Ultraviolet plumage does not signal social status in free-living blue tits; an experimental test

Ultraviolet (UV) signals are suggested to be sexually selectedin a wide range of taxa. Most research, however, has focusedon the role of UV signals in mate choice, whereas possible functionsin intraspecific competition remain largely untested. Studieson other colors indicate that ornaments preferred by femalescan also function as signals of social status in competitiveinteractions between individuals. Whereas these colors are mainlypigment based, UV reflectance is generally caused by selectivereflectance of light from surface structures. Here we test experimentallywhether the structurally based UV-reflective crown plumage inthe blue tit (Cyanistes caeruleus) serves as a signal of statusin interindividual competition. We reduced the crown UV reflectanceof free-living blue tits in winter and compared their probabilityof winning conflicts over food at a feeding table with control-treatedand untreated individuals. Although we controlled for effectsof sex, age, and distance from territory, we found no effectof reduced UV reflectance on the probability of winning norwere conflicts involving UV-reduced individuals more likelyto escalate. Therefore, we conclude that the UV reflectanceof the blue tit's crown does not serve as a signal of statusin competition over food in winter. We suggest that the observedsite-dependent dominance structure may constrain the opportunityfor a status signal to evolve and that enhancing attractivenessin mate choice may be the sole function of the crown's UV reflectance.     

Age-related differences in plumage characteristics of male tree swallows Tachycineta bicolor: hue and brightness signal different aspects of individual quality

Age-related differences in plumage characteristics of birds can be the result of differential survival of more ornamented individuals, within-individual changes in plumage attributes with age, or a combination of both. In this study, we investigated age-class related differences in plumage attributes of male tree swallows Tachycineta bicolor by performing both cross-sectional and longitudinal analyses. Male tree swallows in their first breeding season do not display delayed plumage maturation, and possess, like experienced breeders, metallic green to metallic blue iridescent plumage on their dorsal surface. Our results showed that, at the population level, older males were brighter and reflected light maximally at shorter wavelengths (i.e. were more blue). Differences in plumage brightness were most likely caused by changes within individuals as males increased in brightness between the first time they were captured and the subsequent year. Differences in hue, however, were not due to within-individual changes, but rather appear to be the result of greener individuals having lower survival and/or nest site fidelity. Indeed, relatively dull, greener birds had a lower probability of being recaptured the subsequent year. In contrast, we found that if birds captured in their first year as breeding adults were relatively bright, hue was not related to the probability of recapture. These results suggest that plumage attributes in male tree swallows have the potential of being honest signals of quality. Furthermore, plumage brightness and plumage hue might signal different aspects of male quality in this species.     

Multiple sexual ornaments in satin bowerbirds: ultraviolet plumage and bowers signal different aspects of male quality

Much attention has been devoted to understanding the evolutionof elaborate male ornaments and how they may signal male quality.However, the evolution of multicomponent sexual signals remainspoorly understood, and past research on this type of signalinghas been largely theoretical. Satin bowerbirds, Ptilonorhynchusviolaceus, are polygynous, are sexually dichromatic, and constructsexually selected display structures (bowers): a model systemfor investigating the evolution and signal function of multiplesexual signals. We studied the interrelationship between bowerfeatures, plumage coloration, and indicators of male qualityin this species. To do this, we located the bowers of male satinbowerbirds in rainforest in Queensland, Australia, and quantifiedbower quality. We captured the male bower owners and used reflectancespectrometry to objectively measure the plumage coloration ofseveral body regions. We measured various indicators of malehealth and condition, including the intensity of infection fromectoparasites and blood parasites. Bower quality and male ultravioletplumage coloration were significantly correlated. By using multipleregression analyses, we show that bower quality predicts ectoparasiteload and body size, whereas ultraviolet plumage coloration predictsthe intensity of infection from blood parasites, feather growthrate, and body size. Our findings support the multiple messageshypothesis of multicomponent signals: Female satin bowerbirdsshould assess both male and bower features to choose the highestquality mates.     

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. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 76 , 237–245.     

The separate effects of egg size and parental quality on the development of ornamental plumage coloration

The fitness‐related consequences of egg size, independent of the influences of parental quality, are poorly understood in altricial birds. Not only can egg size and parental quality influence growth and survival, but each could influence the development of condition‐dependent plumage coloration in offspring. The Eastern Bluebird Sialia sialis is an altricial, multi‐brooded, cavity‐nesting passerine in which juveniles display dichromatic UV‐blue plumage. Previous research suggests that plumage coloration acts as a signal of individual quality among juvenile and adult Eastern Bluebirds. Here, we separate the effects of egg size and parental quality (defined by egg size laid) on nestling growth and plumage ornamentation by exchanging clutches of large eggs with clutches of small eggs. Nestlings were significantly larger immediately post‐hatching when hatched from a large egg, but to maintain a larger size, nestlings needed to have hatched from a large egg and to have been reared by high‐quality parents. Nestlings were brighter when reared by high‐quality parents and this relationship was strongest later in the breeding season. Nestlings exhibited greater UV chroma if hatched early in the season, but UV chroma was not significantly affected by egg size or parental quality. These findings demonstrate varying influences of both egg size and parental quality on offspring growth and plumage ornamentation but suggest that quality of post‐hatching investment is more influential than pre‐hatching investment.     

