The adaptive significance of male subadult plumage in purple martins: plumage dyeing experiments

Summary I tested two hypotheses for the adaptive significance of subadult plumage in male purple martins (Progne subis) : the female mimicry and subordinance signaling hypotheses. Subadult males were at a competitive disadvantage in obtaining territories, as they arrived later in the spring than adult males. Contrary to the predictions of both hypotheses, adult male territory owners were not less aggressive toward subadult male than adult male intruders. The subadult plumage was not effective in mimicking females, as adult male owners were significantly more aggressive toward subadult male than female intruders. Summer adaptation hypotheses predict that young males in subadult plumage are more successful in acquiring territories and mates than they would be with an adult plumage. I tested this prediction by dyeing the plumage of floater subadult males to mimic the appearance of adult males. In 13/17 paired experiments, dyed subadults obtained territories before control subadults. There was no difference in the time it took dyed and control males to attract a mate after they obtained a territory. These results suggest that the subadult plumage is not an advantage to young males in competing with adult males for breeding resources. In late winter, subadult males were growing mostly femalelike feathers on their underside, suggesting that the subadult plumage is not the result of a molt constraint. The subadult plumage could enhance survival of yearlings in winter roosts if it improves access to good roost sites or reduces the risk of predation.     

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     

Social environment during molt and the expression of melanin-based plumage pigmentation in male house sparrows (<Emphasis Type="Italic">Passer domesticus</Emphasis>)

Evolutionary biologists have shown much recent interest in the costliness and signal content of colorful plumage displays in birds. Although many studies suggest that both carotenoid- and structurally-based plumage colors are condition-dependent indicators of health and nutritional state at the time ornamental feathers are grown, there is little experimental evidence supporting the idea that melanin pigmentation is a reliable signal of condition during molt. Instead, melanin-based ornamental coloration often reveals the competitive ability and dominance of individuals throughout the year. However, this work does not indicate which proximate environmental factors shape the expression of melanin pigmentation at the time of feather growth. Because of the link between melanin coloration and the social environment, it is possible that the development of brightly colored plumage may be associated with aggressive social interactions during feather molt. Here, we show that melanin-based ornamental coloration in male house sparrows (Passer domesticus) is correlated with the degree to which individuals interact aggressively with conspecifics during molt. Males that were dominant (beta, but not alpha) within captive social groups during molt grew larger badges than subordinates. Groups of males that had higher rates of aggression during molt grew larger badges than less aggressive triads. To our knowledge, this is the first demonstration that melanin pigmentation and plumage-based status badges are related to the competitive history of individuals during feather development. By coupling badge size directly with aggressive experiences during molt, birds can use their status signal to honestly indicate their likelihood of winning agonistic encounters throughout the year.     

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.     

Relative importance of multiple plumage ornaments as status signals in golden whistlers (<Emphasis Type="Italic">Pachycephala pectoralis</Emphasis>)

Status signals are traits that advertise an individual’s competitive abilities to conspecifics during aggressive disputes. Most studies of status signals in birds have focussed on melanin-based plumage signals, but recent research shows that carotenoid-based signals may also play a role in aggressive signaling. We assessed the relative importance of melanin- and carotenoid-based plumage patches as agonistic signals in a small passerine, the golden whistler (Pachycephala pectoralis). Display signals in male golden whistlers include an unpigmented white throat patch, a carotenoid-based yellow breast and nape band, and a melanin-based black chin-stripe. We found that only the white throat patch was correlated with contest-related attributes. Males possessing large throat patches defended larger territories and commenced breeding earlier. When caged males with either experimentally reduced, or unmanipulated throat patches were presented to conspecifics, those with experimentally reduced patches attracted less aggression from male subjects. Focal males also responded faster to caged males with throat patches similar in size to their own, suggesting that they may assess relative throat patch size before engaging in aggressive encounters. Females did not discriminate between “reduced” or “control” treatments. Our data strongly suggest that only the unpigmented throat patch functions as a status signal. As this signal is unlikely to have significant development costs, honesty may be maintained through social costs.     

Sexual selection and the function of a melanin-based plumage ornament in polygamous penduline tits <Emphasis Type="Italic">Remiz pendulinus</Emphasis>

Melanin-based ornaments are often involved in signaling aggression and dominance, and their role in sexual selection is increasingly recognized. We investigated the functions of a melanin-based plumage ornament (facial ‘mask’) in male Eurasian penduline tits Remiz pendulinus in the contexts of male–male aggression, mating success, and parental care. The penduline tit is a passerine bird with a unique mating system in which both sexes may mate with several mates in a breeding season, and one (or both) parent deserts the clutch. Our study revealed that mask size of males is more likely an honest signal used by females in their mate choice decisions than a trait involved in male–male competition. First, mask size increased with both age and body condition, indicating that the mask may signal male quality. Second, males with larger masks paired more quickly and had more mates over the breeding season than males with smaller masks. Third, we found no evidence that male mask size signals male–male aggression or dominance during competitive encounters. The increased mating success of large-masked males, however, did not translate into higher reproductive success, as nestling survival decreased with mask size. Therefore, we conclude that there is either no directional selection on male mask size or males with larger masks receive indirect, long-term benefits.     

