The development of plumage polymorphism in male Madagascar paradise flycatcher Terpsiphone mutata

The development of plumage polymorphism in male Madagascar paradise flycatcher Terpsiphone mutata (Linnaeus, 1766) was investigated in north‐western Madagascar. Four distinct morphs were observed, namely, white‐coloured with long tails (WL), rufous‐coloured with long tails (RL), rufous‐coloured with tails of middle length (RM), and rufous‐coloured with short tails (RS). Females were rufous and had short tails. Indivudual males were marked and re‐observed during 1994–98. RS males changed to RM in the following seasons. Among RM males, some individuals retained RM, others changed to RL or WL. RM males which changed to WL in the next season had possessed white patches on their belly, whereas RM males which changed to RL had not had such patches. Neither WL nor RL males changed their morphs. Nestlings were rufous regardless of their father's morph. One nestling changed to RS in the next season. From these results and the difference of body size among four morphs, it is deduced that RS males are one year old, and change to RM males in the next season. RM males change to RL or WL males after one or two years. RL and WL are two stable terminal plumages, and they may be genetically determined morphs.     

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.     

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.     

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.     

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.     

The effects of elevated testosterone on plumage hue in male House Finches

The majority of studies examining the role of hormones in the proximate mechanisms of plumage coloration in birds have focused on intersexual differences (plumage dichromatism) and on structural- or melanin-based plumage coloration. The relationship between hormones and carotenoid-based plumage color, and in particular intrasexual plumage color variation, has received little attention. We manipulated testosterone levels of both captive and wild male House Finches to determine whether testosterone influences the expression of male plumage color in this species. We found that in captive male House Finches elevated testosterone delayed molt and resulted in drabber, less red plumage, even when birds were supplemented with dietary carotenoids. Elevated testosterone also resulted in drab plumage color in wild males, and appeared to delay molt in wild birds as well. Wild males implanted with testosterone showed wide variation in expression of plumage coloration. Those implanted early in the year molted plumage similar in color to their pre-treatment plumage, but those implanted later molted substantially duller plumage, possibly because delayed molt resulting from elevated testosterone caused these males to molt when carotenoid pigments were not available in sufficient amounts. These observations have the potential to explain previously reported relationships between plumage color and behavior in male House Finches, and highlight the importance of considering the proximate mechanisms of plumage coloration in avian sexual selection.     

Signals of quality and age: the information content of multiple plumage ornaments in male western bluebirds Sialia mexicana

Male structural plumage coloration and UV signals in particular can provide information on individual quality and influence female choice, while melanin-pigmented plumage is largely considered to be important in intrasexual competition. Many avian species demonstrate both types of plumage ornamentation that may convey different information about the signaller's quality or condition in addition to age. We examine rufous and blue plumage ornamentation across multiple body regions in relation to age, condition and reproductive performance in male western bluebirds Sialia mexicana. We demonstrate a strong positive relationship between head plumage brightness and both male age and the mass of the offspring. Older males are in better condition and display a reduced plumage patch on the back while the size of the rufous breast patch increases with increasing condition but not with age. Spectral characters from the wings and rump were not associated with any of the reproductive parameters measured. In conjunction with published evidence showing that females preferentially accept extrapair copulations from older males, these data suggest a need for experimental manipulation of plumage colour in known-aged birds to understand mate choice in this species.     

DELAYED MATURATION,NEOTENY, AND SOCIAL SYSTEM DIFFERENCES IN TWO MANAKINS OF THE GENUS CHIROXIPHIA

Long-tailed manakins (Chiroxiphia linearis) and swallow-tailed manakins (C. caudata) are closely related, sexually dichromatic, lek-breeding species in which male mating success is highly skewed. Males of both species delay plumage maturation. Before reaching the definitive state, they wear a sequence of feather coats less conspicuous than that of the adult. Nondefinitive plumages probably enhance male survival in the two species; in C. caudata they may also enhance breeding success of young males, who may be fully reproductively mature their first year. In C. linearis testicular development is retarded along with that of plumage, although males may be physiologically capable of breeding prior to the acquisition of the definitive plumage. This difference probably reflects differences in the social systems of the two species. Five hypotheses have been proposed to explain the evolution of delayed plumage maturation. The sexual-selection, cryptic-breeder, and winter-adaptation hypotheses suggest that it functions primarily to enhance survival of young males. The juvenile- and female-mimicry hypotheses emphasize enhancement of immediate mating success. Support is provided for all but the female-mimicry hypothesis; it is argued that data are more consistent with juvenile mimicry and a neotenic origin of nondefinitive plumages.     

