THE PLUMAGE OF THE RED AVADAVAT AMANDAVA AMANDAVA AND ITS EFFECT UPON CONTACT BEHAVIOUR

Goodwin (1962) has commented upon the capricious, variable nature of the two plumages of the cock Red Avadavat Amandava amandava. At any time of the year cocks in the 'red' nuptial plumage can be found, but birds moulting into the 'hen' type post-nuptial (eclipse) plumage occur most frequently between December and February. In captivity, the annual rhythm is upset and is inconsistent from year to year. In my experimental colony, all the cocks moulted into their post-nuptial plumage during March 1962 and donned their nuptial plumage in the following May, only to effect a complete change again by early in October. A year previous to this, in October 1961, they were still in their full 'red', nuptial plumage.     

Testosterone treatment of female Superb Fairy-wrens Malurus cyaneus induces a male-like prenuptial moult, but no coloured plumage

When selection strongly favours a testosterone-dependent trait in males, and this trait is not beneficial to females, a correlated response to selection in females, which also circulate some testosterone, could slow the rate of evolution in males. Here I investigate whether experimental testosterone treatment in female Superb Fairy-wrens Malurus cyaneus can induce the testosterone-dependent sexually selected moult into nuptial plumage, as it does in males. Silastic testosterone implants in females rapidly induced a moult akin to the male prenuptial moult, involving all body areas that are colourful in nuptial males (head, upper back, ear tufts, breast). Moreover, the newly moulted feathers had a similar glistening appearance and morphology as male nuptial feathers. However, the treatment failed to induce production of the blue and black structural colours, and the breast and ear tufts were lighter than the normal grey-brown feathers of females and males in eclipse plumage. Microscopic and ultrastructural analyses of these unusual feathers could further our understanding of structural colour production.     

The effects of elevated testosterone on plumage hue in male House Finches

The majority of studies examining the role of hormones in the proximate mechanisms of plumage coloration in birds have focused on intersexual differences (plumage dichromatism) and on structural- or melanin-based plumage coloration. The relationship between hormones and carotenoid-based plumage color, and in particular intrasexual plumage color variation, has received little attention. We manipulated testosterone levels of both captive and wild male House Finches to determine whether testosterone influences the expression of male plumage color in this species. We found that in captive male House Finches elevated testosterone delayed molt and resulted in drabber, less red plumage, even when birds were supplemented with dietary carotenoids. Elevated testosterone also resulted in drab plumage color in wild males, and appeared to delay molt in wild birds as well. Wild males implanted with testosterone showed wide variation in expression of plumage coloration. Those implanted early in the year molted plumage similar in color to their pre-treatment plumage, but those implanted later molted substantially duller plumage, possibly because delayed molt resulting from elevated testosterone caused these males to molt when carotenoid pigments were not available in sufficient amounts. These observations have the potential to explain previously reported relationships between plumage color and behavior in male House Finches, and highlight the importance of considering the proximate mechanisms of plumage coloration in avian sexual selection.     

Timing of prenuptial molt as a sexually selected indicator of male quality in superb fairy-wrens (Malurus cyaneus)

Seasonally dichromatic male superb fairy-wrens (Malurus cyaneus)solicit extra-group fertilizations through displays of theirbrightly colored nuptial plumage during visits to females inneighboring territories. This ornamental plumage is lost atthe end of the breeding season, and reacquired before the nextbreeding season after a highly variable period spent in a cryptic,female-like, "eclipse" plumage. Individuals start visiting femalesas soon as they return to nuptial plumage, often several monthsbefore the start of breeding.We examine variation in the timingof acquisition of nuptial plumage in relation to environmentalconditions and the age, social status, and condition of males.We show that males undergo the prenuptial molt earlier as theyage and when in superior condition, and that molt is delayedin years of more adverse winter weather conditions. We proposethat the purpose of male displays may be to advertise theirquality, and that females use this variation in the timing ofacquisition of nuptial plumage to assess the relative qualityof prospective extra-group mates. Molting earlier is apparentlycostlier, so this handicap would appear to be a reliable reflectionof male quality.     

