Structural Plumage Coloration Research Articles

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.

Plumage iridescence is associated with distinct feather microbiota in a tropical passerine

Birds present a stunning diversity of plumage colors that have long fascinated evolutionary ecologists. Although plumage coloration is often linked to sexual selection, it may impact a number of physiological processes, including microbial resistance. At present, the degree to which differences between pigment-based vs. structural plumage coloration may affect the feather microbiota remains unanswered. Using quantitative PCR and DGGE profiling, we investigated feather microbial load, diversity and community structure among two allopatric subspecies of White-shouldered Fairywren, Malurus alboscapulatus that vary in expression of melanin-based vs. structural plumage coloration. We found that microbial load tended to be lower and feather microbial diversity was significantly higher in the plumage of black iridescent males, compared to black matte females and brown individuals. Moreover, black iridescent males had distinct feather microbial communities compared to black matte females and brown individuals. We suggest that distinctive nanostructure properties of iridescent male feathers or different investment in preening influence feather microbiota community composition and load. This study is the first to point to structural plumage coloration as a factor that may significantly regulate feather microbiota. Future work might explore fitness consequences and the role of microorganisms in the evolution of avian sexual dichromatism, with particular reference to iridescence.

Effect of Prenatal and Natal Administration of Testosterone on Production of Structurally Based Plumage Coloration

Testosterone has been implicated as a developmental mechanism involved in the organization and expression of sexually dimorphic traits, such as plumage coloration, in birds. Although research findings relating testosterone levels to plumage expression is equivocal, few studies have investigated how testosterone may influence the expression of structurally based plumage coloration. Here, we use experimental and correlational evidence to test the hypothesis that testosterone influences the development and maintenance of structurally based plumage coloration in a wild-breeding population of eastern bluebirds (Sialia sialis). First, we experimentally manipulated yolk testosterone and measured the effect on the development of plumage coloration of nestlings. Second, we implanted juvenile bluebirds with testosterone and measured the effect on nestling growth, body condition, and plumage coloration of nestlings. Third, we measured covariation between circulating testosterone and plumage coloration of breeding males. Yolk testosterone injections had no significant effect on nestling plumage coloration. Testosterone implantation, however, caused a reduction in plumage brightness, elevated corticosterone, and slower growth in nestlings. Finally, in breeding adult males we found no significant relationship between structural coloration and testosterone; however, males with higher testosterone levels exhibited duller chestnut (melanin-based) plumage. Our observations lead us to reject the hypothesis that testosterone increases structural plumage coloration in male eastern bluebirds.

Changes in carotenoid‐based plumage colour in relation to age in European Serins Serinus serinus

Yearling birds generally display duller colours than adults. This may be due to selection favouring birds with more intensely coloured plumage or to an increase in colour after the first complete moult. Most research to date on the topic has been carried out on species with structural plumage coloration or with carotenoid‐based coloration that is produced by the unmodified deposition of pigments. However, no study has been carried out on species whose carotenoids are metabolically modified before deposition. In this study, we assess age‐related changes in the carotenoid‐based coloration of European Serins, a species that metabolically processes carotenoids before they can be deposited into feathers. Birds were captured over consecutive years and we carried out both cross‐sectional and longitudinal analysis. Adults had significantly greater values of lightness and chroma than yearling birds. However, there were no changes in plumage colour when analysing the same individuals captured in subsequent seasons. Plumage lightness and chroma of adult males after moult were related to body mass, suggesting a role of body condition on plumage coloration. Our results suggest that changes in plumage coloration with age in European Serins are due to a selection process that favours more intensely coloured individuals.

