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Non-iridescent structural plumage reflectance is a sexually selected indicator of individual quality in several bird species. However, the structural basis of individual differences remains unclear. In particular, the dominant periodicity of the quasi-ordered feather barb nanostructure is of key importance in colour generation, but no study has successfully traced back reflectance parameters, and particularly hue, to nanostructural periodicity, although this would be key to deciphering the information content of individual variation. We used matrix small-angle X-ray scattering measurements of intact, stacked feather samples from the blue tit crown to estimate the sex-dependence and individual variation of nanostructure and its effects on light reflectance. Measures of nanostructural periodicity successfully predicted brightness, ultraviolet chroma and also hue, with statistically similar effects in the two sexes. However, we also observed a lack of overall effect of the nanostructural inhomogeneity estimate on reflectance chromaticity, sex-dependent accuracy in hue prediction and strong sex-dependence in position estimation error. We suggest that reflectance attributes are modified by other feather structures in a sex-specific manner, and that within-individual variation in nanostructural parameters exists within or among feathers and this confounds the interpretation of structure–reflectance relationships at the plumage area level.

Recycling consumed commercial polymers is considered a highly important issue that chemists and engineers must take care of to develop the proper recycling techniques. The main objective of this study is to find a radical solution to the problems of plastic waste by recycling plastic waste (water bottles) and studying the effect of the produced Degraded Polyethylene Terephthalate (DPET) on the properties of paints. DEPT was added in six quantities to the paint mixture (1, 3, 4, 5, 7, and 14 grams). The paints were evaluated using various tests, including scanning electron microscopy (SEM), viscosity, adhesion, brightness, color, ultraviolet reflectance, and accelerated weathering. Testing the paint mixtures showed that the samples were not affected by weather conditions. This indicates the improvement of the paint mixtures by adding quantities of DPET. This study concludes that the catalyst ratios used succeeded in cracking DPET and avoided the need for large quantities of the catalyst. The use of DPET in various material applications reduces the cost due to the low cost of DPET production. The use of PET in sustainable applications conferred a radical solution to plastic waste problems worldwide. In this work, mixing plastic waste products, after their treatment, in the preparation of paint mixtures successfully contributed to improving the required specifications.

Flowers have many traits to appeal to pollinators, including ultraviolet (UV) absorbing markings, which are well‐known for attracting bees at close proximity (e.g., <1 m). While striking UV signals have been thought to attract pollinators also from far away, if these signals impact the plant pollinia removal over distance remains unknown. Here, we report the case of the Australian orchid Diuris brumalis, a nonrewarding species, pollinated by bees via mimicry of the rewarding pea plant Daviesia decurrens. When distant from the pea plant, Diuris was hypothesized to enhance pollinator attraction by exaggeratedly mimicking the floral ultraviolet (UV) reflecting patterns of its model. By experimentally modulating floral UV reflectance with a UV screening solution, we quantified the orchid pollinia removal at a variable distance from the model pea plants. We demonstrate that the deceptive orchid Diuris attracts bee pollinators by emphasizing the visual stimuli, which mimic the floral UV signaling of the rewarding model Daviesia. Moreover, the exaggerated UV reflectance of Diuris flowers impacted pollinators' visitation at an optimal distance from Da. decurrens, and the effect decreased when orchids were too close or too far away from the model. Our findings support the hypothesis that salient UV flower signaling plays a functional role in visual floral mimicry, likely exploiting perceptual gaps in bee neural coding, and mediates the plant pollinia removal at much greater spatial scales than previously expected. The ruse works most effectively at an optimal distance of several meters revealing the importance of salient visual stimuli when mimicry is imperfect. [ABSTRACT FROM AUTHOR]

Non-iridescent structural plumage reflectance is a sexually selected indicator of individual quality in several bird species. However, the structural basis of individual differences remains unclear. In particular, the dominant periodicity of the quasi-ordered feather barb nanostructure is of key importance in colour generation, but no study has successfully traced back reflectance parameters, and particularly hue, to nanostructural periodicity, although this would be key to deciphering the information content of individual variation. We used matrix small-angle X-ray scattering measurements of intact, stacked feather samples from the blue tit crown to estimate the sex-dependence and individual variation of nanostructure and its effects on light reflectance. Measures of nanostructural periodicity successfully predicted brightness, ultraviolet chroma and also hue, with statistically similar effects in the two sexes. However, we also observed a lack of overall effect of the nanostructural inhomogeneity estimate on reflectance chromaticity, sex-dependent accuracy in hue prediction and strong sex-dependence in position estimation error. We suggest that reflectance attributes are modified by other feather structures in a sex-specific manner, and that within-individual variation in nanostructural parameters exists within or among feathers and this confounds the interpretation of structure-reflectance relationships at the plumage area level., Competing Interests: We declare we have no competing interests., (© 2024 The Authors.)

