatism in relation to nesting habits in European passerines: a test of Wallace's hypothesis

Wallace proposed in 1868 that natural rather than sexual selection could explain the striking differences in avian plumage dichromatism. Thus, he predicted that nesting habits, through their association with nest predation, could drive changes in sexual dichromatism by enabling females in cavity nesters to become as conspicuous as males, whereas Darwin (1871, The Descent of Man and Selection in Relation to Sex, John Murray, London) argued that sexual selection was the sole explanation for dichromatism. Sexual dichromatism is currently used as indicating the strength of sexual selection, and therefore testing Wallace's claim with modern phylogentically controlled methodologies is of prime interest for comparing the roles of natural and sexual selection in affecting the evolution of avian coloration. Here, we have related information on nest attendance, sexual dichromatism and nesting habits (open and cavity nesting) to male and female plumage conspicuousness in European passerines. Nest incubation attendance does not explain male or female plumage conspicuousness but nest type does. Moreover, although females of monochromatic and cavity nesting species are more conspicuous than females of other species, males of monochromatic and open nesting species are those with more cryptic plumage. Finally, analyses of character evolution suggest that changes in nesting habits influence the probability of changes in both dichromatism and plumage conspicuousness of males but do not significantly affect those in females. These results strongly suggest a role of nesting habits in the evolution of plumage conspicuousness of males, and a role for sexual selection also in females, both factors affecting the evolution of sexual dichromatism. We discuss our findings in relation to the debate that Darwin and Wallace maintained more than one century ago on the importance of natural and sexual selection in driving the evolution of plumage conspicuousness and sexual dichromatism in birds, and conclude that our results partly support the evolutionary scenarios envisaged by both extraordinary scientists.     

Sexually selected dichromatism in the hihi Notiomystis cincta: multiple colours for multiple receivers

Why do some bird species show dramatic sexual dichromatism in their plumage? Sexual selection is the most common answer to this question. However, other competing explanations mean it is unwise to assume that all sexual dichromatism has evolved by this mechanism. Even if sexual selection is involved, further work is necessary to determine whether dichromatism results from competition amongst rival males, or by female choice for attractive traits, or both. Here, we test whether sexually dichromatic hihi (Notiomystis cincta) plumage is currently under sexual selection, with detailed behavioural and genetic analyses of a free‐living island population. Bateman gradients measured for males and females reveal the potential for sexual selection, whilst selection gradients, relating reproductive success to specific colourful traits, show that there is stabilizing selection on white ear tuft length in males. By correlating colourful male plumage with different components of reproductive success, we show that properties of yellow plumage are most likely a product of male–male competition, whilst properties of the black and white plumage are an outcome of both male–male competition and female choice. Male plumage therefore potentially signals to multiple receivers (rival males and potential mates), and this may explain the multicoloured appearance of one of the most strikingly dichromatic species in New Zealand.     

Structural plumage colour and parasites in satin bowerbirds Ptilonorhynchus violaceus: implications for sexual selection

We investigated whether variation in structural plumage coloration in satin bowerbirds, Ptilonorhynchus violaceus , could reveal the intensity of infection from parasites, as predicted from models of parasite-mediated sexual selection (PMSS). To do this, we captured adult male, female, and juvenile male satin bowerbirds in Queensland, Australia, and objectively measured individual plumage reflectance from four body regions using a spectrometer. We quantified both ectoparasite load and the intensity of infection from blood parasites. In iridescent blue adult males, plumage reflectance is unimodal, with a single peak in the ultraviolet, while in greenish females and juveniles, plumage reflectance is bimodal, with peaks in both the ultraviolet and green portions of the spectrum. In adult males, the intensity of infection from blood parasites was best predicted by plumage brightness (total reflectance), with brighter males having fewer parasites. Similarly, juvenile males exhibiting greater UV chroma (proportion of reflectance in the UV) had fewer blood parasites. Our findings support a key prediction of PMSS models and provide the first evidence that a structural colour ornament can signal the intensity of infection from blood parasites.     

Ultraviolet vision, fluorescence and mate choice in a parrot, the budgerigar Melopsittacus undulatus.

As in many parrots, the plumage of the budgerigar Melopsittacus undulatus reflects near-ultraviolet (UVA) wavelengths (300-400 nm) and exhibits UVA-induced fluorescence. However, there have, to our knowledge, been no tests of whether the yellow fluorescence observed under intense UVA illumination has any role in signalling. Four experiments were carried out on wild-type budgerigars, where the presence and absence of UV reflectance and fluorescence were manipulated using filters. Few studies have attempted to separate the contribution of UV reflectance to plumage hue as opposed to brightness or distinguish between a role in sexual as opposed to social preferences. However, our first experiments show that not only do females consistently prefer UV-reflecting males, but also that the observed preferences are due to removal of UV affecting the perceived hue rather than brightness. Furthermore, we found no effect of the light environment on male response to females, suggesting that the female preferences relate to plumage colour per se. Whilst UV reflectance appears important in heterosexual choice by females, it has no detectable influence on same-sex association preferences. The results from the second series of experiments suggest that enhancement of the budgerigar's yellow coloration through fluorescence has no effect on male attractiveness. However, the fluorescent plumage may play a role in signalling by virtue of the fact that it absorbs UVA and so increases contrast with nearby UV-reflecting plumage. Our study provides convincing evidence that UV reflectances can play a role in mate choice in non-passerines, but no evidence that the yellow fluorescence observed under UVA illumination is itself important as a signal.     

麻雀两性羽色的比较

鸟类对色彩有较强的区分能力。基于鸟类视觉模型的研究发现,在人类看来类似的羽色,在鸟类眼中存在差别。本研究通过量化麻雀（Passer montanus saturatus）羽毛的反射光谱以及身体量度和喉部、耳羽的黑色斑块面积,比较其在雌鸟和雄鸟之间的差异。研究发现,麻雀雌鸟和雄鸟的身体量度、喉部和耳羽的斑块面积在繁殖季和非繁殖季均无显著差异。基于鸟类的视觉模型,麻雀头顶、喉部、耳羽、腰部的羽色,在雌鸟和雄鸟间无明显分化。基于上述结果,我们认为麻雀的雌鸟和雄鸟在外形上没有表现出性二型。