The cheek plumage patch is an amplifier of dominance in great tits

Amplifiers are signals that enhance the perception of other signals or cues, but no studies to date provide empirical evidence for the role of these signals in a reproductive context. Here we use the white cheek patch of great tits as a model for studying this issue. Aggressive interactions decrease patch immaculateness, so patch size may be an amplifier of dominance, that is, more clearly reveal status. If so, in high-quality individuals patch size should correlate positively with reproductive success (here estimated by laying date, assuming that the earlier the better), whereas low-quality individuals with a large patch should only more clearly reveal their low quality and thus suffer low reproductive success, which is exactly the pattern found in males. In contrast, the cheek patch does not seem to function as an amplifier in female great tits.     

Parid foraging choices in urban habitat and their consequences for fitness

Urban environments are habitat mosaics, often with an abundance of exotic flora, and represent complex problems for foraging arboreal birds. In this study, we used compositional analysis to assess how Blue Tits Cyanistes caeruleus and Great Tits Parus major use heterogeneous urban habitat, with the aim of establishing whether breeding birds were selective in the habitat they used when foraging and how they responded to non‐native trees and shrubs. We also assessed whether they showed foraging preferences for certain plant taxa, such as oak Quercus, that are important to their breeding performance in native woodland. Additionally, we used mixed models to assess the impact of these different habitat types on breeding success (expressed as mean nestling mass). Blue Tits foraged significantly more in native than non‐native deciduous trees during incubation and when feeding fledglings, and significantly more in deciduous than evergreen plants throughout the breeding season. Great Tits used deciduous trees more than expected by chance when feeding nestlings, and a positive relationship was found between the availability of deciduous trees and mean nestling mass. Overall, the breeding performance of both species was poor and highly variable. Positive relationships were found between mean nestling mass and the abundance of Quercus for Great Tits, but not for Blue Tits. Our study shows the importance of native vegetation in the complex habitat matrix found in urban environments. The capacity of some, but not all, species to locate and benefit from isolated patches of native trees suggests that species vary in their response to urbanization and this has implications for urban ecosystem function.     

Variation in effects of male plumage ornaments: the case of Iberian Pied Flycatchers

It has been proposed that mate preferences by female Pied Flycatchers Ficedula hypoleuca differ between southern (Iberian) and northern (Scandinavian) European populations. Whereas the size of the white forehead patch, but not plumage colour, has been reported to be a sexually selected trait in the former, only plumage darkness apparently acts as an ornament in the latter. In addition, northern male Pied Flycatchers become darker with age, a trend not detected until the present study in southern birds. Here we show that in an Iberian population of Pied Flycatchers breeding only a few tens of kilometres from previously studied populations, plumage darkness is associated with mating success and increases with age, whereas the size of the white forehead patch is not related to mating success and is only weakly correlated with age, trends similar to those reported for Scandinavian rather than other Iberian Pied Flycatcher populations. This represents a case of variation in sexually selected traits between geographically close populations of Pied Flycatchers that cannot be explained by sympatry with closely related species. It is proposed that differences in the identity and abundance of environmental stressors may be the cause of this regional variation in sexually selected traits.     

Reproductive correlates of plumage coloration of female Mountain Bluebirds

Many studies have shown that the plumage coloration of male birds can act as an honest signal of quality, indicating benefits that a female could gain from pairing with a specific male. In some species, females also display ornamental plumage, but less is known about the function and potential adaptive significance of female coloration because most research has focused on male coloration. Male Mountain Bluebirds (Sialia currucoides) display full body, ultraviolet (UV)‐blue plumage, whereas female plumage is more subdued, with blue color focused on the rump, wing, and tail. During the 2011 and 2012 breeding seasons (May–July) near Kamloops, BC, Canada, we examined coloration of the rump and tail of female Mountain Bluebirds to determine if their plumage could act as an indicator of direct reproductive benefits (e.g., enhanced parental care or reproductive success) to potential mates. We found no relationship between female plumage coloration and either provisioning rate or fledging success. However, female coloration varied with age, with after‐second‐year (ASY) females having brighter, more UV‐blue tail feathers than second‐year (SY) females. In addition, ASY females with brighter, more UV‐blue tails had larger clutches. We also observed positive assortative mating by tarsus length. Because previous work with other species suggests that female body size may be a good predictor of breeding success, males could potentially benefit from pairing with larger females. However, reproductive success did not vary with female size in our study. Although our evidence that structural plumage coloration of female Mountain Bluebirds is a signal of direct reproductive benefits for males (e.g., higher reproductive success) is limited, our results (i.e., ASY females with brighter tails than SY females, and ASY females with brighter tails having larger clutches) do suggest the potential for sexual selection to act on female coloration.     