SEXUAL DICHROMATISM IN BIRDS: IMPORTANCE OF NEST PREDATION AND NEST LOCATION FOR FEMALES VERSUS MALES

Examinations of variation in plumage dichromatism in birds have focused on male plumage brightness and largely neglected variation in female plumage brightness. Nest predation previously was concluded to constrain male brightness and thereby reduce dimorphism in ground-nesting birds based on an incorrect assumption that nest predation is greater for ground nests. Correlations of plumage brightness and dichromatism with nest predation have never been tested directly and we do so here with data for warblers (Parulinae) and finches (Carduelinae). We show that male plumage brightness varies among nest heights, but in a pattern that is not correlated with nest predation. Female plumage brightness also varies among nest heights, but in a pattern that differs from males, and one in which variation in female plumage brightness was negatively correlated with nest predation. These results suggest that nest predation may place greater constraints on female than male plumage brightness, at least in taxa where only females incubate eggs and brood young. These results also show that female plumage patterns vary at least partly independently of male patterns and emphasize the need to include consideration of both female and male plumage variation in tests of plumage dimorphism. Plumage dimorphism differs between ground and off-ground nesters as previously described and, if anything, the relationship between plumage dimorphism and nest predation was positive rather than negative as previously argued.     

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.     

Reversed plumage ontogeny in a female hummingbird: implications for the evolution of iridescent colours and sexual dichromatism

In polygynous birds, bright plumage is typically more extensive in the sexually competitive males and develops at or after sexual maturity. These patterns, coupled with the importance of male plumage in sexual displays, fostered the traditional hypothesis that bright plumages and sexual dichromatism develop through the actions of sexual selection on males. This view remains problematic for hummingbirds, all of which are polygynous, because their bright iridescent plumages are also important non-sexual signals associated with dominance at floral nectar sources. Here I show that female amethyst-throated sunangels [ Heliangelus amethysticollis (d'Orbigny & Lafresnaye)], moult from an immature plumage with an iridescent gorget to an adult plumage with a non-iridescent gorget. This 'reversed' ontogeny contradicts the notion that iridescent plumage has a sexual function because sexual selection in polygynous birds should be lowest among non-reproductive immature females. Moreover, loss of iridescent plumage in adult females indicates that adult sexual dichromatism in H. amethysticollis is due in large part to changes in female ontogeny. I suggest that both the ontogeny and sexual dichromatism evolved in response to competition for nectar.     

Delayed plumage maturation and delayed reproductive investment in birds

Delayed plumage maturation is the delayed acquisition of a definitive colour and pattern of plumage until after the first potential breeding period in birds. Here we provide a comprehensive overview of the numerous studies of delayed plumage maturation and a revised theoretical framework for understanding the function of delayed plumage maturation in all birds. We first distinguish between hypotheses that delayed plumage maturation is attributable to a moult constraint with no adaptive function and hypotheses that propose that delayed plumage maturation is a component of an adaptive life‐history strategy associated with delayed reproductive investment. We then recognize three potential benefits of delayed plumage maturation: crypsis, mimicry and status signaling. Evidence suggests that delayed plumage maturation is not a consequence of developmental constraints and instead represents a strategy to maximize reproductive success in circumstances where young adults cannot effectively compete with older adults for limited resources, particularly breeding opportunities. A multi‐factorial explanation that takes into account lifespan and the degree of competition for limited breeding resources and that combines the benefits of an inconspicuous appearance with the benefits of honest signaling of reduced competitiveness provides a general explanation for the function of delayed plumage maturation in most bird species. Delayed plumage maturation should be viewed as a component of alternative reproductive strategies that can include delay in both plumage and sexual development. Such strategies are frequently facultative, with individuals breeding prior to the acquisition of definitive plumages when conditions are favourable. Presumably, the benefits of delayed plumage maturation ultimately enhance lifetime reproductive success, and studying delayed plumage maturation within the context of lifetime reproductive success should be a goal of future studies.     

Cryptic sexual dichromatism occurs across multiple types of plumage in the Green-backed Tit Parus monticolus

Several recent studies have found instances of cryptic sexual dichromatism within avian taxa. Although this dichromatism has been found in plumage produced through a variety of proximate mechanisms, little is known about how dichromatism varies across these types of plumage within a single species. We used a reflectance spectrometer to measure colour within the Green-backed Tit Parus monticolus , a species which displays multiple types of pigment and structural colours. We found significant differences in spectral measurements corresponding to hue, chroma, and brightness between male and female carotenoid, melanin, structural white, grey and structural blue plumage. The only plumage that did not appear to show sexual dichromatism was the olive plumage of the back. These findings suggest that the mechanism(s) producing cryptic dichromatism in the Green-backed Tit are non-specific and act across multiple types of plumage, rather than within a single type, such as carotenoid-based or structurally produced.     