Achieving luster: prenuptial molt pattern predicts iridescent structural coloration in Blue-black Grassquits

Colors in feathers are produced by pigment deposition or by nanostructures within barbs or barbules. In the absence of pigments or nanostructures, light is scattered incoherently, producing white coloration. Honest advertisement models predict that ornamental colors evolve if they reliably signal individual properties such as viability, health, or nutritional state. In this study, we tested if (1) iridescent structural and (2) unpigmented plumage coloration signal male quality in the Blue-black Grassquit (Volatinia jacarina). During three reproductive seasons, we captured males and measured morphological variables and nuptial plumage coverage, and collected feathers for spectrometry. We found that saturation of the iridescent coloration was positively related to relative molting speed, indicating that males investing more in prenuptial molt also produced more saturated, UV-shifted plumage. Body condition was not related to brightness or saturation of the iridescent plumage, and no male morphological attributes were associated with the white underwing patch coloration or size. Our results suggest that patterns of molt, and possibly feather growth, may affect the organization of optical nanostructures responsible for iridescent coloration, and that the ontogeny of iridescent plumage ornaments must be taken into account in hypotheses concerning honest advertisement of such signals. Thus, coloration in this species may reliably reflect energy allocation to molting, constituting an honest indicator of male quality, life history decisions, or endocrine state. To our knowledge, this is the first report of natural variation in molting pattern being associated with sexually-selected structural color plumage.

No consistent female preference for higher crown UV reflectance in Blue Tits Cyanistes caeruleus: a mate choice experiment

Although male UV structural plumage coloration can indicate male quality (e.g. Keyser & Hill 2000) and female reproductive investment strategies (e.g. Sheldon et al. 1999, Griffith et al. 2003), unambiguous evidence that such plumage is a direct target of female choice is still lacking. A straightforward way of testing this is by conducting controlled mate choice experiments that exclude confounding factors such as male–male competition or territory quality. The first experiments on the role of structural colours in mate choice used UV-blocking windows (e.g. Bennett et al. 1996, Hunt et al. 1999), thereby completely removing male UV reflectance. A better approach is to vary UV reflectance within the natural range under natural light conditions. Only two such mate choice experiments have been undertaken to date (Ballentine & Hill 2003, Liu et al. 2007). In neither study did females show a preference for more UV-ornamented males. In the present study, we manipulated crown reflectance of first-year male Blue Tits Cyanistes caeruleus to test the hypothesis that females prefer males with a higher crown UV reflectance, as predicted by patterns of sex allocation, paternity and female parental investment in this species (Johnsen et al. 2005, Delhey et al. 2007a,b). In a mate choice experiment, females were offered a choice of two males (matched for yellow breast colour), one with a UV-reduced crown reflectance and one control-manipulated male. The results indicated that female preference was inconsistent and may be context-dependent

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.

Seasonal changes in a ultraviolet structural colour signal in blue tits, Parus caeruleus

Recent studies of blue tits, Parus caeruleus, have found sexual selection and a viability-indicating function of the structural ultraviolet and blue crown plumage, but the reasons for this signal variation are not understood. Furthermore, studies in England and Sweden have yielded somewhat different results (particularly with regard to the spectral position of the reflectance peak). Here we investigate whether the blue tit UV/blue ornament varies with time of year since such variation might be relevant to the signalling function as well as the apparent difference between populations. From 400 blue tits captured at two different localities in Sweden, we found that objective measures of ‘hue’ (spectral location), ‘chroma’ (spectral purity) and ‘brightness’ (spectral intensity), varied substantially with season. Just after moult (October), crown ‘hue’ is maximally UV-shifted (359 nm for males and 373 nm for females). Thereafter the peak drifts upwards and by the time of nestling feeding (June) male reflectance peaks at 404 nm and female at 413 nm. This change is probably due to feather wear as well as fat and dirt accumulation, which might constitute an additional male quality cue. Our results suggest that it is important to consider plumage age when exploring variation in structural plumage coloration, and that it can largely explain the difference between the British and Swedish studies.