The relative importance of male tail length and nuptial plumage on social dominance and mate choice in the red-backed fairy-wren Malurus melanocephalus: evidence for the multiple receiver hypothesis

Understanding why males of many species exhibit two or more sexual ornaments depends upon identifying both the information conveyed and the intended receiver(s) for each signal. Here we focus on identifying the intended receivers for two sexual signals exhibited by male red-backed fairy-wrens Malurus melanocephalus , extent of nuptial plumage and tail length. In doing so we test the multiple receiver hypothesis, which predicts that each trait is directed toward a different type of receiver (e.g., males vs females). Male red-backed fairy-wrens in nuptial plumage exhibit reversed sexual dimorphism for tail length in the breeding season, when their tails are significantly shorter than those of females or males in eclipse plumage. Using both aviary-based experiments and indices of mate choice and social dominance from a natural population, we found that extent of nuptial plumage and age primarily affected female mate choice and that shorter tails were primarily associated with male:male dominance signaling. The field and aviary studies combined are consistent with the multiple receiver hypothesis, in that each trait appears to be directed primarily to a different set of receivers (plumage for females and tail length for males), though each trait may also signal information to the other set of receivers as well. We propose that sexual selection may favor shorter tail lengths in male red-backed fairy-wrens through social competition mechanisms.     

Fluctuations in population composition dampen the impact of phenotypic plasticity on trait dynamics in superb fairy-wrens

1. In structured populations, phenotypic change can result from changes throughout an individual's lifetime (phenotypic plasticity, age-related changes), selection and changes in population composition (environment- or density-driven fluctuations in age-structure). 2. The contribution of population dynamics to phenotypic change has often been ignored. However, for understanding trait dynamics, it is important to identify both the individual- and population-level mechanisms responsible for trait change, because they potentially reinforce or counteract each other. 3. We use 22 years of field data to investigate the dynamics of a sexually selected phenological trait, the timing of nuptial moult in superb fairy-wrens Malurus cyaneus. 4. We show that trait expression is both climate- and age-dependent, but that phenotypic plasticity in response to climate variability also varies with age. Old males can acquire nuptial plumage very early after high rainfall, but 1- to 2-year-olds cannot. However, males of all ages that defer moult to later in the year acquire nuptial plumage earlier when conditions are warmer. 5. The underlying mechanism appears to be that old males may risk moulting in the most challenging period of the year: in autumn, when drought restricts food abundance and during the cold winter. By contrast, young males always moult during the spring transition to benign - warmer and generally wetter - conditions. Temperature changes dominate this transition that heralds the breeding season, thereby causing both young and late-moulting older birds to be temperature sensitive. 6. Climate and age also affect trait dynamics via a population dynamical pathway. The same high rainfall that triggers early moulting in old males concurrently increases offspring recruitment and thereby reduces the average age of males in the population. Consequently, effects of rainfall on trait dynamics through phenotypic plasticity of old males are dampened by synchronous rejuvenation of the age-structure. 7. A long-term trend towards drier environments prompted phenotypic change because of plasticity, but this was masked by climate-driven demographic change (causing apparent stasis). This suggests a novel explanation for why trait change may fail to reflect the observed pattern of directional selection or phenotypic plasticity.     

Zur Wirkung von Androgenen auf die Induktion des Schlichtkleids bei kastrierten Stockerpeln (Anas platyrhynchos)

Zusammenfassung An 15 seit mehreren Jahren kastrierten Stockerpeln wurde die Wirkung der Androgene Testosteron (T) und 5-Dihydrotestosteron (DHT) auf die Umfärbung in das Schlichtkleid untersucht, die nach Literaturangaben von Estrogenen abhängig sein soll. Von Anfang März bis Anfang Mai wurden 5 Vögeln täglich 5 mg T subcutan injiziert. 5 weitere erhielten im gleichen Zeitraum täglich 5 mg DHT, und die restlichen 5 dienten als Kontrollen. Gegen Ende der postnuptialen Mauser in der zweiten Julihälfte trugen die Kontrollerpel (wie alle 15 Vögel in den Vorjahren) das Prachtkleid. Die mit T behandelte Gruppe hatte dagegen ein fast vollständiges Schlichtkleid angelegt. Die DHT injizierten Erpel zeigten ein individuell sehr variables Mischgefieder mit durchschnittlich etwa 70% Schlichtkleid- und 30% Prachtkleidanteil. Die schwächere Wirksamkeit des DHT im Vergleich zu T bei der Induktion des Schlichtkleids wird in Zusammenhang mit einer Aromatisierbarkeit der Androgene zu Estrogenen diskutiert.

