Nestling colouration is adjusted to parent visual performance in altricial birds

Hitherto, most of the investigation on the perceptual efficacy of begging signals has dwelled on how patterns of nestling colouration adjust to predominant nest luminosity. However, visual sensitivity of birds varies across species, which raises the question of whether colouration of traits involved in begging displays is adjusted to parent visual capacities. Here, by comparing nestling colouration and visual sensitivity across 22 altricial bird species, we provide a first test of this hypothesis. Firstly, we assessed differences in performance of typical UV‐tuned and violet‐tuned bird eyes when looking at the nestling traits under the light regimes prevailing at their nests. Secondly, while controlling for common ancestry in a comparative approach, we explored variation in colouration of nestlings in relation to parent visual system. The colour discrimination model indicated a general higher performance of the ultraviolet over the violet eye at detecting gape and body skin traits in either open‐ or hole‐nest light conditions. Gape colouration was associated with parental visual system as the nestlings of UVS species displayed more yellow and less pure ultraviolet mouths than the nestlings of VS species. Thus, our results agree with an adaptive parent–offspring communication scenario where the nestlings’ colours tuned the perception capacities of their parents.     

Nestling detectability affects parental feeding preferences in a cavity-nesting bird

In many avian species, nestlings have evolved striking plumage, behaviours and mouth colours to obtain a greater share of parental investment. Studies revealing parental feeding preferences for nestlings with red gapes have proposed that red mouth colour in songbirds can act as a signal of nestling need or condition. Alternative hypotheses suggest that bright nestling mouths in cavity-nesting birds evolved to increase nestling detectability by the parents. We tested whether nestling mouth colour affects parental feeding preferences in great tits, Parus major L. In broods of six young, we experimentally painted mouth gapes and flanges either red or yellow and tested the effect of mouth colour on nestlings' mass gain under two lighting conditions. In nests with high luminosity, there was no significant effect of mouth colour on mass gain. In nests with low luminosity, nestlings with red gapes and flanges gained less mass than nestlings with red gapes and yellow flanges or both yellow gapes and flanges. Our results suggest that, in nests with low luminosity, red mouths decreased nestling detectability to the feeding parents and support the hypothesis that poor luminosity in nesting cavities can select for pale mouths. Overall, our results do not support the hypothesis that red mouth colour signals nestling need or condition to parent great tits.     

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

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

Nestling birds put their best flange forward

The offspring of caring parents may evolve specialized traits uniquely adaptive during their dependence on parental care. For example, the mouths of passerine nestlings are often bordered by enlarged and colorful rictal flanges expressed only during the nestling period. Although these traits are commonly hypothesized to act as visual signals during begging, non‐communication functions for the specialized mouth have been proposed as well. To test the hypothesis that nestling flange colors have evolved largely or exclusively as visual signals, I compared the reflectance of flange tissue that would be visible to parents during begging to that of flange tissue not exposed during begging in nestling house sparrows Passer domesticus and cliff swallows Petrochelidon pyrrhonota. Specifically, I tested the prediction that both condition‐dependent color parameters and those associated with visual conspicuousness would be expressed more intensely in tissue displayed during begging. Consistent with this prediction, flange tissue exposed during begging was brighter (reflected more total light), more UV‐rich, and had more intense carotenoid‐based coloration than hidden tissue. These differences do not exclude a non‐signaling function for flanges, but are consistent with the hypothesis that flange colors have evolved as visual signals.     

Mouth coloration of nestlings covaries with offspring quality and influences parental feeding behavior

Altricial nestlings compete with their nest mates for resourcesdelivered by parents. Parents may allocate food to nestlingsbased on reproductive value of offspring. To test the hypothesisthat mouth coloration acts as a signal of nestling conditionin the barn swallow Hirundo rustica, we investigated whethergape coloration is correlated with offspring quality and age.We also examined the role of ultraviolet (UV) flange colorationin parental allocation in a manipulative experiment. Mouth colorationchanged with age, probably due to accumulation of dietary carotenoidsin the tissue and an increase in the number of collagen layers.Highly UV and redder palates and brighter flanges were associatedwith longer tarsi and greater body mass at day 6 and with feathergrowth at day 12 posthatching. Although we did not find evidencethat UV coloration of flanges is associated with nestling quality,parents preferentially fed young whose flanges reflected higherUV light, compared with experimentally UV-filtered nestlings.These results support the hypothesis that mouth coloration isa reliable signal of nestling condition. In addition, they showthat UV flange coloration influences parental decisions regardingfood allocation.     

