Brood sex ratio and male UV ornamentation in blue tits (<Emphasis Type="Italic">Cyanistes caeruleus</Emphasis>): correlational evidence and an experimental test

Sex-allocation theory predicts that females paired to attractive males should bias the brood sex ratio towards male offspring, as these would inherit the attractiveness of their father. We studied sex allocation based on male ornamentation in blue tits. Brood sex ratios varied with male UV coloration in an age-dependent manner. For juvenile males, the proportion of sons increased with increasing UV ornamentation, which is in agreement with previous findings from a Swedish population. However, the relationship between UV ornamentation and brood sex ratio was reversed for adult males, with females paired to less UV-ornamented adult males producing more sons. This pattern fits with the observation that, in our population, less UV-ornamented adult males sire the majority of extra-pair young. To test the causality of the association between brood sex ratio and male coloration, we experimentally manipulated crown colour largely within the natural range. We created two groups of males: one with higher and one with lower UV reflectance, UV(+) and UV(−), respectively. Contrary to our expectations, there was no significant treatment effect. However, in UV(−), but not UV(+) males, the proportion of sons was negatively correlated with male coloration before manipulation. This suggests that the UV(−) treatment caused males that were more UV ornamented to decline more in attractiveness, as shown in a similar experiment in Sweden. However, given that correlational patterns differ between these populations, similarities in experimental results should not be taken as evidence for consistent patterns of adaptive sex allocation in this species.     

The effect of rearing environment on blue structural coloration of eastern bluebirds (<Emphasis Type="Italic">Sialia sialis</Emphasis>)

We used a brood-size manipulation to test the effect of rearing environment on structural coloration of feathers grown by eastern bluebird (Sialia sialis) nestlings. Ultraviolet (UV)-blue structural coloration has been shown to be sexually selected in this species. Our experimental design took advantage of the growth of UV-blue wing feathers in nestlings that are retained as part of the first nuptial plumage. We cross-fostered nestlings to create enlarged and reduced broods with the purpose of manipulating parental feeding rates and measured the effect on nestling growth and plumage coloration. Brood size influenced feeding rates to offspring, but the effect varied with season. In general, male nestlings reared in reduced broods were fed more often, weighed more, and displayed brighter structural plumage compared to nestlings reared in enlarged broods. Female nestlings appeared to experience less adverse affects of brood enlargement, and we did not detect an effect of brood-size manipulation on the plumage coloration of female nestlings. Measures of plumage coloration in both males and females, however, were correlated to hatching date and nestling mass during feather development. These data provide empirical evidence that environmental quality can influence the development of the blue structural coloration of feathers and that males may be more sensitive to environmental fluctuations than females.     

Testosterone is involved in acquisition and maintenance of sexually selected male plumage in superb fairy-wrens, Malurus cyaneus

Testosterone has been proposed as a physiological link between the level of sexual signalling and male condition. Bright plumage is one of the most noticeable sexual signals and is often used by females as a basis for mate choice. Yet bright male plumage is not necessarily testosterone dependent. We investigated the role of testosterone in the moult into seasonal nuptial plumage in male superb fairy-wrens. Early pre-nuptial moult is under intense intersexual selection and males can acquire the bright plumage any time between autumn and the next spring. Testosterone was always undetectable or very low in males in dull eclipse plumage. During the pre-nuptial moult, both the number of males with detectable testosterone and average testosterone levels increased sharply. High testosterone was more correlated with nuptial plumage than with presence of the cloacal protuberance (indicative of sperm storage). Subcutaneous testosterone implants always induced the pre-nuptial moult within 2–3 weeks after implantation, even well outside the natural time range of moulting. Moreover, removal of the implants before the nuptial plumage was completed, arrested the moult process. The evidence suggests that development of the nuptial plumage is testosterone dependent, although we cannot exclude that testosterone exerts its action after conversion to a metabolite such as oestrogen. Once the nuptial plumage was completed, all males maintained substantially elevated testosterone, sometimes months before the onset of breeding. These high levels could be necessary to maintain the plumage, and/or are involved in courtship displays. The results are discussed with respect to potential costs involved in acquiring and maintaining the nuptial plumage. Received: 17 January 2000 / Received in revised form: 24 February 2000 / Accepted: 25 February 2000     