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Effects of androgens on the induction of eclipse colouration in castrated wild Mallard drakes (Anas platyrhynchos)

Summary According to the literature the eclipse colouration in wild Mallard drakes seems to depend on estrogens. I studied the effects of the 2 androgens testosterone (T) and 5-dihydrotestosterone (DHT) on the induction of eclipse colouration in 15 wild Mallard drakes which were castrated since several years. From the beginning of March to the beginning of May 5 castrates were daily injected with 5 mg T, 5 birds received daily injections of 5 mg DHT, and the remaining served as controls. In the second half of July, near the end of the postnuptial moult, the 5 controls (like all castrates in previous years) exhibited the nuptial colouration. The T treated group had moulted into an almost complete eclipse plumage. The DHT injected birds showed a mixed colouration with distinct individual variation, the estimated average being about 70% eclipse and 30% nuptial plumage. The weaker effect of DHT, compared with T, on the induction of the eclipse colouration is discussed with regard to an aromatization of the androgens to estrogens.

     

Effects of testosterone and corticosterone on immunocompetence in the zebra finch

The original immunocompetence handicap hypothesis (ICHH) suggested that testosterone has a handicapping effect in males by both promoting the development of sexual signals and suppressing immune function. A modified version, the stress-linked ICHH, has recently proposed that testosterone is immunosuppressive indirectly by increasing production of corticosterone. To test both the original and stress-mediated versions of the ICHH, we implanted male zebra finches taken from lines selected for divergent maximum stress-induced levels of corticosterone (high, low and control) with either empty or testosterone-filled implants. Their humoral and cell-mediated immune responses were then assessed by challenge with diphtheria:tetanus vaccine and phytohemagglutinin respectively. We found no effect of the hormone manipulations on either PHA or tetanus antibody responses, but found a significant interaction between titers of both testosterone and corticosterone on diphtheria secondary antibody response; antibody response was greatest in individuals with high levels of both hormones. There was also a significant interactive effect between testosterone treatment group and corticosterone titer on body mass; the body mass of males in the elevated testosterone treatment group decreased with increasing corticosterone titer. These results suggest that, contrary to the assumption of the stress-mediated version of the ICHH, high plasma levels of corticosterone are not immunosuppressive, but are in fact immuno-enhancing in the presence of high levels of plasma testosterone. Equally, the central assumption of the ICHH that testosterone is obligately immunosuppressive is also not supported. The same individuals with the highest levels of both hormones and consequently the most robust antibody response also possessed the lowest body mass.     

GROWTH, AND ANNUAL CYCLES IN WEIGHTS, PLUMAGES AND REPRODUCTIVE ORGANS OF GOOSANDERS IN EASTERN CANADA

A study of Goosanders, in connection with a Fisheries shooting programme on Cape Breton Island, has shown the pattern of growth in young birds, and of plumage and other annual cycles in fully grown birds. The plumage cycles show little difference between the sexes in the first year, with males thereafter moulting about one month earlier than females. Virtually no moult occurs from January to March, nor in adults during April to mid-June, in this area. Weights and measurements permit ageing of flightless young of known sex, and sexing of all fully grown birds, but they do not distinguish first-year from older individuals. Gross examination suggests that males in the second calendar year (11 to 12 months old) may come into reproductive condition, at least under conditions of year-round hunting. Variations in annual moult cycles may be due to differing geographic origin of the birds; I suggest that birds killed in other Goosander shooting programmes should be used for comparative studies parallel to this one.     