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.     

Behavioral tactics of subadult female floaters in the tree swallow

Summary There are large numbers of reproductively mature female tree swallows (Tachycineta bicolor) which do not breed due to limits of suitable nesting cavities. Many of these floaters are one-year-old females that have a distinctive subadult plumage. This study examines the behavioral tactics that these subadult female floaters use to obtain breeding opportunities. Early in the season, subadult floaters tended to intrude briefly (Fig. 4) on many nest sites in succession (Figs. 2, 3), although they rarely gained close access to nest sites (Fig. 5). Subadults responded very quickly to vacant nest sites, where the resident female had been experimentally removed, by entering the nest cavity and defending it from conspecifics. We argue that the early season exploratory behavior increases a subadult's chances of discovering a vacant nest site, rather than increasing its success in evicting resident females or laying eggs in other females' nests. During the nestling period, subadult females intruded on fewer nest sites for longer periods, and often gained close access to the nest site. Late in the season, subadult floaters may be gathering information on the quality of nest sites for the next breeding season, rather than searching for current breeding opportunities. The reproductive tactics of subadult female tree swallows are consistent with the breeding threshold model for the evolution of delayed plumage maturation in passerines.     

Testosterone and melanin-based black plumage coloration: a comparative study

Despite the functional significance of melanin-based plumage coloration in social and sexual signaling, the mechanisms controlling its information content are poorly understood. The T-regulation hypothesis proposes that melanin ornaments signal competitive abilities via the effects of testosterone (T) mediating both melanization and sexual/aggressive behaviors. Using the phylogenetic comparative approach, we tested whether frontal black melanization is associated with elevated T around the time of breeding plumage development across all bird species with available T-data. We found a context-dependent relationship between melanization and T, varying with the type of ornamentation (patchy or full-black) and with the presumed taxonomic distribution of the hormonal control of plumage dichromatism. Within two taxa in which male plumage development is assumed androgen-dependent (Charadriiformes, Corvida), evolutionary increases in male melanization, and melanin dichromatism correlated with increases in T in most analyses but not within the basal lineage (ratites, Galloanseriformes) with androgen-independent male plumage. Among Passeroidea with presumably genetically or luteinizing-hormone-based male plumage, melanization and its dichromatism correlated with T only in species with <100% frontal melanization. These results were robust as we controlled for several confounding variables such as mating and parental behaviors. This study is the first to test and support the T-regulation hypothesis interspecifically, suggesting that among-species differences in melanization may have evolved in response to differences in circulating T in certain avian taxa. Our results imply that the extent of black ornamentation may serve as an honest indicator of male competitiveness in those species that evolved an appropriate hormonal basis (T dependence) for color production. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Achromatic color variation in black-capped chickadees,Poecile atricapilla: black and white signals of sex and rank

Sexual dichromatism and phenotypic variation in elaborate male traits are common products of sexual selection. The spectral properties of carotenoid and structurally-based plumage colors and the patch sizes of melanin-based plumage colors have received considerable attention as sexual signals in birds. However, the importance of variation in achromatic plumage colors (white, gray and black) remains virtually unexplored, despite their widespread occurrence. We investigated a potential signal function of the achromatic black and white plumage of black-capped chickadees (Poecile atricapilla). We captured and color-banded 178 free-living chickadees and assessed winter flock dominance hierarchies by tabulating pairwise interactions at feeders. We recaptured 73 of these birds and measured plumage coloration for six body regions using a reflectance spectrometer and the area of melanin-based plumage patches from standardized photographs. We found extensive individual variation in chickadee plumage traits and considerable sexual dichromatism. Male black-capped chickadees have significantly brighter white plumage than females, larger black patches, and greater plumage contrast between adjacent white and black plumage regions. We also found rank differences in the plumage reflectance of males; high-ranking males, who are preferred by females as both social and extra-pair partners, exhibit significantly darker black plumage and grow their feathers more rapidly than low-ranking males. This variation among individuals reveals a potential signal function for achromatic plumage coloration in birds.     