Despite the wide interest in flower colours, only after the end of the nineteenth-century studies started to comprise floral UV reflection, which is invisible to humans but visible to the major groups of pollinators. Many flowers and inflorescences display colour patterns, an important signal for pollinators, promoted by the presence of at least two different colours within flowers or inflorescences, including colours in the UV waveband. For Neotropical savanna plant species, we characterised floral UV features using UV-photography and reflectance measurements. We tested (i) whether floral UV features were constrained by their shared ancestry, (ii) whether floral UV features were associated with pollinators, and (iii) whether floral UV features were associated with floral traits mediating these interactions, including floral resource, type of attraction unit and presence/absence of non-UV colour patterns. Of 80 plant species, ca. 70% were UV-patternless, most of them UV-absorbing. Approximately 30% presented one of three types of UV-patterns: bullseye, contrasting corolla markings oriented toward floral resources or contrasting reproductive structures, which were all considered as floral guides. Floral UV features were phylogenetically constrained and were associated with pollinators, floral resources and attraction unit, but not with non-UV colour patterns. UV-patternless flowers were associated with most of the pollination systems, while UV-patterned flowers were mainly associated with bee-pollination. UV-absorbing flowers comprised the only category with hawkmoth- and butterfly-pollinated flowers, and a high percentage of hummingbird-pollinated species. Nocturnal pollinated species were also commonly UV-absorbing, except for one UV-reflecting bat-pollinated species and one beetle-pollinated species with UV-reflecting stigmas. All types of floral UV features were associated with nectar; however, flowers with contrasting reproductive structures were mainly associated with pollen. There was an association between UV-absorbing species and the presence of inflorescences and intermediate attraction units. Our results evince that phylogenetic relatedness can constraint floral UV features' diversification, but combinations of evolutionary and ecological processes may be expected in this scenario. [ABSTRACT FROM AUTHOR]

High inflows of freshwater from the Mississippi and Atchafalaya rivers into the northern Gulf of Mexico during spring contribute to strong physical and biogeochemical gradients which, in turn, influence phytoplankton community composition across the river plume–ocean mixing zone. Spectral features representative of bio-optical signatures of phytoplankton size classes (PSCs) were retrieved from underway, shipboard hyperspectral measurements of above-water remote sensing reflectance using the quasi-analytical algorithm (QAA_v6) and validated against in situ pigment data and spectrophotometric analyses of phytoplankton absorption. The results shed new light on sub-km scale variability in PSCs associated with dynamic and spatially heterogeneous environmental processes in river-influenced oceanic waters. Our findings highlight the existence of localized regions of dominant picophytoplankton communities associated with river plume fronts in both the Mississippi and Atchafalaya rivers in an area of the coastal margin that is otherwise characteristically dominated by larger microphytoplankton. This study demonstrates the applicability of underway hyperspectral observations for providing insights about small-scale physical-biological dynamics in optically complex coastal waters. Fine-scale observations of phytoplankton communities in surface waters as shown here and future satellite retrievals of hyperspectral data will provide a novel means of exploring relationships between physical processes of river plume–ocean mixing and frontal dynamics on phytoplankton community composition.

Reducing the use of insecticides is an important issue for agriculture today. Sowing wildflower strips along field margins or within crops represents a promising tool to support natural enemy populations in agricultural landscapes and, thus, enhance conservation biological control. However, it is important to sow appropriate flower species that attract natural enemies efficiently. The presence of prey and hosts may also guide natural enemies to wildflower strips, potentially preventing them from migrating into adjacent crops. Here, we assessed how seven flower traits, along with the abundance of pollen beetles (Meligethes spp., Coleoptera: Nitidulidae) and true weevils (Ceutorhynchus spp., Coleoptera: Curculionidae), affect the density of parasitoids of these two coleopterans in wildflower strips sown in an oilseed rape field in Gembloux (Belgium). Only flower traits, not host (i.e. pollen beetles and true weevils) abundance, significantly affected the density of parasitoids. Flower colour, ultraviolet reflectance and nectar availability were the main drivers affecting parasitoids. These results demonstrate how parasitoids of oilseed rape pests react to flower cues under field conditions. Similar analyses on the pests and natural enemies of other crops are expected to help to develop perennial flower mixtures able to enhance biological control throughout a rotation system. [ABSTRACT FROM AUTHOR]