An experimental test of female choice relative to male structural coloration in eastern bluebirds

Several experimental studies have shown that female birds use ornamental melanin and carotenoid plumage coloration as criteria in mate choice. Whether females choose mates based on natural variation in structural coloration, however, has not been well established. Male eastern bluebirds (Sialia sialis) display brilliant ultraviolet (UV)-blue plumage coloration on their head, back, wings, and tail, which is positively correlated with condition, reproductive effort, and reproductive success. We experimentally tested the hypothesis that female eastern bluebirds prefer as mates males that display brighter structural coloration by presenting breeding-condition females with males of variable coloration. We conducted two types of mate-choice experiments. First, females chose between males whose coloration was manipulated within the natural range of variation in the population; feathers were either brightened with violet marker or dulled with black marker. Second, females chose between males with naturally dull or bright plumage coloration. In both manipulated and unmanipulated coloration trials, female choice did not differ significantly from random with respect to structural coloration. We found no support for the hypothesis that the UV–blue coloration of male eastern bluebirds functions as a criterion in female mate choice.     

Age before beauty? Relationships between fertilization success and age-dependent ornaments in barn swallows

When males become more ornamented and reproduce more successfully as they grow older, phenotypic correlations between ornament exaggeration and reproductive success can be confounded with age effects in cross-sectional studies, and thus say relatively little about sexual selection on these traits. This is exemplified here in a correlative study of male fertilization success in a large colony of American barn swallows (Hirundo rustica erythrogaster). Previous studies of this species have indicated that two sexually dimorphic traits, tail length and ventral plumage coloration, are positively correlated with male fertilization success, and a mechanism of sexual selection by female choice has been invoked. However, these studies did not control for potential age-related variation in trait expression. Here, we show that male fertilization success was positively correlated with male tail length but not with plumage coloration. We also show that 1-year-old males had shorter tails and lower fertilization success than older males. This age effect accounted for much of the covariance between tail length and fertilization success. Still, there was a positive relationship between tail length and fertilization success among older males. But as this group consisted of males from different age classes, an age effect may be hidden in this relationship as well. Our data also revealed a longitudinal increase in both tail length and fertilization success for individual males. We argue that age-dependent ornament expression and reproductive performance in males complicate inferences about female preferences and sexual selection.     

Female European green lizards (Lacerta viridis) prefer males with high ultraviolet throat reflectance

The role of ultraviolet (UV) signals in intraspecific communication has been identified in a number of vertebrate taxa. In lizards, the signalling role of UV has only been shown in male–male competition and male mate choice. Here, we investigated whether male UV colour can be a basis of female association preference in European green lizards (Lacerta viridis), a species where males develop blue nuptial throat colouration with high UV reflectance. We experimentally manipulated the UV colour of male pairs, where the members of the pair did not differ significantly in body length, body weight, head size, throat UV chroma and brightness or throat blue chroma and brightness measured prior to colour manipulation. By providing these pairs of males to females (only visual stimuli could be perceived by the females), we assessed the role of UV in female association preference irrespective of other potentially important visual traits. We found that unmated but receptive females preferred males of higher UV reflectance. Our results show for the first time that UV colour can be an important male signal in female preference in reptiles.     

Male-male competition ensures honest signaling of male parental ability in the three-spined stickleback (Gasterosteus aculeatus)

The importance of socially imposed costs for the evolution and maintenance of honest sexual signals has received less attention than other costs. Here I show that male-male competition can increase the honesty of sexual signaling in relation to male parental ability in a species with flexible signaling. When four three-spined stickleback (Gasterosteus aculeatus) males were allowed to court a female first separately and then in competition, red nuptial coloration under competition reflected male parental ability more accurately than red coloration when separated. Parental ability was determined as the ability of the male to raise a clutch of eggs to the hatching stage under interaction with other breeding and non-breeding males. The increased honesty under competition was probably due to subordinate males of poor parental ability decreasing their color expression under interaction to reduce the risk of fights with superior males. However, socially imposed costs of signaling were probably not the main factors maintaining honest signaling, as red coloration reflected male parental ability also in the absence of competition, although less accurately. Nevertheless, the small-scale differences that male-male competition induced can significantly facilitate adaptive female choice and have large impacts on sexual selection. Received: 7 July 2000 / Revised: 31 August 2000 / Accepted: 4 September 2000     