Female coloration indicates female reproductive capacity in blue tits

It is poorly understood whether female morphological and behavioural traits can be used as 'signals'. In particular, experimental tests of the hypothesis that female ornaments reflect quality are scarce. Here, we experimentally examine whether female plumage coloration might signal maternal quality in the blue tit, Cyanistes caeruleus by forcing half of the females breeding in our population to produce a replacement clutch. Using statistical models that controlled for the effects of male coloration, and the effects of age and condition of both parents, we found that carotenoid-based female coloration was positively linked to key proxies of bird lifetime reproductive success: clutch size, fledgling success and recruitment. Importantly, the relationships between maternal yellow carotenoid coloration and both clutch size and recruitment were stronger in the experimental group than in the control group, indicating that breeding females with higher values of yellow coloration were better able to handle the cost of producing a second clutch. Finally, UV-blue female coloration was positively linked to female survival and marginally linked to laying date. Taken together, these results show for the first time in a natural population that female coloration can indicate individual and maternal quality under natural and adverse reproductive conditions. They highlight the potential for the evolution of female ornamental traits through sexual selection.     

Haemosporidian parasites depress breeding success and plumage coloration in female barn swallows Hirundo rustica

Parasites are major effectors of natural selection and also play a role in sexual selection processes. Haemosporidian blood parasites are common in vertebrates and have been shown to vary in their effects depending on both the parasite and host species, on the host trait investigated as well as on host condition and stage of infection. Here we investigated infection of adult barn swallows Hirundo rustica by Plasmodium, Leucocytozoon and Haemoproteus species during the chronic stage of infection and the consequences for host fitness traits. Prevalence was higher than 10% only for Plasmodium. Chronic stage infection by Plasmodium was associated with reduced female breeding success, but did not affect breeding dates. Infection did not affect the expression of male secondary sexual traits (tail length and melanin‐based plumage coloration), but was associated with paler coloration of females. Finally, we found a negative effect of infection by Plasmodium on feather growth rate in older but not in yearling individuals. Because feathers are moulted during wintering in sub‐Saharan Africa where infection of barn swallows by Plasmodium occurs, our results suggest that male secondary sexual traits have little potential to reveal acute‐stage infection whereas plumage coloration of females may advertise their infection status. In addition, these results suggest that infection by Plasmodium can influence the course of plumage moult. Thus, our results add to the observations of negative effects of haemosporidian infection on fitness traits in birds and provides evidence that these effects can vary among traits and in relation to age and sex.     

Consistency of passerine embryo development and the use of embryonic staging in studies of hatching failure

Hatching failure is widespread in birds, and is usually the result of embryo death rather than infertility. Embryo death can result from both intrinsic and extrinsic factors, some of which may vary across the developmental period. Determining the point at which an embryo died during development may therefore help us to understand the underlying cause of death. Here we describe simple criteria that can be used by field ornithologists to establish the developmental stage of dead embryos found in unhatched passerine eggs, and explain how this can be used to estimate the date of embryo death. We compared the pattern of embryo development over the incubation period for three species, the Zebra Finch Taeniopygia guttata, Blue Tit Cyanistes caeruleus and Great Tit Parus major. We also compared rates of Zebra Finch embryo development under artificial and standard (parental) incubation. Embryo development rates were remarkably similar across the three species and between Zebra Finch embryos under artificial and natural incubation conditions. We therefore suggest that the pattern of embryo development in the Zebra Finch may provide a model for other small passerines with similar incubation periods, but acknowledge that further interspecific comparisons are required before this model is considered more widely applicable. By estimating embryo death dates using our approach, ornithologists will be able to determine temporal patterns of embryo mortality in relation to extrinsic environmental conditions. This approach may shed light on how extrinsic factors such as climate and parental behaviour influence embryo survival in wild birds.     

Revealing the colourful side of birds: spatial distribution of conspicuous plumage colours on the body of Australian birds

In many species of birds, different body parts often display very different colours. This spatial distribution of coloured plumage patches may be determined, among other factors, by the balance between being cryptic to predators, and conspicuous to intended receivers. If this is the case, ventral and anterior body parts in birds – which are less visible to predators but more prominent to conspecifics – should present more conspicuous and sexually dichromatic plumage colours. Here, I test these predictions using reflectance spectrometric measurements of standardised plumage patches across males and females for nearly an entire avifauna (Australian landbirds, n = 538 species). My data show that, as predicted, conspicuous and sexually dichromatic colours are mainly located near the head, while the plumage of the back is the most cryptic. One clear exception to this pattern is the conspicuous rump coloration. In many species, this patch can be concealed by wings, and therefore exposed only when necessary. In addition, conspicuous rump coloration could deflect or confuse predators in case of attack. However, there is considerable variation across species, and this makes position on the body a very poor predictor of plumage elaboration (R² < 0.02). Future studies should try to determine whether differences between species in the distribution of colours across the plumage are due to variation in ecological factors (predation risk, habitat, etc.).     