Shield characteristics are testosterone-dependent in both male and female moorhens

The immunocompetence handicap hypothesis proposes that the expression of secondary sexual characteristics is positively related to testosterone levels, but that elevated testosterone levels also impose costs from immune suppression. Hence, testosterone-dependent characteristics should accurately reflect male quality because only high-quality males are able to invest in large sexual characteristics without detrimental effects upon their own immune system. Most studies to date have focused on the role of testosterone in the expression of male ornaments and on the possible immunosuppressant effects of androgens in males. In the moorhen (Gallinula chloropus), a sexually monomorphic monogamous bird species showing a partial sex-role reversal, both sexes have a prominent frontal shield. We implanted both sexes with testosterone-filled implants to examine the effects of testosterone on shield characteristics and immune function. Shield size, thickness, and color were significantly increased by an experimental increase in testosterone concentrations in both males and females. Likewise, removal of the implants led to a rapid decrease in shield size and thickness in both males and females, suggesting that both sexes responded quickly to an increase or a decrease in testosterone. Moorhens implanted with testosterone had higher intensities of ectoparasite infestations than control birds, but other indirect measures of immunocompetence did not differ significantly between the two categories of birds.     

Testosterone and oxidative stress: the oxidation handicap hypothesis

Secondary sexual traits (SST) are usually thought to have evolved as honest signals of individual quality during mate choice. Honesty of SST is guaranteed by the cost of producing/maintaining them. In males, the expression of many SST is testosterone-dependent. The immunocompetence handicap hypothesis has been proposed as a possible mechanism ensuring honesty of SST on the basis that testosterone, in addition to its effect on sexual signals, also has an immunosuppressive effect. The immunocompetence handicap hypothesis has received mixed support. However, the cost of testosterone-based signalling is not limited to immunosuppression and might involve other physiological functions such as the antioxidant machinery. Here, we tested the hypothesis that testosterone depresses resistance to oxidative stress in a species with a testosterone-dependent sexual signal, the zebra finch. Male zebra finches received subcutaneous implants filled with flutamide (an anti-androgen) or testosterone, or kept empty (control). In agreement with the prediction, we found that red blood cell resistance to a free radical attack was the highest in males implanted with flutamide and the lowest in males implanted with testosterone. We also found that cell-mediated immune response was depressed in testosterone-treated birds, supporting the immunocompetence handicap hypothesis. The recent finding that red blood cell resistance to free radicals is negatively associated with mortality in this species suggests that benefits of sexual signalling might trade against the costs derived from oxidation.     

Concordant evolution of plumage colour, feather microstructure and a melanocortin receptor gene between mainland and island populations of a fairy-wren

Studies of the patterns of diversification of birds on islands have contributed a great deal to the development of evolutionary theory. In white-winged fairy-wrens, Malurus leucopterus, mainland males develop a striking blue nuptial plumage whereas those on nearby islands develop black nuptial plumage. We explore the proximate basis for this divergence by combining microstructural feather analysis with an investigation of genetic variation at the melanocortin-1 receptor locus (MC1R). Fourier analysis revealed that the medullary keratin matrix (spongy layer) of the feather barbs of blue males was ordered at the appropriate nanoscale to produce the observed blue colour by coherent light scattering. Surprisingly, the feather barbs of black males also contained a spongy layer that could produce a similar blue colour. However, black males had more melanin in their barbs than blue males, and this melanin may effectively mask any structural colour produced by the spongy layer. Moreover, the presence of this spongy layer suggests that black island males evolved from a blue-plumaged ancestor. We also document concordant patterns of variation at the MC1R locus, as five amino acid substitutions were perfectly associated with the divergent blue and black plumage phenotypes. Thus, with the possible involvement of a melanocortin receptor locus, increased melanin density may mask the blue-producing microstructure in black island males, resulting in the divergence of plumage coloration between mainland and island white-winged fairy-wrens. Such mechanisms may also be responsible for plumage colour diversity across broader geographical and evolutionary scales.     