Avian eggshells are variedly coloured, yet only two pigments, biliverdin and protoporphyrin IX, are known to contribute to the dramatic diversity of their colours. By contrast, the contributions of structural or other chemical components of the eggshell are poorly understood. For example, unpigmented eggshells, which appear white to the human eye, vary in their ultraviolet (UV) reflectance, which may be detectable by birds. We investigated the proximate mechanisms for the variation in UV-reflectance of unpigmented bird eggshells using spectrophotometry, electron microscopy, chemical analyses, and experimental manipulations. We specifically tested how UV-reflectance is affected by the eggshell cuticle, the outermost layer of most avian eggshells. The chemical dissolution of the outer eggshell layers, including the cuticle, increased UV-reflectance for only eggshells that contained a cuticle. Our findings demonstrate that the outer eggshell layers, including the cuticle, absorb UV-light, probably because they contain higher levels of organic components and other chemicals, such as calcium phosphates, compared to the predominantly calcite-based eggshell matrix. These data highlight the need to examine factors other than the known pigments in studies of avian eggshell colour.

A comparison of 95 species of Central European moths, representing 11 families and inhabiting various habitats, was carried out in order to detect the potential impact of biotope on the ultraviolet (UV) light reflectance of their wings. Based on digitized photographs taken under UV light conditions, a phylogeny-controlled redundancy analysis relating UV reflectance to preferred habitat type (xerophilous, mesophilous, and hygrophilous) and habitat openness (open, semiopen, and closed) was carried out. Species preferring hygrophilous habitats displayed significantly higher UV wing reflectance than species inhabiting xerothermic and mesic habitats, and this pattern remained significant even after controlling for phyletic relationships. In contrast, UV wing reflectance displayed no pattern related to habitat openness. Given the higher UV reflectance of water and humid surfaces, we interpret these results, which are based on the first comprehensive sampling of UV reflectance in Central European moths, in terms of predator avoidance under habitat-specific light conditions. We conclude that the moisture content of the environment may markedly contribute to the variation of appearance of moth wings for better imitation habitat characteristics and therefore to increase protection. [ABSTRACT FROM AUTHOR]

Screening-based photoprotection is a first-line defence of plants against potentially harmful solar radiation. In the Iberian Peninsula, most natural fleshy fruits are red, purple or black, although there do exist a few plants that naturally present white fruit, such as Corema album (L.) D.Don (Ericaceae). This is a wild shrub of Atlantic coastal areas, which presents white fleshy fruit that ripens throughout the summer. We analysed the reflectance of fully ripened fruit in the field, and laboratory (before and after hexane washing and pericarp removal). The aqueous and ethyl acetate extract was successively obtained from the crushed fruit and submitted to column chromatography on Silicagel. The fractions were analysed through gas chromatography/mass spectrometry, and two compounds were separated and identified as oleanolic and ursolic acids. The reflectance measurements were taken with the UniSpec-SC Spectral Analysis System. The fruit in the field and laboratory exhibited elevated UV reflectance. The ethyl acetate extract, fractions, and isolated compounds showed high reflectance in UV, which indicated that these triterpene acids could be responsible for reflectance in natural berries. Ursolic and oleanolic acids can play a photoprotective role in berries of Corema album . In sunny environments white fruit can be more visible to dispersers. [ABSTRACT FROM AUTHOR]

Predation has profound effects on the phenotypes of animal prey and, in lizards, the relationship between coloration and antipredatory behaviour has been studied in depth. However, studies that address the relationships between dorsal patterns and tail coloration with escape behaviour in polymorphic lizards are absent in the literature. We describe dorsal morphs and measured tail coloration and escape behaviour in hatchling Iberian wall lizards, Podarcis hispanicus, a species with a previously undescribed female-restricted dorsal polymorphism (reticulated-blotched males, and either striped or reticulated-blotched females) and juvenile tails with conspicuous blue coloration, which is probably used to divert predator attacks towards the autotomizable tail. Overall we provide evidence for the existence of sexual dimorphism in tail ultraviolet reflectance between reticulated females and males, with striped females being intermediate. We identified sex/dorsal morph, body size and tail brightness as predictors of different aspects of escape behaviour and suggest the existence of two alternative escape strategies between striped and reticulated-blotched females that may be dependent on dorsal morph differences, independently of sex. Reticulated-blotched females, and also males (all reticulated-blotched), ran faster and spent less time paused than striped females, which might reflect an escape behaviour strategy based on endurance in striped females. In addition, lowland males displayed tail waving as a 'last resort' antipredator strategy that may be related to fatigue. We concluded that hatchling antipredatory behaviour is influenced by both dorsal pattern and tail conspicuousness. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113, 1094-1106. [ABSTRACT FROM AUTHOR]