Parental Effort in Relation to Structural Plumage Coloration in the Mountain Bluebird (Sialia currucoides)

AbstractIndicator models of sexual selection suggest that costly ornaments signal reliable information regarding an individual's quality to potential mates. In species that produce altricial offspring, the amount of parental care provided by both males and females can impact reproductive success. The Good Parent Hypothesis proposes that ornamentation in biparental species can act as an honest signal of parental ability to potential mates. We tested this hypothesis using the mountain bluebird (Sialia currucoides), a sexually dichromatic, socially monogamous species in which both sexes have structurally based ornamental plumage coloration. A male's plumage color predicted neither the rate at which it provisioned nestlings nor brood growth rate. The same was true for females. We also found no indication of assortative mating by color or body condition. Feeding rates within pairs were positively correlated, which we suggest may be due to pairs responding similarly to the perceived needs of nestlings or to local area prey availability. In sum, our results do not support the Good Parent Hypothesis as an explanation for the evolution of ornamental plumage color in mountain bluebirds. We suggest alternative hypotheses for the evolution of ornamental plumage color in this species.

Bacteria as an Agent for Change in Structural Plumage Color: Correlational and Experimental Evidence

Recent studies have documented that a diverse assemblage of bacteria is present on the feathers of wild birds and that uropygial oil affects these bacteria in diverse ways. These findings suggest that birds may regulate the microbial flora on their feathers. Birds may directly inhibit the growth of harmful microbes or promote the growth of other harmless microbes that competitively exclude them. If keratinolytic (i.e., feather-degrading) bacteria degrade colored feathers, then plumage coloration could reveal the ability of individual birds to regulate microbial flora. We used field- and lab-based methods to test whether male eastern bluebirds (Sialia sialis) with brighter blue structural plumage coloration were better able to regulate their microbial flora than duller males. When we sampled bluebirds in the field, individuals with brighter color had higher bacterial loads than duller individuals. In the lab, we tested whether bacteria could directly alter feather color. We found that keratinolytic bacteria increased the brightness and purity, decreased the ultraviolet chroma, and did not affect the hue of structural color. This change in spectral properties of feathers may occur through degradation of the cortex and spongy layer of structurally colored barbs. These data suggest that bacteria can alter structural plumage color through degradation.

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.

The effect of coccidial infection on iridescent plumage coloration in wild turkeys

Wild turkeys, Meleagris gallopavo , are among the most ornamented birds in North America, displaying vividly coloured fleshy ornaments on their heads and bright, iridescent structural coloration in their plumage. We investigated the effect of experimental inoculation with coccidian parasites on the expression of iridescent structural coloration in yearling male turkeys. Prior to moult of ornamental feathers, we assigned turkeys to three experimental groups: we maintained six turkeys free of coccidial infection, inoculated four turkeys with a single species of coccidial oocysts, and inoculated six turkeys with multiple species of coccidial oocysts. We used reflectance spectrometry to quantify the plumage coloration of wing covert and breast feathers in the breeding plumage of males in each treatment protocol. We found significant treatment-based variation in the iridescent plumage coloration of yearling male turkeys such that infected males showed proportionately less UV reflectance in their wing covert and breast feathers and had duller breast feathers. This is the first experimental evidence that parasites can suppress the expression of structural plumage coloration, and our findings suggest that, in wild turkeys, iridescent coloration could serve as a condition-dependent signal of male health.

UV-blue structural coloration and competition for nestboxes in male eastern bluebirds

Recent studies suggest that structural plumage coloration can indicate male quality and is used in female mate choice decisions. Whether or not structural coloration functions as a signal in male–male competitive interactions, however, has not been studied. Male eastern bluebirds, Sialia sialis, have brilliant ultraviolet-blue plumage on the head, back, wings and tail that is correlated with both reproductive effort and reproductive success. Bluebirds cannot excavate their own nest cavities, and as a consequence of limited nest sites, often engage in intense competition for nestboxes. We experimentally tested the hypothesis that structural coloration reflects male competitive ability by manipulating the number of available nestboxes. We erected a limited number of nestboxes in early spring and, after birds has established residency in those nestboxes, we added more nestboxes to the study site. We found that the reflective properties of the ultraviolet-blue plumage differed between males that acquired nestboxes early versus late in the spring, indicating that more colourful birds won competitions for access to nest sites. We also found that more colourful males fledged more offspring. These observations support the hypothesis that structural plumage colour is a condition-dependent trait in male eastern bluebirds that could be used to accurately assess the fighting ability of competitors.