Dark nests and conspicuousness in color patterns of nestlings of altricial birds

Nests of altricial birds exhibit variable spectral properties that may affect the efficacy (conspicuousness) of the colored begging traits that a nestling displays to its parents. Here we explored whether selection for efficient perception has favored the evolution of nestling color designs that maximizes nestling detectability in variable light environments. Visual models were used to estimate how parents perceive the coloration of mouths, flanges, heads, and breasts of nestlings within their nest in 21 species of European birds. We show that the largest chromatic and achromatic contrasts against the nest background appeared for nestling mouths and flanges, respectively. Nestlings of open-nesting species showed a larger general achromatic contrast with the nest than did nestlings of hole-nesting species. However, nestlings of hole nesters showed a more evident achromatic contrast between flanges and other traits than did nestlings of open nesters. In addition, species with larger clutch sizes showed larger general achromatic contrasts with the nest. Gaping traits of open-nesting species contrasting with the nest background were better perceived under rich light regimes than under poor ones. These findings are consistent with a scenario in which selection for nestling detectability in dark environments has favored the evolution of particular achromatic components of gape coloration but also nestling traits that enhance signal efficacy by maximizing color contrasts within a nestling.     

Covariation between bill colouration and fitness components in a nocturnal bird

Vividly coloured chromatic signals play a key role in social and sexual signalling in diurnal birds, but their role is considered negligible in favour of achromatic (i.e. white, pure grey and black colourations) signals in nocturnal species. Here we studied colour variation and potential signalling of the yellow bill – a trait functioning as sexual signal in diurnal raptors during the breeding season in females of a truly nocturnal raptor, the little owl Athene noctua. We found that yellow‐red chroma of the bill was highly variable between individuals and positively correlated with female fitness prospects (i.e. brood size at fledging). In addition, we found that females with brighter bills were larger in size and produced owlets with a higher mass at fledging. This study suggests that yellow bill colouration in female little owls may potentially play a role in sexual signalling and may constitute the first evidence of chromatic colour signalling in a nocturnal bird.     

The effects of diet quality and quantity on plumage colour and growth of great tit Parus major nestlings: a food manipulation experiment along a pollution gradient

The yellow carotenoid-based plumage coloration of great tit Parus major nestlings is found to be paler in polluted and urban environments. Because carotenoid pigmentation is often considered to be a condition dependent trait in birds we wanted to find out whether food-limitation and poor nestling condition could explain the pale plumage colour in a polluted area. P. major nestlings were supplemented with variable diets along a well known heavy metal pollution gradient around a copper smelter: two food treatments with carotenoids, one food treatment with little carotenoid and one unsupplemented control. Our field experiment showed that nestlings in the polluted area grew better with carotenoid rich diets, while such effect was not found in the unpolluted area. Nestlings showed higher plasma carotenoid (lutein) levels and higher plumage carotenoid chroma values in the unpolluted area than in the polluted area. However, plasma lutein levels or plumage colour were not associated with heavy metal levels in nestling faeces (a proxy for dietary exposure). Our results provide only weak evidence for carotenoid-based colouration to be condition-dependent in great tit nestlings as we found a positive relationship between body mass and carotenoid chroma in the non-supplemented control group only. The positive relationship between body mass and plumage colour intensity is more likely to be produced by the fact that good availability of caterpillars, an important food source for P. major, also means a good availability of carotenoids to nestlings. Our results suggest that main reason for pale nestling plumage in the polluted area is lower quality invertebrate food, and not nutrition-related oxidative stress.     

Egg colouration predicts brood size,telomere length and body condition of spotless starling fledglings

Understanding the impressive interspecific variation in avian eggshell colouration has attracted the attention of evolutionary ecologists for more than a century. Several functional explanations predict positive covariation between eggshell pigmentation and phenotypic quality of nestlings. We test this prediction in spotless starlings Sturnus unicolor by using biometric measurements and telomere length of hatchlings and fledglings as proxies of phenotypic quality. Female spotless starlings lay immaculate blue‐green eggs, a sexually selected signal directed to males. Pigmentation predicts positive associations with concentration of antioxidants and testosterone in the yolk and with paternal provisioning effort during nestling growth. Eggshell pigmentation (blue‐green chroma) is not associated with telomere length of hatchlings, which suggests weak maternal effects on this trait. However, we find negative associations of eggshell colouration with both body condition and telomere length of fledglings. Moreover, we find positive associations between eggshell colouration and clutch size, which suggests that sibling competition is higher in nests with more coloured eggshells. Previous works demonstrated that level of sibling competition is positively related to telomere erosion and, thus, the detected negative associations between eggshell colouration, body condition and telomere length of fledglings would reflect higher level of competition in nests with more coloured eggshells. We therefore speculate with the possibility that females that lay larger clutches also lay more coloured eggshells that elicit increased paternal provisioning effort and, thus, raise larger broods at the expense of telomere erosion of their offspring.     