The males of the Brimstone butterfly ( Gonepteryx rhamni) have ultraviolet pattern on the dorsal surfaces of their wings. Using geometric morphometrics, we have analysed correlations between environmental variables (climate, productivity) and shape variability of the ultraviolet pattern and the forewing in 110 male specimens of G. rhamni collected in the Palaearctic zone. To start with, we subjected the environmental variables to principal component analysis (PCA). The first PCA axis (precipitation, temperature, latitude) significantly correlated with shape variation of the ultraviolet patterns across the Palaearctic. Additionally, we have performed two-block partial least squares (PLS) analysis to assess co-variation between intraspecific shape variation and the variation of 11 environmental variables. The first PLS axis explained 93 % of variability and represented the effect of precipitation, temperature and latitude. Along this axis, we observed a systematic increase in the relative area of ultraviolet colouration with increasing temperature and precipitation and decreasing latitude. We conclude that the shape variation of ultraviolet patterns on the forewings of male Brimstones is correlated with large-scale environmental factors. [ABSTRACT FROM AUTHOR]

By laying their eggs in the nests of other birds, avian brood parasites impose the cost of rearing young upon their hosts. The recognition and rejection of foreign eggs are primary host defenses against costly brood parasitism. Hosts of parasitic brown-headed cowbirds ( Molothrus ater) challenge coevolutionary theory because most cowbird hosts accept parasitic eggs despite their drastically different appearance from the hosts' own eggs. American robins ( Turdus migratorius) are one of only 10 % of the over 200 potential cowbird host species to robustly reject parasitic eggs, but the mechanisms driving the sensory bases of foreign egg rejection remain elusive. Our research combined avian visual perceptual modeling and behavioral experimentation to investigate chromatic cues eliciting parasitic egg rejection in American robins. We assessed the effects of perceivable background color differences between real host and model parasite eggs, across all four avian photoreceptors, on rates of rejection of model eggs spanning in color across the entire avian spectral sensitivity range, and including immaculate model eggs matching the natural colors of robin and cowbird eggs. The results suggest that egg rejection in robins is driven by the overall perceivable difference in color between own and artificial eggs, and input from all four single-cone avian photoreceptors affects the rejection decision. The results, however, also reveal that when viewed by the avian eye, natural cowbird eggs appear more similar in background color to robin eggs than predicted by the high rejection rate of these parasitic eggs. This suggests that robins respond specifically to parasitism by cowbirds, despite an apparent lack of sensory tuning toward the detection of the background color of cowbird eggs. [ABSTRACT FROM AUTHOR]

Raptors have excellent vision, yet it is unclear how they use colour information. It has been suggested that raptors use ultraviolet (UV) reflections from vole urine to find good hunting grounds. In contrast, UV plumage colours in songbirds such as blue tits are assumed to be 'hidden' communication signals, inconspicuous to raptors. This ambiguity results from a lack of knowledge about raptor ocular media transmittance, which sets the limit for UV sensitivity. We measured ocular media transmittance in common buzzards (Buteo buteo). sparrowhawks (Accipiter nisus), red kites (Milvus milvus) and kestrels (Falco tinnunculus) so that, for the first time, raptor UV sensitivity can be fully described. With this information, and new measurements of vole urine reflectance, we show that (i) vole urine is unlikely to provide a reliable visual signal to hunting raptors and (ii) blue tit plumage colours are more contrasting to blue tits than to sparrowhawks because of UV reflectance. However, as the difference between blue tit and sparrowhawk vision is subtle, we suggest that behavioural data are needed to fully resolve this issue. UV cues are of little or no importance to raptors in both vole and songbird interactions and the role of colour vision in raptor foraging remains unclear. [ABSTRACT FROM AUTHOR]