Sex,size, and plumage redness predict house finch survival in an epidemic

Mycoplasma gallisepticum is a well-known disease of poultry but until 1994 had not been observed in passerine birds. From 1994 to 1996, tens of millions of house finches (Carpodacus mexicanus) are believed to have died in an epidemic of mycoplasmal conjunctivitis, similar to ''pinkeye'' in humans. The outbreak of Mycoplasma gallisepticum affected finches of both sexes but disproportionately killed males, shifting the sex ratio from male-biased to female-biased. This differential male mortality is consistent with a cost of testosterone, which is a key prediction of the immunocompetence handicap hypothesis. Males and females that survived the epidemic weighed significantly less and had significantly shorter wing chords, tarsi, and bills than did individuals before the epidemic. Male survivors also had significantly redder plumage than males that did not survive, supporting the idea that plumage brightness serves as an indicator of condition, as proposed by the honest advertisement model of sexual selection.     

Carotenoid-based plumage coloration of male greenfinches reflects health and immunocompetence

Hypotheses of parasite-mediated sexual selection (PMSS) propose that elaborate male ornaments have evolved due to female preferences. Females would benefit from mating with more ornamental males if males' ornamentation signals their health status and ability to provide parasite resistance genes for the offspring. Carotenoid-based plumage coloration of birds has been hypothesised to honestly reflect an individual's health status due to trade-off in allocation of carotenoids between maintenance and signalling functions. The prediction of this hypothesis, namely that individuals with brighter plumage are able to mount stronger immune responses against novel antigens and reveal generally better health state, was tested in captive male greenfinches (Carduelis chloris). Greenfinches with brighter yellow breast feathers showed stronger humoral immune response against novel antigen (SRBC) while no relationship between plumage coloration and an estimate of cell-mediated immune responsiveness (PHA response) was detected. Elaborately ornamental individuals had better general health state as indicated by the negative correlations between plumage brightness and heterophil haemoconcentration. Consistent with the concept of PMSS, these results suggest that carotenoid-based plumage coloration in greenfinches honestly signals immunocompetence and health status.     

Color expression in experimentally regrown feathers of an overwintering migratory bird: implications for signaling and seasonal interactions

Plumage coloration in birds plays a critical role in communication and can be under selection throughout the annual cycle as a sexual and social signal. However, for migratory birds, little is known about the acquisition and maintenance of colorful plumage during the nonbreeding period. Winter habitat could influence the quality of colorful plumage, ultimately carrying over to influence sexual selection and social interactions during the breeding period. In addition to the annual growth of colorful feathers, feather loss from agonistic interactions or predator avoidance could require birds to replace colorful feathers in winter or experience plumage degradation. We hypothesized that conditions on the wintering grounds of migratory birds influence the quality of colorful plumage. We predicted that the quality of American redstart (Setophaga ruticilla) tail feathers regrown after experimental removal in Jamaica, West Indies, would be positively associated with habitat quality, body condition, and testosterone. Both yearling (SY) and adult (ASY) males regrew feathers with lower red chroma, suggesting reduced carotenoid content. While we did not observe a change in hue in ASY males, SY males shifted from yellow to orange plumage resembling experimentally regrown ASY feathers. We did not observe any effects of habitat, testosterone, or mass change. Our results demonstrate that redstarts are limited in their ability to adequately replace colorful plumage, regardless of habitat, in winter. Thus, feather loss on the nonbreeding grounds can affect social signals, potentially negatively carrying over to the breeding period.     

The correlation between immunocompetence and an ornament trait changes over lifetime in Panorpa vulgaris scorpionflies

The immunocompetence-handicap hypothesis posits that costly male ornament traits might function to signal superior heritable immunocompetence to females. Quite a number of studies have aimed at testing this hypothesis. Yet the empirical data obtained so far are ambiguous. Many studies analysed the phenotypic correlation between handicap expression and immunocompetence at the same time point. However, since immunocompetence may change drastically over an individual's lifetime, such a singular measurement may not represent genetic differences among males and the benefits of choosing handicapped males for females might thus be weak. Here, I tested the correlation of a potential immunocompetence-handicap, the production of salivary secretions as nuptial gifts in a scorpionfly (Panorpa vulgaris), with immunocompetence at two different time points. I found a positive correlation with the handicap, but only if immunocompetence was measured shortly after expression of the handicap, i.e. briefly after mating in 2 weeks old scorpionflies. By contrast, there was no correlation with immunocompetence of the same flies at a younger age, i.e. shortly after adult emergence and a weak, insignificant trend for increased immunocompetence in offspring. These results are in agreement with positive phenotypic correlations between immunocompetence and handicap expression reported from other species, but advise caution when generalizing such one-time correlations.     

