Plumage color degradation indicates reproductive effort: an experiment

Plumage color has traditionally been regarded as a static ornamental trait, but evidence is accumulating for significant color changes without molt that typically reduce the conspicuousness of ornamentation. In some species, the social partner seems to increase its reproductive investment if the color trait is experimentally enhanced, suggesting that color change could act as a signal. However, the information content of this signal is so far unclear. For example, birds in poor condition or making greater effort may deteriorate more severely. We used brood size manipulations to alter the reproductive effort of male and female collared flycatchers Ficedula albicollis. Both sexes showed less severe decline in some reflectance attribute of their white breast when their brood was experimentally reduced. In each sex, greater deterioration of the reflectance trait affected by the manipulation was accompanied by increased feeding rate by the partner. These feeding patterns do not prove, but are consistent with, a compensatory response by the partner to induced degradation. The manipulation effects on color change we detected confirm for the first time that plumage color deterioration can indicate current reproductive effort, thereby providing a potential fitness advantage to social partners that react to such deterioration.

Not so black, not so white: differences in microorganism load of contiguous feathers from white stork chicks

Many organisms are characterized by strikingly contrasting black and white coloration, but the function of such contrasts has been inadequately studied. In this article, we tested the function of black and white contrasting plumage in white stork Ciconia ciconia chicks. We found greater abundance and diversity of microorganisms on black compared with adjacent white feathers. In addition, nest size was positively correlated with the abundance and diversity of microorganisms on white feathers. Flight initiation distance (FID), defined as the distance at which adult white storks took flight when approached by a human, was negatively correlated with most measurements of microorganism abundance. Breeding success was generally positively correlated with the abundance and diversity of microorganisms on black feathers. The feather growth rate was positively correlated with some and negatively correlated with other measurements of microbial abundance and diversity. Finally, chick growth was negatively correlated with the number of microbial species on black feathers and positively with the abundance and diversity of microorganisms on white feathers. These findings are consistent not only with the role of microorganisms in the maintenance of a benign microbial environment which differs between black and white feathers, but also with the hypothesis that several taxa of microorganisms found in black and white plumage are virulent, with negative effects on the fitness of their hosts.

Relationships among plumage coloration, blood selenium concentrations, and immune responses of adult and nestling tree swallows

In a number of taxa, males and females both display ornaments that may be associated with individual quality and could be reliable signals to potential mates or rivals. We examined the iridescent blue/green back and white breast of adult tree swallows (Tachycineta bicolor) to determine if plumage reflectance was related to adult or offspring immune responses. We simultaneously addressed the influence of blood selenium levels and the interaction between blood selenium and plumage coloration on adult and nestling immunity. Selenium is a well-known antioxidant necessary for mounting a robust immune response but its importance in wild birds remains poorly understood. In females, the brightness of white breast coloration was positively associated with bactericidal capacity, but there was no association with blood selenium. In contrast, male bactericidal capacity was associated with an interactive effect between dorsal plumage coloration and blood selenium concentrations. Males with bluer hues and greater blue chroma showed increased bactericidal capacity as blood selenium concentrations increased, while bactericidal capacity declined in greener males at higher blood selenium concentrations. In nestlings, bactericidal capacity was positively associated with nestling blood selenium concentrations and white brightness of both social parents. These results suggest that white plumage reflectance is indicative of quality in tree swallows and that greater attention should be paid to the reflectance of large white plumage patches. Additionally, the role of micronutrients, such as selenium, in mediating relationships between physiology and signals of quality, should be explored further.

Female blue tits with brighter yellow chests transfer more carotenoids to their eggs after an immune challenge

Female ornaments are present in many species, and it is more and more accepted that sexual or social selection may lead to their evolution. By contrast, the information conveyed by female ornaments is less well understood. Here, we investigated the links between female ornaments and maternal effects. In birds, an important maternal effect is the transmission of resources, such as carotenoids, into egg yolk. Carotenoids are pigments with antioxidant and immunomodulatory properties that are crucial for females and developing offspring. In blue tits, we evaluated whether ultraviolet (UV)/blue and yellow feather colouration signals a female's capacity to allocate carotenoids to egg yolk. Because mounting an immune response is costly and trade-offs are more detectable under harsh conditions, we challenged the immune system of females before laying and examined the carotenoid level of their eggs afterward. A positive association between feather carotenoid chroma and egg carotenoid level would be expected if yellow colouration signals basal immunity. Alternatively, if female colouration more generally reflects maternal capacity to invest in reproduction under challenging conditions, then other components of colouration (i.e. yellow brightness and UV/blue colouration) could be linked to maternal capacity to invest in eggs. No association between egg carotenoid levels and UV/blue crown colouration or female yellow chest chroma was found; the latter result suggests that yellow colouration does not signal immune capacity at laying in this species. By contrast, we found that, among females that mounted a detectable response to the vaccine, those with brighter yellow chests transmitted more carotenoids into their eggs. This result suggests yellow brightness signals maternal capacity to invest in reproduction under challenging conditions, and that male blue tits may benefit directly from choosing brighter yellow females.