The handicap hypothesis broadly argues that ornamental traits and displays are costly to produce and are a function of individual quality and condition. Consequently, the extent of ornament expression can act as an honest indicator of quality, condition or susceptibility to parasites, and influence the behavior of mates or competitors. We used reflectance spectrometry to quantify non-iridescent, ultraviolet (UV) and blue structurally-based plumage color and character size in relation to individual condition (feather growth rate) and occurrence of disease caused by a parasite infestation (knemidokoptic mange) in wild male Steller's jays (Cyanocitta stelleri). We then attempted to experimentally reduce parasite load prior to molt and quantified the relative change in plumage and condition. Those birds with faster feather growth rates (i.e., in better condition) had lower hues (i.e., reflected shorter wavelengths, displaying a more intense, brilliant UV-blue color), suggesting the possibility of condition or feather growth rate-dependent production of color. We found that UV reflectance (UV chroma) was positively related to the occurrence of disease. We detected no effect of experimental treatment on plumage or condition. In conclusion, our results are suggestive of possible condition-dependent development of structurally-based plumage, and the positive relationship between macro-parasite-caused disease intensity and UV blue plumage color, demonstrated for the first time in this study, did not support the parasite-mediated handicap hypothesis. [ABSTRACT FROM PUBLISHER]

Sex and population differences in the ultraviolet (UV) reflectance patterns of two phenotypically divergent lacertids from Tenerife, Gallotia galloti eisentrauti and G. g. galloti, are described. Adults in both populations have rows of lateral and ventro-lateral blue patches on either side of the trunk that show a single peak of reflectance in the near UV. The lateral patches of males are more intensely reflective than those of females, but females have more chromatic and UV-shifted lateral patches than males. Most G. g. eisentrauti and some G. g. galloti also have blue cheek patches on either side of the gular area. Male cheek patches have reflectance peaks in the near UV, whereas those of females peak in the visible part of the spectrum. The lateral patches of males from the two populations differ in hue but not in intensity, those of G. g. galloti being more UV-shifted than those of G. g. eisentrauti. Female G. g. galloti have more intensely reflective lateral patches than female G. g. eisentrauti. Interindividual variation in peak wavelength and/or UV chroma is present in both sexes and populations. This is the first demonstration of sexual dichromatism in the UV reflectance of colour patches in a lacertid. [ABSTRACT FROM AUTHOR]

Increasing attention has focused on the treatment and reversal of ultraviolet- induced photodamage. This study was designed to determine whether ultraviolet reflectance photography could quantify the clinical effects of erbium:YAG laser resurfacing on photoaged skin. Twenty patients (ages 46–67) with moderate to severe photodamage underwent dual-mode variable pulsed erbium:YAG laser facial resurfacing. Pretreatment and post-treatment evaluations at 3 and 6 months were performed clinically and photographically with white and ultraviolet reflectance photography. Image conversion of the ultraviolet reflectance imaging data demonstrated an average of 36% and 42% reduction in photodamage at 3 and 6 months, respectively. This correlated well with the wrinkle reduction and correction of dyspigmentation observed in all patients following laser resurfacing. Histologic examination confirmed a thicker, more orderly epidermis and dermal collagen remodeling after treatment. Reduction in photodamage using the dual-mode variable pulsed erbium:YAG laser can be quantitatively measured with ultraviolet reflectance photography using digital imaging. This model should prove useful in developing treatment regimens for photodamage. [ABSTRACT FROM AUTHOR]

In seabirds, colors of feathers and external tissues have only recently been studied, and ultraviolet (UV) color has not yet been detected. Using live individuals as well as museum skins, we found UV peaks of reflectance in two large Aptenodytes species, King (A. patagonicus) and Emperor (A. forsteri) Penguins. UV reflectance did not occur on the feathers, claws, or skin of these species, nor did we find UV reflectance in five other genera of penguins (11 species). UV peaks overlapped with spots of color on the lower beak that appeared orange for human observers, and beak spots differed slightly in location between the two species. Adults of both sexes possessed these UV markings, but they were lacking in juveniles, as was the orange color of the beak spot, and auricular patches used for selecting mates. Finally, measurements of free-ranging King Penguins showed that recently paired birds had higher UV reflectance than courting ones, suggesting possible roles of UV beak spots in pairing and as an indicator of sexual maturity. [ABSTRACT FROM AUTHOR]