Variation in melanin pigmentation of a sexually selected plumage trait and its adaptive value in the Common Snipe Gallinago gallinago

There is increasing evidence that melanin‐based plumage coloration correlates with different components of fitness and that it may act as a social or sexual signal of individual quality. We analysed variation in melanin pigmentation in the outermost tail feathers of the Common Snipe Gallinago gallinago. During courtship flights, male Snipe use their outermost tail feathers to generate a drumming sound, which plays a role in territory establishment and mate choice. As the outermost tail feathers are displayed to females during these flights, we predicted that conspicuous variation in their rusty‐brown (pheomelanin‐based) coloration may act as an honest signal of individual quality. To test this prediction, we spectrophotometrically measured brightness (an indicator of total melanin content) and red chroma (an indicator of pheomelanin content) of the outermost tail feathers in 180 juvenile and adult Common Snipe. An age‐related decline in feather brightness was found exclusively in females, suggesting that melanization could have evolved by natural selection to camouflage incubating birds. In both sexes, brightness of the tail feathers was inversely correlated with their structural quality (as measured with mass–length residuals), suggesting that melanization could increase mechanical properties of feathers and, in males, enhance the quality of courtship sonation. Red chroma positively correlated with total plasma protein concentration, supporting our prediction that pheomelanin pigmentation of tail feathers may act as an honest signal of condition. Our study indicated that variation in the melanin‐based coloration of the outermost tail feathers in the Common Snipe could have evolved as a result of several different selection pressures and it emphasizes the complexity of the processes that underlie the evolution of melanin‐based plumage coloration in birds.     

Structurally based plumage coloration is an honest signal of quality in male blue grosbeaks

We investigated the signaling function of blue plumage in maleblue grosbeaks (Guiraca caerulea) to determine if structurallybased coloration may act as a reliable signal of quality toconspecifics. Blue plumage results from the microstructureof feather barbules rather than from pigment granules, andthus it is possible that structurally based plumage ornamentsmay function differently from sexually selected ornamental coloration that is pigment based. The plumage of male blue grosbeaksreflects maximally in the blue-ultraviolet range, so most variationin plumage coloration among males is invisible to human observers.In previous research, we showed that increased area of blueplumage on the body is associated with a shift in the wavelengthof maximum feather reflectance toward the ultraviolet and withhigh intensity of light reflected at that maximum, and thatextreme expression of the male ornament is condition dependent.These observations suggest that blue plumage may be an honestadvertisement of male quality. We tested this hypothesis ina wild population of blue grosbeaks. We quantified male qualityin three broad categories. (1) Physical condition was assessed from subcutaneous fat deposits, ectoparasite load, and bodysize. (2) Territory quality was assessed from territory area,prey abundance, and predation risk. (3) Paternal investmentwas assessed from male feeding rate. We found that the bluestmales have the largest body size, maintain the largest territorieswith the greatest prey abundance, and feed nestlings in thefirst nest of the season at the highest rates. We conclude that structurally based plumage coloration functions as an honest,intraspecific signal of quality.     

Winter male plumage coloration correlates with breeding status in a cooperative breeding species

The function of colored ornaments is usually related to thesignaling of individual quality in intra- and intersexual interactions.In cooperative breeding species, where only a fraction of themale population access the breeding status and the other fractionhas the option to help breeding pairs, colored traits mightprovide the females with a reliable information on the qualityof potential mate. Males of the cooperative breeding azure-wingedmagpies (Cyanopica cyanus) display conspicuous blue plumagecoloration. Here we explored the role played by structural bluecoloration of males and the probability of becoming a breederor a helper. Birds were trapped during 4 consecutive years,and feather coloration was measured with a spectrometer. Malesthat became breeders had a more brilliant and saturated bluecoloration and showed a more violet hue in the nonbreeding periodcompared with birds that became helpers. Breeding males alsoshowed a seasonal decline in blueness, whereas the color propertiesof helpers were constant throughout the year. Blueness of individualstrapped in the nonbreeding period was positively correlatedwith body size and condition. These findings are consistentwith a scenario in which nonbreeding blue plumage colorationmay function as a signal of individual quality in the azure-wingedmagpie at the pair formation time and add to growing evidencesuggesting that the nonbreeding season appears particularlyimportant in impacting breeding roles in cooperative breedingbirds.