DELAYED PLUMAGE MATURATION IN PASSERINE BIRDS: RELIABLE SIGNALING BY SUBORDINATE MALES?

Three hypotheses (Cryptic, Female Mimicry, and Winter Adaptation) have been proposed to explain the occurrence of delayed plumage maturation (DPM) in passerine birds. We show that each of these hypotheses is really a composite of two different questions about: 1) the proximate function of dull plumage in second year (SY) males and 2) the selective mechanism that has favored that proximate function. We review the three hypotheses in the context of this distinction, and we find little evidence clearly supporting any of them. We propose a new Status Signaling Hypothesis (SSH) suggesting that dull SY male plumage is a reliable signal of subordinate. We suggest that female choice based on male plumage color (as an index of male quality) is the selective mechanism that has favored subordinate status signaling by SY males. If females prefer bright males, then dull plumage may be a reliable signal of subordinance and SY males may experience reduced levels of aggression from adult males. Male characters (like plumage color) are most likely to be the object of female choice when males defend simple nesting territories with little or no variation in territory quality. In such a system, SY males with low resource-holding potential would benefit (via matings or experience) by signaling subordinance and being allowed to settle among more brightly colored adults. Thus, DPM is expected to be more prevalent when males defend simple nesting territories. This prediction of the SSH is supported by data from the literature—a significantly higher proportion of species with DPM defend simple nesting territories (versus all-purpose territories) than do species without DPM.     

Sexual dichromatism in the yellow-breasted chat Icteria virens: spectrophotometric analysis and biochemical basis

Sexual dimorphism or dichromatism has long been considered the result of sexual selection. However, for many organisms the degree to which sexual dichromatism occurs has been determined within the confines of human perception. For birds, objective measures of plumage color have revealed previously unappreciated sexual dichromatism for several species. Here we present an unbiased assessment of plumage dichromatism in the yellow-breasted chat Icteria virens . Chats exhibit yellow to orange throat and breast plumage that to the unaided human observer differs only subtly in color. Spectrophotometric analyses revealed that chat throat and breast feathers exhibited reflective curves with two peaks, one in the ultraviolet and one in the yellow end of the spectrum. We found differences in both the shape and magnitude of reflectance curves between males and females. Moreover, for feathers collected from the lower edge and middle of the breast patch, male plumage reflected more light in the ultraviolet and yellow wavelengths compared to females, whereas male throat feathers appeared brighter than those of females only in the ultraviolet. Biochemical analyses indicated that the plumage pigmentation consisted solely of the carotenoid all- trans lutein and we found that males have higher concentrations of plumage carotenoids than females. Feathers that were naturally unpigmented reflected more UV light than yellow feathers, suggesting a potential role of feather microstructure in UV reflectance.     

Plumage color as a status signal in male–male interaction in the red-flanked bushrobin, Tarsiger cyanurus

Recent studies have suggested that structural-based coloration is an honest signal of male genetic and/or conditional quality in sexual selection. However, whether structural coloration functions in intrasexual competition is unknown. We examined whether plumage color functions as a status signal during intrasexual interactions in the red-flanked bushrobin Tarsiger cyanurus; adult males have many blue plumes as structural coloration whereas yearling males and females are olive brown with few blue plumages. Blue males did not always dominate olive-brown males. The number of interactions did not differ with the colors of the two birds involved. The interactions of a blue male and an olive-brown male were less aggressive than those of two blue or of two olive-brown males. In this study, we found that structural plumage coloration may serve as a signal of aggressive intent and lower the escalation level of an aggressive interaction in a manner consistent with hypotheses regarding the evolution of delayed plumage maturation.     

Ontogenetic colour change signals sexual maturity in a non-territorial damselfly

Conspicuous colouration increases male reproductive success through female preferences and/or male–male competition. Despite the advantages of conspicuous colouration, inconspicuous male morphs can exist simultaneously in a population due to genetic diversity, condition dependence or developmental constraints. We are interested in explaining the male dichromatism in Xanthagrion erythroneurum damselflies. We reared these damselflies in outdoor insectaries under natural conditions and showed that this species undergoes ontogenetic colour changes. The younger males are yellow and change colour to red 6–7 days after their emergence. We took red and yellow male reflectance spectra and found that red males are brighter than yellow males. Next, we aimed to determine whether ontogenetic colour change signals sexual maturity with field observations and laboratory experiments. Our field observational data showed that red males are in higher abundance in the breeding territory, and they have a higher mating frequency than yellow males. We confirmed these field observations by enclosing a red and a yellow male with two females and found that yellow males do not mate in presence of red males. To determine whether colour change signals sexual maturity, we measured mating success of males before and after colour changes by enclosing a single male at different age (day 3-day 7) and colour (yellow, intermediate and red) with a single female in a mating cage. Males did not mate when yellow but the same male mated after it changed colour to red, suggesting the ontogenetic colour change signals sexual maturity in this species. Our study shows that male dichromatism can be age-dependent and ontogenetic colour change can signal age and sexual readiness in non-territorial insects.     