Condition‐dependent expression of carotenoid‐ and melanin‐based plumage colour of northern flicker nestlings revealed by manipulation of brood size

Carotenoid‐based colouration in feathers is widely accepted to be a reliable signal of the health of an individual, but the condition‐dependence of melanin‐based plumage ornaments has been highly debated. Using broods that were manipulated in size, we tested whether nutritional stress during rearing affected the carotenoid pigmentation in secondary feathers and the size, shape, and symmetry of melanin spots on breast plumage of northern flicker Colaptes auratus nestlings. Two measures of carotenoid colour (chroma and brightness) of secondary flight feathers did not vary according to brood size treatment, but in a larger dataset from the population, carotenoid chroma was positively associated with nestling mass. Nestlings from experimentally enlarged broods had smaller melanin spots than those from reduced broods, which is some of the first experimental evidence that melanin ornament size in growing nestlings is condition‐dependent. However, the shape and symmetry of the melanin breast spots was not associated with nestling mass. Sexual dimorphism was apparent in both types of pigmentation and future studies should investigate whether there are any trade‐offs for nestlings between investing in carotenoid colouration and melanisation and whether trade‐offs differ between the sexes.     

Cross‐fostering reveals that among‐brood differences in ornamental mouth coloration mostly reflect rearing conditions in nestling house sparrows

Dependent offspring use specialized traits to attract parental care. In birds, this includes morphological ornaments (e.g. colourful plumage or mouthparts) that are associated with nestling condition and shape the allocation of parental care. Ornament expression often differs among broods, even after differences in individual condition are accounted for statistically. Understanding how this variation arises is important for understanding the information content of these signals, their functional importance, and their evolution. The present study used a cross‐fostering experiment to assess the relative contributions of parental effects to among‐brood differences in the mouth coloration of nestling house sparrows, specifically the carotenoid‐richness, overall brightness, and ultraviolet (UV) coloration of rictal flanges. The expression of carotenoid‐based coloration was explained by synchronous breeding, nest‐of‐rearing and nest‐of‐origin. Brightness and relative UV intensity, however, were explained only by synchronous breeding, and there was substantial unexplained variation in all three colour parameters. Among‐brood variation in mouth coloration, then, may primarily contain information about the environment in which offspring are reared. At the individual level, ontogenetic changes in the carotenoid‐richness and brightness of flanges positively reflected mass gain (a proxy for food intake). Larger and yellower chicks gained more mass, consistent with parental preferences for these traits. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 169–179.     

Carotenoid-based nestling colouration and parental favouritism in the great tit

While elaborate carotenoid-based traits in adult birds may have evolved as honest signals of individual quality in the context of sexual selection or other social interactions, the function of carotenoid-based colours in juveniles is less well understood. We investigated the hypothesis that carotenoid-based nestling colouration has evolved in response to parental preference of intensely coloured offspring during food provisioning. In a field experiment, we manipulated nestling plumage colouration by a carotenoid-supplementation and analysed the parental food provisioning behaviour before feather appearance and at the end of the nestling stage. Carotenoids per se did not influence the nestlings begging behaviour or parental feeding decisions and we found no evidence that carotenoid-based colouration in nestling great tits has a signalling function in parent-offspring interactions. Parents did not discriminate between intensely coloured and control offspring in their food provisioning and in accordance with this finding intensely coloured nestlings were not heavier or larger at the end of the nestling stage. Alternative explanations for the evolution of carotenoid-based colours in nestling birds are discussed.     

Colour also matters for nocturnal birds: owlet bill coloration advertises quality and influences parental feeding behaviour in little owls