Racketed tail of the male and female turquoise-browed motmot: male but not female tail length correlates with pairing success,performance, and reproductive success

Both males and females of many avian species maintain elaborate plumage traits, and elaborate monomorphic plumage may convey adaptive benefits to one or both sexes as inter- or intraspecific signals. Both sexes of the turquoise-browed motmot (Eumomota superciliosa) are elaborately plumed with long racket-tipped tail. I investigated whether the racketed tail functions as a sexually selected signal in one or both sexes by testing the predictions that males and/or females with the largest tails have: (1) greater pairing success, (2) greater reproductive performance (clutch-initiation date, clutch size, and hatching success), and (3) greater reproductive success. Yearling males with longer denuded rachises (wires) on the central tail feathers had greater pairing success. In addition, adult males with longer wires paired with females who laid larger clutches, had greater hatching success independent of clutch size, and fledged more young. There was no relationship between female tail plumage and pairing success, reproductive performance, or fledgling success. These results are consistent with the hypothesis that male tail plumage functions as a mate choice or status signal, but that the tail of the female does not function in a sexually selected context. I discuss alternative hypotheses for the evolutionary maintenance of the elaborate female tail plumage.     

Female pied flycatchers Ficedula hypoleuca choose male characteristics in homogeneous habitats

Summary The paper reports the results of a 2-year study of pairing success of male pied flycatchers in a homogeneous habitat. A handicapping experiment was carried out in which certain wing and tail feathers were removed from randomly selected males. Handicapped males had reduced pairing success, they lost weight, and they sang less frequently than control males. Male pairing success was positively correlated with the darkness of the plumage, body-size, and previous breeding experience. Earlier studies on the same species have failed to detect any relationships between pairing success and male characteristics, possibly because of habitat heterogeneity and variation in nest site quality. The evolutionary basis for female choice of male characteristics is discussed. There are reports that males with attractive traits (e.g. black plumage) provide a high quality of parental care. However, the fact that male pairing success was related to male conspicuousness makes it difficult to discriminate between active and passive female choice.     

Delayed maturation in plumage colour: Evidence for the female-mimicry hypothesis in the kestrel

Summary In many sexually dichromatic species, young males have female-like plumage during their first potential breeding year. The female-mimicry hypothesis (FMH) supposes that by possessing female-like plumage young males deceive older conspicuous males into believing that they are females, thus reducing competition from adult males. The status-signalling hypothesis (SSH) supposes that adult males can distinguish sex, but postulates that young males reduce competition from adult males by reliably signaling low status with their dull plumage. We tested these hypotheses in the European kestrel (Falco tinnunculus). Female-like young males settled to breed closer to adult males than did other adult males (Figs. 1a, b). By settling near adult males, young males seemed to increase their chance of mating with adult females. Adult female-young male pairs had better reproductive success than yearling-yearling pairs. These results suggest that there is an adaptive value in possessing a female-like plumage colour in the breeding season. To test the FMH, we measured sexual preference of adult males when adult females and young males were simultaneously shown in an aviary. Adult males were unable to recognize sex, because in half the cases they preferred young males (Fig. 3). However, when adult males and females were shown simultaneously, males preferred females (Fig. 2). Our results support the FMH rather than the SSH, because young males successfully deceived older males by their plumage.     

Philopatry,reproductive success of females,and maternal investment in the red-necked wallaby

Summary Female red-necked wallabies settle within their mothers' home ranges, apparently for life, while males disperse at about two years of age. However, sons spend much more time with their mothers before dispersing than do daughters of similar ages. Females who associate regularly with their subadult offspring are less likely to reproduce successfully at their next breeding attempt than are females who spend little time with their subadults, and sons therefore impose greater short-term reproductive costs on their mothers than do daughters. Females who are generally gregarious also suffer reduced reproductive success, even though reproductive success is independent of local density. It is suggested that the reproductive costs to females of associating with their subadult offspring, and other relatives, are incurred through tolerance of ecological competition from those kin, and therefore reflect a form of prolonged maternal investment, which is initially heaviest in sons but is sustained for longer periods in daughters. Females produce equal numbers of male and female offspring, and spend equal amounts of time suckling them in infancy.     

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.     

Female mallard mating preferences for multiple male ornaments

Mating preferences were studied in captive wild-stock female mallards. Independent observers scored ornament brightness for the bill and eight plumage areas of male mallards on a scale of 1 to 6. Ornament scores were generally positively correlated with each other as indicated by univariate correlations and principal component analysis. Males with higher bill scores were significantly more likely to obtain pairings. None of eight individual plumage ornaments were significant predictors of pairing success, but males with higher average plumage scores had significantly higher pairing success. Males that completed the molt first were also significantly more likely to be paired. Thus females may have paid some attention to plumage brightness. Stepwise multiple regression produced a model that included only bill and molt, both of which contributed a similar degree (r ² = 0.39); plumage ornaments and size measurements did not enter the model. These results are interpreted in light of hypotheses for the evolution of multiple ornaments, and phylogenetic plumage patterns in dabbling ducks. Received: 18 December 1995 / Accepted after revision: 22 September 1996     