Elaborately ornamented males avoid costly parental care in the house finch (<Emphasis Type="Italic">Carpodacus mexicanus</Emphasis>): a proximate perspective

Selection should favor flexibility in reproductive tactics when the combination of sexual traits and reproductive behaviors that achieve the highest fitness differs between males within a population. Understanding the functional significance of variation in male reproductive tactics can provide insight into their evolution. Male house finches (Carpodacus mexicanus) in a Montana population display continuous variation in parental tactics: males with more elaborated (redder) plumage color provide little or no parental care compared to less elaborated (dull) males. Here, we first determined whether elevation of prolactin (a pituitary hormone) was related to variation in male parental tactics and, second, we used the relationship between prolactin levels and parental behavior to investigate why redder males avoid a high investment in parental care. We found that prolactin elevation was closely associated with paternal care. In addition, males with redder plumage color had low prolactin levels, whereas dull males, which provision twice as frequently, had high levels of prolactin. We also found that male condition was unrelated to plumage color but negatively related to prolactin levels. These results suggest that the low provisioning of redder males was not due to physiological constraints, but instead reflected a tactic to avoid the costs associated with parental care. The condition benefits accrued by redder males may explain their higher post-breeding survival compared to dull males. Moreover, dull males were previously shown to have higher pairing success than redder males, suggesting that the relationship between male plumage color and parental care may reflect individually optimized parental tactics.Communicated by H. Kokko     

Experimental evidence that bright coloration is not important for territory defense in purple martins

Summary I tested the hypothesis that bright breeding plumage in territorial males acts as a badge of fighting ability or aggressive motivation to intimidate intruders. Territorial male purple martins (Progne subis) whose iridescent blue plumage was lightened to mimic the appearance of subadult males did not suffer an increase in intruder pressure or loss of territory compared with control males. Bright plumage color itself did not deter intruders and was not important for successful territory defense. Furthermore, a bright coloration of owners was not associated with an increased level of aggression toward intruders. Results from parallel studies on this species suggest that bright coloration is important in territory acquisition. The effectiveness of badges of fighting ability and aggressive motivation in territory defense is limited by whether intruders benefit from assessing these traits in owners. Differences in signaling systems between species are due in part to differences in floater tactics and the mode of territory acquisition.     

Egg coloration in ring-billed gulls (<Emphasis Type="Italic">Larus delawarensis</Emphasis>): a test of the sexual signaling hypothesis

Although many avian eggs appear to be cryptically colored, many species also lay vibrant blue green eggs. This seemingly conspicuous coloration has puzzled biologists since Wallace, as natural selection should favor reduced egg visibility to minimize predation pressure. The sexual signaling hypothesis posits that blue green egg coloration serves as a signal of female quality and that males exert post-mating sexual selection on this trait by investing more in the nests of females laying more intensely blue green eggs. This hypothesis has received mixed support to date, and most previous studies have been conducted in cavity-nesting species where male evaluation of his partner’s egg coloration, relative to that of other females, may be somewhat limited. In this study, we test the sexual signaling hypothesis in colonially nesting ring-billed gulls (Larus delawarensis) where males have ample opportunity to assess their mate’s egg coloration relative to that of other females. We used correlational data and an experimental manipulation to test four assumptions and predictions of the sexual signaling hypothesis: (1) blue green pigmentation should be limiting to females; (2) extent of blue green egg coloration should relate to female quality; (3) extent of blue green egg coloration should relate to offspring quality; and (4) males should provide more care to clutches with higher blue green chroma. Our data provide little support for these predictions of the sexual signaling hypothesis in ring-billed gulls. In light of this and other empirical data, we encourage future studies to consider additional hypotheses for the evolution of blue green egg coloration.     