Chromatic signals of offspring quality have been shown to play a role in parent–offspring communication in diurnal birds, but are assumed to be useless in dim light conditions because colour-based discrimination probably requires more light. A major ecological and evolutionary conundrum in this scenario is why the nestlings of some nocturnal owls display colourful beaks. Here, we test the hypothesis that yellow bill coloration of owlets of the nocturnal little owl Athene noctua may function as a chromatic signal revealing to parents aspects of quality of their offspring. In a first step, we examined physical variation in bill coloration and its covariation with owlet quality. Secondly, we studied parental provisioning in relation to an experimental manipulation of bill coloration of owlets. Bills of owlets showed higher within-nest variation in yellow–red chroma than in brightness. Plasma carotenoid concentration and nestling immunological status were not associated with chromatic or achromatic features of the bill. Interestingly, however, heavier owlets displayed more yellow bills than lighter ones. The effect of bill coloration on parental favouritism changed with brood size. Parents holding large broods preferentially fed owlets with enhanced over reduced yellow bill coloration, whereas those with small broods did not significantly bias feeding in relation to owlet bill coloration. Our results, based on integration of objective spectrophotometric assessment of colour and experimental procedures, confirm that parent little owls use bill coloration to reveal information on owlet body mass to adjust their feeding strategies, thus highlighting the importance of considering potential chromatic signals for a full comprehension of parent–offspring communication processes in nocturnal bird species.     

Mouth colour is a reliable signal of need in begging canary nestlings

Begging passerine chicks display brightly coloured mouths as they solicit food from their parents. Despite a range of hypotheses, the function of vivid nestling mouth colour remains unknown. Here I report that mouth colour functions as a signal of need in canary nestlings, in the days immediately following hatching. Changes in mouth colour accurately reflect a nestling''s state of need: the more food deprived the chick, the more intensely coloured its mouth. In controlled experiments with two nestlings, parents were offered the opportunity to choose which nestling to feed. When the mouth colour of one offspring was artificially reddened using food colouring, parents gave it more food. These results demonstrate a novel function for nestling mouth colour and are consistent with recent models of the resolution of parent–offspring conflict.     

Ultraviolet-blue reflectance of some nestling plumage patches mediates parental favouritism in great tits Parus major

The colouration exhibited by nestling birds is generally cryptic, but in some species it has a conspicuous appearance. The adaptive function of these nestling displays is poorly known, especially in altricial species. We performed an experimental study in which the ultraviolet-blue reflectance of the carotenoid-based plumage of great tit Parus major nestlings was reduced in order to find possible evidence of parental favouritism. Tarsus length increment in a three-day period was significantly lower in manipulated nestlings than in their control siblings. As tarsus length may be a good predictor of survival, the parents may increase their fitness return by investing more in those nestlings reflecting more in the UV-blue spectral region. Interestingly, this effect was only found in a yellow patch located at the nape of nestlings that has been previously overlooked, as all the studies on plumage colouration in this species have focused on the breast plumage. Some quality indicators are proposed for the colour of nestling plumage, though the possibility exists that selection may operate through aspects of signal efficacy instead of aspects of signal content.     

Colouration in Atlantic puffins and blacklegged kittiwakes: monochromatism and links to body condition in both sexes

Sexual dimorphism is widely used as an indirect measure of the intensity of sexual selection. It is also a way to evaluate whether different selective pressures act on males and females. Dichromatism, defined as a difference in colouration between males and females, may for instance result from selection for crypsis in females and selection for conspicuousness in males. Here, we conducted a study to investigate whether differential sexual selective pressures might act on the colour traits of two colonial seabird species, the Atlantic puffin Fratercula artica and the black‐legged kittiwake Rissa tricactyla. First, we used spectrophotometry and visual modelling to determine whether these presumed monomorphic birds are really monochromatic from an avian perspective (birds and humans have a different vision). Second, we estimated whether some of their colourations have the potential to be sexually or socially selected by determining whether these colourations were related to body condition in males and females, and whether the yellow, orange and red colourations may contain carotenoid pigments. Our results indicated that both species were fully monochromatic from an avian perspective. Moreover, our preliminary analyses suggested that the yellow, orange and red colours of these birds contained carotenoids. Lastly, some indices of colouration were positively linked to estimates of condition. Birds in better condition had redder gape (both species) and bill (puffins). In puffins, the relation between condition and gape colouration was significantly stronger in females than males. By contrast, the size of the gape rosette was larger in males than females. The positive links we found between colour indices and condition, together with the absence of sexual dichromatism, suggest that mutual sexual selection may act in these two species.     