High inflows of freshwater from the Mississippi and Atchafalaya rivers into the northern Gulf of Mexico during spring contribute to strong physical and biogeochemical gradients which, in turn, influence phytoplankton community composition across the river plume–ocean mixing zone. Spectral features representative of bio-optical signatures of phytoplankton size classes (PSCs) were retrieved from underway, shipboard hyperspectral measurements of above-water remote sensing reflectance using the quasi-analytical algorithm (QAA_v6) and validated against in situ pigment data and spectrophotometric analyses of phytoplankton absorption. The results shed new light on sub-km scale variability in PSCs associated with dynamic and spatially heterogeneous environmental processes in river-influenced oceanic waters. Our findings highlight the existence of localized regions of dominant picophytoplankton communities associated with river plume fronts in both the Mississippi and Atchafalaya rivers in an area of the coastal margin that is otherwise characteristically dominated by larger microphytoplankton. This study demonstrates the applicability of underway hyperspectral observations for providing insights about small-scale physical-biological dynamics in optically complex coastal waters. Fine-scale observations of phytoplankton communities in surface waters as shown here and future satellite retrievals of hyperspectral data will provide a novel means of exploring relationships between physical processes of river plume–ocean mixing and frontal dynamics on phytoplankton community composition. [ABSTRACT FROM AUTHOR]

Abstract: In many species, male coloration signals aggressiveness and/or fighting ability. Males of the Tenerife lizard (Gallotia galloti) have conspicuous ultraviolet (UV)-blue cheek and lateral color patches that are brighter in the breeding season and larger than those of females. We analyzed experimentally the effect of morphological and behavioral traits, including spectral variables from UV-reflecting color patches, on the outcome of staged dyadic contests between males of two subspecies of G. galloti. We performed two experiments: (1) using pairs of unmanipulated males and (2) reducing the UV reflectance of the UV-blue patches of one of the contestants with sunscreen lotion. Results from experiment 1 showed no significant difference between subspecies in the effect of any variable on contest outcome. Overall, winners had larger body mass, head width, and fore-limb length and showed a higher rate of aggressive behavior patterns than losers, whereas losers showed a higher rate of “tail shake,” which is considered a submissive behavior. Winners also had lateral UV-blue patches with higher (more blue-biased) hue than losers, but no other spectral trait had a significant effect on contest outcome. Results from experiment 2 showed that reducing patch reflectance in the UV range had no effect in one subspecies but significantly increased fighting success in the other. The probability of winning was positively associated with the frequency of bites (irrespective of whether individuals had manipulated patches or not). Results from both experiments suggest that while multiple traits (morphometric, coloration, and behavioral) may influence the outcome of male contests in G. galloti, behavioral traits take prevalence over morphological, including coloration, traits.Significance statement: The effect of several traits on the outcome of contests has been previously analyzed in males of several lizard species. These analyses have commonly considered the effect of a single trait on contest outcome. However, as even males of the same age and experience often vary across multiple morphological (including coloration), performance, and behavioral traits, it has been suggested that contest outcome could be influenced by several interacting traits. We tested this hypothesis by staging dyadic contests between males of two subspecies of the Tenerife lizard. Results from a first experiment showed that larger body mass, head width, fore-limb lengths, higher frequency of bites, throat extension, and higher peak reflectance (hue) of UV-blue patches significantly affected the probability of winning, while a large frequency of tail shakes was associated with losing. Results from a second experiment, in which we experimentally manipulated patch UV reflectance in one of the contestant lizards, showed that bite frequency and reduction of UV reflectance affected the probability of winning; we discuss the complex effect of this latter trait on contest outcome. [ABSTRACT FROM AUTHOR]

Avian eggshells are variedly coloured, yet only two pigments, biliverdin and protoporphyrin IX, are known to contribute to the dramatic diversity of their colours. By contrast, the contributions of structural or other chemical components of the eggshell are poorly understood. For example, unpigmented eggshells, which appear white to the human eye, vary in their ultraviolet (UV) reflectance, which may be detectable by birds. We investigated the proximate mechanisms for the variation in UV-reflectance of unpigmented bird eggshells using spectrophotometry, electron microscopy, chemical analyses, and experimental manipulations. We specifically tested how UV-reflectance is affected by the eggshell cuticle, the outermost layer of most avian eggshells. The chemical dissolution of the outer eggshell layers, including the cuticle, increased UV-reflectance for only eggshells that contained a cuticle. Our findings demonstrate that the outer eggshell layers, including the cuticle, absorb UV-light, probably because they contain higher levels of organic components and other chemicals, such as calcium phosphates, compared to the predominantly calcite-based eggshell matrix. These data highlight the need to examine factors other than the known pigments in studies of avian eggshell colour., (© 2015. Published by The Company of Biologists Ltd.)