Absence of status signalling by structurally based ultraviolet plumage in wintering blue tits (<Emphasis Type="Italic">Cyanistes caeruleus</Emphasis>)

Structurally based ultraviolet (UV)-reflective plumage parts can be important cues in mate choice. However, it remains largely unknown if UV plumage variation can also function as a signal of social status during competitive interactions. In blue tits (Cyanistes caeruleus), the UV-reflective crown plumage functions as a female mate choice cue that probably indicates male quality, as males with higher UV reflectance have been shown to have better chances of over-winter survival. Possibly, the UV crown plumage acts as a status signal in the competition over scarce food sources during winter. To test this idea, we related dominance of individuals at an artificial food source during adverse winter conditions to spectrophotometric measurements of their crown plumage. However, while controlling for the confounding effects of sex, age, and distance from territory, we found no significant effect of crown UV reflectance on dominance. Consistent with this result, we also found no relation between crown UV reflectance and over-winter survival. We conclude that the structurally based UV reflectance of the blue tit crown feathers plays little role in competition between individuals during winter despite its importance as a cue in mate choice.     

Mediating male–male interactions: the role of the UV blue crest coloration in blue tits

Badges of status, usually color patches, are hypothesized to serve as important signals within natural populations by communicating an individual’s fighting ability or aggressiveness before an interaction ever takes place. These signals, which may evolve via sexual and/or social selection, mediate intra-specific competition by influencing the outcome or escalation of contests between individuals. The last 10 years saw the rise of interest in the role of ultraviolet (UV)-based coloration in intra-sexual communication. However, the rare experimental studies that tested this hypothesis found opposite results, which may originate from the different methodological procedures used to assess the badge of status theory. We present here the results of an experiment testing whether male blue tits (Cyanistes caeruleus) respond differently to unfamiliar conspecifics presenting contrasted UV crest coloration. In an aviary, we simultaneously presented two caged blue tits with enhanced (UV+) or reduced (UV−) crest coloration to a focal bird. We found that focal males acted more aggressively towards the UV− males than UV+ males. In addition, focal males fed more often close to males that were similar in brightness or duller than themselves. We conclude that, in blue tits, UV blue crest coloration affects both social and aggressive responses towards unfamiliar individuals, and thus it has some properties of a badge of status.     

Bill color, not badge size, indicates testosterone-related information in house sparrows

The honesty of ornamental signals of quality is often argued to be enforced via costs associated with testosterone. It is still poorly understood, however, how seasonal variation of testosterone within individuals is related to the timing and extent of ornament development. Here, we studied inter- and intra-individual variability of plasma testosterone levels in a population of 150 captive male house sparrows (Passer domesticus) through the course of a full year. We further analyzed the relationship between plasma testosterone levels and two sexually dimorphic ornaments: badge size and bill coloration. Also, because of a known negative relation between molt and circulating testosterone levels, we analyzed the relationship between ornamentation and molt status during the fall. We found that testosterone levels increased towards the breeding season and decreased before the onset of annual molt. However, within individuals, relative testosterone titers demonstrated low repeatability between seasons. Plasma testosterone levels were not correlated with badge size in any season but were correlated strongly with bill coloration during all periods, except the breeding season when variation in bill color was low. Finally, we found that bill coloration strongly correlated with molt status during fall. Our results indicate that bill coloration, not badge size, is the best ornamental indicator of a “running average” of male testosterone in house sparrows and therefore the best potential indicator of qualities and/or behavioral strategies associated with testosterone.     

A novel method for quantifying the glossiness of animals

The glossy sheen of healthy hair is an ideal of human beauty; however, glossiness has never been quantified in the context of non-human animal signaling. Glossiness, the specular reflectance characteristic of polished surfaces, has the potential to act as a signal of quality because it depends upon material integrity and cleanliness. Here, we undertook two studies of glossiness in avian plumage to determine (a) the repeatability of a recently developed measure of glossiness, (b) the relationship between glossiness and conventional measures of coloration, and (c) how glossiness is associated with quality signaling. Using museum specimens of three North American bird species with glossy plumage (red-winged blackbird, Agelaius phoeniceus; great-tailed grackle, Quiscalus mexicanus; Chihuahuan raven, Corvus cryptoleucus), we found that the glossiness measure was highly repeatable for all species and was significantly correlated with plumage coloration (e.g., chroma, brightness) in male great-tailed grackles. We then used wild-caught grackles to examine sexual dimorphism in plumage glossiness and its correlation to a potentially sexually selected trait in this species, male tail length. We found that males were significantly glossier than females and that male, but not female, glossiness correlated positively with tail length. This study provides a repeatable method to measure glossiness and highlights its potential as a signal of individual quality in animals.     