Phenotypic variation in nestlings of a bird of prey under contrasting breeding and diet conditions

Environmental conditions often vary in space and time, and this may explain variation in the expression of phenotypic traits related to individual quality, such as ornamental coloration. Furthermore, the direction and strength of the relationship between coloured trait expression and individual quality might vary under contrasting conditions. These issues have been explored in adult birds but much less so in nestlings, which are more likely to experience different selective pressures and different physiological trade‐offs than adults. Here, we empirically investigated the effects of contrasting breeding and diet conditions on the expression of carotenoid‐based colour traits displayed by marsh harrier (Circus aeruginosus) nestlings. We studied the variation in coloration, body condition, and immune responsiveness of nestlings in four populations over a 5‐year period. We characterized spatiotemporal differences in rearing conditions experienced by C. aeruginosus nestlings in terms of breeding (laying date, clutch size, and number of nestlings hatched and fledged) and diet (percentage of mammal in diet and prey diversity) conditions. We found that breeding conditions influenced the co‐variation between coloration and immune responsiveness in female nestlings, and that diet conditions influenced the condition‐dependence of nestling coloration in later‐hatched nestlings. In addition, breeding conditions influenced nestling body condition and immune responsiveness, whereas diet conditions influenced nestling coloration and body condition. Our study highlights that nestling phenotype (levels of signalling, circulating carotenoids, and immunity) varies both spatially and temporally, and that some of this variation is related to differences in breeding and diet conditions. Moreover, under contrasting conditions, the direction of the relationships between nestling carotenoid‐based coloration and nestling quality may also vary. In order to fully understand the evolution and maintenance of colour traits in nestling birds, studies and experiments should ideally be replicated under contrasting rearing conditions. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ?? , ??–??.     

Condition-dependent effects of corticosterone on a carotenoid-based begging signal in house sparrows

Begging is a complex display involving a variety of different visual and auditory signals. Parents are thought to use these signals to adjust their investment in food provisioning. The mechanisms that ensure the honesty of begging displays as indicators of need have been recently investigated. It has been shown that levels of corticosterone (Cort), the hormone released during the stress response, increase during food shortage and are associated with an increased begging rate. In a recent study in house sparrows, although exogenous Cort increased begging rate, parents did not accordingly adjust their provisioning rate. Here, we tested the hypothesis that Cort might affect the expression of other components of begging displays, such as flange color (a carotenoid-based trait). We experimentally increased levels of circulating Cort and investigated the effects of the treatment on (1) the flange coloration of the nestlings, (2) the behavioral response and (3) the parental allocation of food and (4) nestling condition and cell-mediated immune response. We found that Cort affected flange coloration in a condition-dependent way. Cort-injected nestlings had less yellow flanges than controls only when in poor body condition. Parental feeding rate was also affected by the Cort treatment in interaction with flange color. Feeding rate of Cort-injected nestlings was negatively and significantly correlated with flange color (nestlings with yellower flanges receiving more food), whereas feeding rate and flange color were not correlated in control chicks. We also found that nestlings injected with Cort showed a weaker immune response than controls. These results suggest that, indeed, Cort has the potential to affect multiple components of the begging display. As Cort levels naturally raise during fasting, parents have to take into account these multiple components to take a decision as to optimally share their investment among competing nestlings.     

Tail colour signals performance in blue tit nestlings

Indirect sexual selection arises when reproductive individuals choose their mates based on heritable ornaments that are genetically correlated to fitness. Evidence for genetic associations between ornamental colouration and fitness remains scarce. In this study, we investigate the quantitative genetic relationship between different aspects of tail structural colouration (brightness, hue and UV chroma) and performance (cell‐mediated immunity, body mass and wing length) in blue tit (Cyanistes caeruleus) nestlings. In line with previous studies, we find low heritability for structural colouration and moderate heritability for performance measures. Multivariate animal models show positive genetic correlations between the three measures of performance, indicating quantitative genetic variation for overall performance, and tail brightness and UV chroma, two genetically independent colour measures, are genetically correlated with performance (positively and negatively, respectively). Our results suggest that mate choice based on independent aspects of tail colouration can have fitness payoffs in blue tits and provide support for the indirect benefits hypothesis. However, low heritability of tail structural colouration implies that indirect sexual selection on mate choice for this ornament will be a weak evolutionary force.     

Mouth Color Signals Thermal State of Nestling Dark-Eyed Juncos (Junco hyemalis)

In many species of birds, nestlings have brightly colored mouths. Some studies have found that mouth color is related to hunger, and may serve to solicit feedings from parents. We devised two experiments to test the hypothesis that mouth color is an indicator of hunger in nestling dark‐eyed juncos (Junco hyemalis), and neither experiment produced results to support the hypothesis. We did find, however, that mouth redness saturation increased for the duration of our experiments (60 min). We devised a third experiment to investigate the effect of a different stressor, temperature. In the third experiment, mouth redness decreased in saturation when microenvironment temperature increased following a period of cooling. These findings suggest that mouth color indicates thermal state of nestling dark‐eyed juncos and may function as a signal to the female to brood them.