Structural plumage colour and parasites in satin bowerbirdsPtilonorhynchus 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.

Female Mate Choice in Relation to Structural Plumage Coloration in Blue Grosbeaks

AbstractPlumage blueness in Blue Grosbeaks (Passerina caerulea) is related to nutritional condition during molt, and bluer males hold larger territories with more food resources. We tested the hypothesis that females use male plumage brightness as a criterion in choosing mates. In a mate-choice aviary, we presented females with a choice between males whose feathers were either brightened with blue marker or dulled with black marker. Treatment resulted in an increase in the plumage intensity (reflectance at peak wavelength) of blue-treated males and a decrease in the intensity of black-treated males within the natural range of variation, while the wavelength of peak reflection (hue) remained unchanged. During mate-choice trials, females consistently preferred particular males, but choice was random with respect to treatment. Thus, the intensity of blue coloration of male Blue Grosbeaks appears not to function as a criterion in female mate choice.Elección de Pareja en Hembras de Passerina caerulea con Relación a la Coloración Estructural del PlumajeResumen. Estudios previos han demostrado que la coloración azul del plumaje de Passerina caerulea está relacionada con la condición nutricional durante la muda y que los machos de coloración más azul mantienen territorios más grandes y con mayores fuentes de alimento. En el presente estudio, examinamos la hipótesis de que las hembras usan el brillo del plumaje como criterio para la selección de parejas. En una arena de selección de parejas, presentamos a las hembras con la opción de elegir entre machos de plumaje coloreado artificialmente con marcador azul o machos de plumaje artificialmente coloreado con marcador negro. El tratamiento del plumaje produjo cambios en la intensidad de color de tal forma que incrementamos la intensidad de color en los machos tratados con marcador azul y redujimos la intensidad de aquellos tratados con marcador negro. La longitud de onda del pico de reflección (tono) no cambió. Durante las pruebas de selección de pareja, las hembras mostraron una preferencia consistente por ciertos machos, pero la elección fue aleatoria con respecto al tratamiento. Por lo tanto, la intensidad de coloración azul de P. caerulea no parece servir como criterio selectivo en la elección de pareja.

Structural Plumage Coloration, Male Body Size, and Condition in the Blue-Black Grassquit

AbstractIn birds, the colors ultraviolet, blue, and green originate from feather microstructure rather than pigmentation. Structural plumage coloration may be maintained by sexual selection, yet there exists only limited evidence that structural colors can honestly signal individual quality. In this study, I found considerable individual variation in the blue-black to ultraviolet coloration of the wing coverts and rump of male Blue-black Grassquits (Volatinia jacarina), thereby revealing the potential for sexual selection on structural plumage coloration in this species. I used a principal components analysis (PCA) of four color characteristics to combine reflectance data into a single color score. Birds with high color scores have brighter, more intensely colored, and more saturated blue-black feathers. There was a significant positive relationship between color scores for both body regions and male condition. There was also a positive trend between the color score of the rump region and male body size. These results support recent findings that structural plumage coloration is condition-dependent, and this type of coloration may be an honest signal of male quality in the Blue-black Grassquit.Coloración Estructural del Plumaje, Tamaño Corporal y Condición Física en Machos de Volatinia jacarinaResumen. En las aves, los colores ultravioleta, azul y verde están dados por la microestructura de las plumas y no por la pigmentación. La coloración estructural del plumaje puede ser mantenida por selección sexual, aunque existe poca evidencia que indique que los colores estructurales son un indicador confiable de la calidad de un individuo. En este estudio encontré variación individual considerable en la coloración negro-azulada a ultravioleta de las cobertoras alares y la rabadilla de machos de Volatinia jacarina, revelando que la coloración estructural del plumaje en esta especie podría estar sujeta a selección sexual. Utilicé un análisis de componentes principales (PCA) de cuatro características de color para combinar datos de reflectancia en un solo valor de color. Las aves con altos valores de color tienen plumas más brillantes e intensamente coloreadas y más saturadas de color negro-azulado. Existió una relación positiva significativa entre los valores de color para ambas regiones del cuerpo y la condición física de los machos. También hubo una tendencia positiva entre el valor del color de la región de la rabadilla y el tamaño corporal de los machos. Estos resultados apoyan descubrimientos recientes que demuestran que la coloración estructural del plumaje depende de la condición física, y que este tipo de coloración puede ser una señal confiable de la calidad de los machos de V. jacarina.