Female yellowhammers (Emberiza citrinella) prefer yellower males: a laboratory experiment

The importance of male plumage coloration as a signal of male dominance and a cue for female choice in the monogamous yellowhammer, Emberiza citrinella, was tested in two sets of experiments in an indoor aviary. Dominance was tested by introducing two individuals, with no previous experience of each other, in an aviary with food and water. Aggression occurred more often between two males than between a male and a female and more frequently between pairs of males including at least one old male than between two young males. Dominance was not related to male colour in trials between males of the same age class, but it was in trials between an old and a young male, often differing in colour. Thus, age may be a more important determinant of dominance than colour. Female preference for more colourful males was tested by allowing hormone-induced females to choose between a more and a less colourful dummy male. Females spent more time in front of more colourful than drabber males and also more often perched beside colourful males than duller individuals. Although male colour cannot be ruled out as a dominance signal, the results suggest that male colour is primarily used as a signal in mate choice. Female choice may hence be responsible for maintenance of bright plumage in the male yellowhammer.     

The effects of preen oils and soiling on the UV–visible reflectance of carotenoid-pigmented feathers

Plumage coloration, particularly when carotenoid-based, is important in social signaling in birds. Although feather color is a relatively stable trait, individuals may modify it with “cosmetic” substances such as preen oils. In addition, dirt accumulation may influence plumage coloration and further affect signal perception by receivers. Here, we analyze the separate potential effects of preen oils and soil accumulation on the reflectance properties of carotenoid-pigmented feathers across the visual range of most bird species, which includes the ultraviolet (UV). Using the yellow portion of tail feathers of Bohemian waxwings (Bombycilla garrulus), we performed two separate experiments where: (a) preen oils and/or soil were removed, or (b) preen oils (from black-billed magpies Pica pica or eagle owls Bubo bubo) were added. Preen oil addition reduced brightness but increased UV hue and yellow chroma. UV chroma was reduced by the addition of magpie (but not owl) preen oil. Soil accumulation had little effect on plumage reflectance in the UV range but significantly reduced yellow chroma. According to models of avian vision, both of these effects are detectable by birds and biologically meaningful when compared with natural variation between the sexes and age classes. We conclude that preen oil and soil accumulation can significantly affect the UV–visible reflectance of carotenoid-based plumages. As such traits typically advertise individual quality, preening and soiling have the potential to modify the information content of carotenoid-based plumage traits and how these signals are perceived by receivers.     

Delayed plumage maturation in Lazuli buntings: tests of the female mimicry and status signalling hypotheses

The evolutionary importance of delayed plumage maturation (DPM) in passerines, the condition when more than 1 year is required to achieve adult-like coloration, remains highly contentious. Adaptive hypotheses propose that aggression from after 2nd-year (ASY) males or predation favors DPM in 2nd-year (SY) males, thereby increasing SY male survivorship or reproductive success. However, each hypothesis suggests a distinct selective mechanism explaining “how” this is accomplished. Alternatively, DPM may be a consequence of a nonadaptive molt constraint. We tested the female mimicry and status signalling hypotheses in territorial ASY male lazuli buntings (Passerinaamoena) using three sets of model presentation experiments. The female mimicry hypothesis proposes that dull SY male plumage deceptively mimics female plumage, and predicts that ASY males can not distinguish SY male from female plumage. The status signalling hypothesis proposes that dull SY male plumage honestly signals low competitive threat, and predicts that ASY males respond less aggressively to dull versus bright, ASY-like plumage. Contrary to the female mimicry hypothesis, ASY males distinguished between SY male and female plumage, as they were aggressive to SY male models exclusively and attempted to copulate with female models. Supporting the status signalling hypothesis, ASY males were significantly less aggressive to SY versus ASY male plumage. While DPM may result from a physiological constraint on bright SY male plumage, our results support the idea that dull plumage in an SY male's first breeding season may be maintained by selection to reduce aggression from ASY males, serving as a signal of competitive status. Received: 21 February 1997 / Accepted after revision: 16 June 1997     