Nest shape explains variation in sexual dichromatism in New World blackbirds

Following Charles Darwin, research on sexual dichromatism has long focused on sexual selection driving ornamentation in males. However, Alfred Russel Wallace proposed another explanation – that dichromatism evolves as a result of selection favoring crypsis in incubating females. Many recent studies suggest that evolutionary changes in sexual dichromatism often result from changes in female, in addition to male, plumage, yet the evolutionary mechanisms driving changes in female plumage remain largely unexplained. To test Wallace's hypothesis, we examined variation in sexual dichromatism and nest shape, a proxy for predation risk, among New World blackbirds (Aves: Icteridae). Phylogenetic models reveal an evolutionary correlation between sexual dichromatism and nest exposure. Specifically, we found that transitions in monochromatic lineages with exposed nests toward either concealed nests or dichromatism were common. Although this evidence supports Wallace's hypothesis that female incubation leads to selection for crypsis or concealment, we also found that transitions to monomorphism were common, even in lineages with exposed nests – a result suggestive of a role for positive selection on female ornamentation. These patterns of plumage evolution support a growing body of work emphasizing the importance of developing and testing hypotheses to explain evolutionary changes in female, as well as male, ornamentation.     

Melanin‐based sexual dichromatism in the Western Palearctic avifauna implies darker males and lighter females

Melanins are the most common pigments providing coloration in the plumage and bare skin of birds and other vertebrates. Numerous species are dichromatic in the adult or definitive plumage, but the direction of this type of sexual dichromatism (i.e. whether one sex tends to be darker than the other) has not been thoroughly investigated. Using color plates, we analysed the presence of melanin‐based color patches in 666 species belonging to 69 families regularly breeding in the Western Palearctic. Sexual dichromatism based on melanins in at least one integumentary part involved 205 (30.7%) species. The body parts contributing more frequently to dichromatism were the dorsal areas, head and breast, whereas the less dichromatic body parts were the belly and the exposed integumentary parts (i.e. bill and legs). Regarding the phylogenetic spread of dichromatisms, 37 (53.6%) families contained at least one species with melanin‐based sexual dimorphism in the definitive adult plumage. As for the direction of the color difference, males are darker than females in a majority of species, meaning that males tend to produce more eumelanin and females tend to synthesize more pheomelanin. This survey has revealed the high prevalence of melanins in the emergence of sexual dichromatism in birds, at least in the Western Palearctic. Whether the described pattern is due to sexual selection promoting more conspicuous males or to natural selection for more cryptic females remains to be determined. Given that pheomelanin synthesis concurrently consumes the antioxidant glutathione but may also reduces toxic cysteine, sex‐biased physiological factors should also be given consideration in the evolution of bird plumages.     

Different modes of evolution in males and females generate dichromatism in fairy‐wrens (Maluridae)

Sexual dichromatism in birds is often attributed to selection for elaboration in males. However, evolutionary changes in either sex can result in plumage differences between them, and such changes can result in either gains or losses of dimorphism. We reconstructed the evolution of plumage colors in both males and females of species in Maluridae, a family comprising the fairy‐wrens (Malurus, Clytomias, Sipodotus), emu‐wrens (Stipiturus), and grasswrens (Amytornis). Our results show that, across species, males and females differ in their patterns of color evolution. Male plumage has diverged at relatively steady rates, whereas female coloration has changed dramatically in some lineages and little in others. Accordingly, in comparisons against evolutionary models, plumage changes in males best fit a Brownian motion (BM) model, whereas plumage changes in females fit an Ornstein Uhlenbeck (OU) multioptimum model, with different adaptive peaks corresponding to distributions in either Australia or New Guinea. Levels of dichromatism were significantly associated with latitude, with greater dichromatism in more southerly taxa. Our results suggest that current patterns of plumage diversity in fairy‐wrens are a product of evolutionary changes in both sexes, driven in part by environmental differences across the distribution of the family.