STRUCTURAL PLUMAGE COLORATION, MALE BODY SIZE, AND CONDITION IN THE BLUE-BLACK GRASSQUIT

Abstract In birds, the colors ultraviolet, blue, and green originate from feather microstructure rather than pigmentation. Structural plumage coloration may be maintained by sexual selection, yet there exists only limited evidence that structural colors can honestly signal individual quality. In this study, I found considerable individual variation in the blue-black to ultraviolet coloration of the wing coverts and rump of male Blue-black Grassquits (Volatinia jacarina), thereby revealing the potential for sexual selection on structural plumage coloration in this species. I used a principal components analysis (PCA) of four color characteristics to combine reflectance data into a single color score. Birds with high color scores have brighter, more intensely colored, and more saturated blue-black feathers. There was a significant positive relationship between color scores for both body regions and male condition. There was also a positive trend between the color score of the rump region and male body size....

Behavioural and evolutionary implications of ultraviolet reflectance by gorgets of sunangel hummingbirds

Wild turkeys, Meleagris gallopavo, are among the most ornamented birds in North America, displaying vividly coloured fleshy ornaments on their heads and bright, iridescent structural coloration in their plumage. We investigated the effect of experimental inoculation with coccidian parasites on the expression of iridescent structural coloration in yearling male turkeys. Prior to moult of ornamental feathers, we assigned turkeys to three experimental groups: we maintained six turkeys free of coccidial infection, inoculated four turkeys with a single species of coccidial oocysts, and inoculated six turkeys with multiple species of coccidial oocysts. We used reflectance spectrometry to quantify the plumage coloration of wing covert and breast feathers in the breeding plumage of males in each treatment protocol. We found significant treatment-based variation in the iridescent plumage coloration of yearling male turkeys such that infected males showed proportionately less UV reflectance in their wing covert and breast feathers and had duller breast feathers. This is the first experimental evidence that parasites can suppress the expression of structural plumage coloration, and our findings suggest that, in wild turkeys, iridescent coloration could serve as a condition-dependent signal of male health.

Does a novel bright colour patch increase or decrease predation? Red wings reduce predation risk in European blackbirds

Bright coloration in animals may be explained by sexual selection, defence against predators (aposematism), and other factors. However, the initial evolution of colourful coats is unclear. Theoretical models often assume that rare, bright mutants suffer increased predation risk. In the blackbird Turdus merula , I tested whether a novel colourful plumage, resembling that of some American and African birds, increases or decreases the risk of predation. Pairs of stuffed blackbirds were exposed in the field, one with a bright red wing patch and one with normal wing colour. Predators (mostly goshawks) attacked normal mounts more often than mounts with red wings. Thus, in contrast with what is often assumed, a colourful mutant may be favoured by a decreased risk of predation. In addition, apostatic selection by predators may favour such mutants over longer time periods. These possibilities have important consequences for the evolution of bright coloration in profitable as well as unprofitable prey.