Display rate by male fairy-wrens (Malurus cyaneus) during the fertile period of females has little influence on extra-pair mate choice

Empirical and theoretical studies have only recently begun to examine how females use complex multi-component displays when selecting mates. Superb fairy-wrens are well suited to the study of female choice because females have control over extra-group paternity and cuckold their mates at high rates, while males possess a variety of sexually selected traits. Available evidence suggests that females base their extra-group mate choice on the timing of male moult into breeding plumage or the onset of display. However, males continue to perform elaborate displays throughout the season, and direct most displays to females during their fertile period. We therefore conducted focal observations on fertile females to quantify the frequency of male display and used microsatellite genotyping to compare the role of display rate during the breeding season and the timing of male moult on female mate choice. We show that the addition of data on male display rate does not improve our ability to predict which males obtain extra-group paternity. The timing of male moult into breeding plumage remains the only predictor of male extra-group reproductive success. Nevertheless, we found that males displayed more to females that were unable to select extra-group mates on the basis of the timing of moult or the onset of display. This raises the possibility that there are circumstances when females use display rate to discriminate between potential extra-group sires. Overall this study supports the theoretical prediction that females are more likely to base their mate choice on reliable indicators of male quality such as fixed morphological traits and displays of endurance, in this case an early moult into breeding plumage and the performance of an elaborate display during the winter, than a flexible behavioural trait such as display rate during the breeding season. Received: 26 January 2000 / Revised: 1 August 2000 / Accepted: 26 August 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.     

Delayed maturation of multiple signals in a migratory songbird

Many animals use multiple signals in sexual communication, but our understanding of the interactions between multiple signals, particularly in inexperienced breeders, is limited. In birds, delayed plumage maturation (DPM) is well documented; young birds appear duller than adults, despite reaching sexual maturity. Age-related changes in song structure are also common in songbirds, though the extent to which songs of yearling males differ from those of adults (delayed song maturation, DSM) and its prevalence in species with DPM is unknown. We tested for DSM in the mate-attraction (repeat) song of a species with dramatic DPM, the American redstart (Setophaga ruticilla). Repeat song structure of territorial yearling and adult males differed significantly, based on discriminant analysis of nine non-collinear song features. Combined with previous evidence of delayed maturation in territorial (serial) song, we provide the first evidence for DSM in different song types used in different behavioral contexts during the breeding season of a migratory songbird. Within adults, variation in repeat song was associated with pairing, earlier onset of breeding, and number of offspring sired, suggesting a potential benefit for expressing more adult-like song. We found no relationship between the expression of adult-like repeat song and plumage in either age class. These results indicate that delayed maturation of repeat song in yearling redstarts is not due to a generalized delay in signal development, and suggest further work is needed to determine whether mate attraction (for both yearling and adult males) provides greater pressure to sound more “adult-like” than to look more “adult-like”.     

Permanent exposure versus facultative concealment of sexual traits: an experimental study in the house sparrow

In many bird species the cryptic winter plumage is due to the presence of light feather tips that conceal conspicuous colorations. The gradual abrasion of these tips that makes sexual traits visible has been interpreted as a strategy to improve mating success (here referred as the permanent exhibition hypothesis). However, under some circumstances, the maintenance of a full plumage that facultatively enables the bird to exhibit or cover aggression-inducing traits has proved to be advantageous (the coverable badge hypothesis). In a population of house sparrows where black throat patches (here called badges) are used in intrasexual competition, the degree of abrasion of dull feather tips that conceal bright colour early in the breeding season correlated neither with badge size nor with traits indicating morphology and body condition. These results are only in accordance with predictions of the coverable badge hypothesis. In 1992 experimental clipping of badge feather tips, which forced permanent exposure of badges, negatively affected birds with the largest badges in terms of nest acquisition, but an opposite trend was indicated for birds with the smallest badges. In 1993, when more novel birds in the study area were competing for nesting sites than in 1992, only badge size, but not the experimental manipulation, affected nesting success. These results suggest that the loss of the ability to conceal badges was disadvantageous, and more so if most competing individuals had already been resident in the colony in previous years and the larger their badges were. There is a striking contrast between the results reported for this study population, where badges mainly signal fighting ability, and those reported for another house sparrow population, where badges are mainly used in mate choice. In this last population, tip abrasion is advantageous for dominant individuals showing the largest badges. This suggests that the trade-off between conserving versus wearing off the feathers that conceal ornaments could have different optimal resolutions depending on the relative importance of inter and intrasexual selection on indicator traits. Received: 29 September 1995/Accepted after revision: 14 July 1996