The annual testosterone profile in cooperatively breeding superb fairy-wrens, Malurus cyaneus, reflects their extreme infidelity

Superb fairy-wrens are cooperatively breeding birds that combine stable, socially monogamous pair bonds and high levels of paternal care, with extreme levels of extra-pair mating and high levels of sexual competition. Our aim was to determine which testosterone correlates would prevail in such a life history that combines features that are conventionally associated with divergent hormone profiles. Unlike the situation in other species with monogamous pair bonds and high levels of paternal care, testosterone was elevated for a very long period of several months. During breeding there was a broad peak in testosterone followed by a gradual decline: this resembles the profile found in polygynous and promiscuous species. We found that three factors correlated with testosterone: development of the sexually selected nuptial plumage, social status and extra-group mating opportunities. Testosterone started increasing months prior to breeding, when the males that are later preferred as extra-group sires develop their nuptial plumage. Although these males did not have higher testosterone levels during breeding, they sustained high testosterone for much longer, and this might lend reliability to this sexual signal. Dominant males in groups had higher testosterone than pair-dwelling males and subordinate helpers. This was not due to differences in age, reproductive capability or mating opportunities, but was presumably associated with the assertion of dominance. In contrast to findings in other species, male testosterone level was not correlated with whether the resident female was fertile or had dependent nestlings. However, testosterone was strongly correlated with the total number of fertile females in the population, and hence with the opportunities for extra-group mating.     

Parental effects on carotenoid-based plumage coloration in nestling great tits, Parus major

Carotenoid pigments have attracted much interest in behavioural and evolutionary ecology because of their dual function in immune physiology and as color signals. In vertebrates, carotenoids must ultimately be obtained from the diet, and the mechanisms and magnitude of this environmental dependence are central for understanding carotenoid signal functions and evolution. In the present cross-fostering experiment with great tits Parus major, we investigate pre- and postnatal parental effects (egg yolk carotenoids, parental coloration) on nestling size and carotenoid coloration, using HPLC analysis of egg yolk carotenoids, and a reflectance-based measure of ‘chroma’ that reflects the plumage pigment concentration. Both rearing environment and origin influenced offspring size and plumage chroma. Maternal allocation of carotenoids to eggs had a weak positive effect on nestling plumage chroma, whereas we found no prenatal maternal effects (egg size or yolk carotenoid concentration) on size. Nestling plumage chroma was also significantly predicted by the chroma of the rearing father, but not by the color of the rearing mother or either of the original (genetical) parents. Thus, both prenatal maternal effects and postnatal paternal effects influence the carotenoid-based plumage coloration of nestling great tits. Future studies will reveal if parental effects have long-term consequences for plumage development and associated fitness components.     

Carotenoid-derived ornaments reflect parental quality in male and female yellow-eyed penguins (<Emphasis Type="Italic">Megadyptes antipodes</Emphasis>)

The handicap principle suggests that ornamental traits that function as honest signals in mate selection must be costly to be effective. We evaluated in the sexually monochromatic yellow-eyed penguin (Megadyptes antipodes) whether the carotenoid-derived plumage and eye coloration predicts parental quality and whether males and females within pairs mate assortatively in relation to these carotenoid-derived ornaments. In addition, we investigated whether age or body condition was related to the coloration of the ornamental traits. In yellow-eyed penguins, parental quality of males and females was predicted by eye and head plumage coloration. Even when we controlled for gender- and age-specific differences, eye and head plumage coloration reflected honestly parental quality. Males and females mated assortatively in relation to these ornamental traits. While age influenced coloration of both the eye and head plumage, body condition was related only to the saturation of plumage coloration. These results provide evidence that the carotenoid-derived ornaments in yellow-eyed penguins reflect the parental abilities of birds and, therefore, may be costly signals. Potentially, female and male yellow-eyed penguins could use eye and plumage coloration as an indirect cue in assessing age and quality of individual birds during mate choice. This is only the second study to examine plumage coloration in relation to sexual selection in penguins, while conspicuous ornamental traits in other species of penguin beg the question whether they also play a role in sexual selection.Electronic Supplementary Material Supplementary material is available in the online version of this article at Communicated by C.R. Brown