Breeding plumage honestly signals likelihood of tapeworm infestation in females of a long-distance migrating shorebird, the bar-tailed godwit

The indicator mechanism for sexual selection proposed by Hamilton and Zuk (i.e. that sexually selected ornaments signal parasite resistance) has received rather little observational support, and none in the case of long-distance migrant birds. Here we present a test by examining the association between helminth infestations and breeding plumage quality in bar-tailed godwits Limosa lapponica taymyrensis during their spring staging period in the Wadden Sea, The Netherlands. After a non-stop flight from West Africa, these shorebirds refuel in the Wadden Sea in preparation for a second flight to the central Siberian Arctic breeding grounds. Earlier studies have shown that only relatively heavy and well ornamented birds carry out a "top-up" moult during stopover, in which part of the contour feathers recently grown in West Africa are replaced by even fresher ones. Active body moult was therefore taken as the primary indicator of ornament quality. Of 78 birds collected between 1992 and 1997, 42% carried helminths, including four species of digenean trematodes (flukes), three species of cestodes (tapeworms) and an acanthocephalan (spiny-headed worm). Faecal samples examined for helminth eggs in another 92 birds in 1998 and 2000 showed similar rates of infestation. Actively moulting bar-tailed godwits were confirmed to be heavier and to show more extensive breeding plumage than non-moulting birds. In females, but not in males, active moult was associated with fewer cestodes and acanthocephalans. Also, breeding plumage and presence of cestodes were negatively associated in females. We argue that the quality of the breeding plumage reliably indicates parasite resistance in female godwits. The repeatability of plumage scores of females between years is consistent with such resistance having a heritable component. In contrast, male ornaments may demonstrate other qualities, e.g. an ability to combine adequate fuelling and flight performances with moult during the time-stress of migration.     

Testosterone-mediated trade-offs in the old age: a new approach to the immunocompetence handicap and carotenoid-based sexual signalling

The immunocompetence handicap hypothesis proposes that testosterone mediates a trade-off between sexual signalling and immunocompetence in males. Such a trade-off could favour the reliability of sexual signals on the basis that testosterone required for signal expression also promotes immunosuppression. However, the immunosuppressive activity of testosterone has not been convincingly demonstrated. We propose that the optimal solution to the testosterone-mediated trade-off should change with age, explaining ambiguous results in the past. Testosterone and ageing would promote two simultaneous immunosuppressive challenges unaffordable for low-quality males. Oxidative stress, as intimately related to ageing and immunosenescence, could contribute to enhance signal reliability. In this context, traits coloured by carotenoids (yellow–red traits) could play a crucial role due to the immunostimulatory and antioxidant properties of these pigments. Here, old and middle-aged male red-legged partridges were treated with testosterone or manipulated as controls. In the presence of high-testosterone levels, middle-aged males increased both circulating carotenoid levels and colour expression, whereas their cell-mediated immunity was not significantly altered. However, in old males, neither circulating carotenoids nor sexual signalling increased when treated with testosterone, but immunosuppression was detected. The link between testosterone and carotenoids could favour the reliability of sexual signals throughout the life.     

Sexual dimorphism in immunocompetence: what does life-history theory predict?

Sexual dimorphism in immunocompetence, usually in the directionof inferior male immunocompetence, has historically been explainedas the result of proximate physiological mechanisms such asthe immunosuppressive effects of the male hormone testosterone.More recently, it has been argued that this pattern is bestunderstood as a result of resource-based trade-offs betweenmale mating effort and immune defense, a trade-off that femalesdo not make. The central prediction of this hypothesis is thatas the strength of sexual selection on males increases, themagnitude of the sex differences in immunocompetence will increase.Two implicit assumptions of this argument are that 1) longevityis of more importance for female than for male fitness and 2)that the primary benefit of immunocompetence is increased longevity.However, both of these assumptions may not be as broadly applicableas has been argued. We have modeled the optimal allocation toimmunocompetence for males and females without making theseassumptions. We find that the optimal allocation to immune defensefor males decreases as the strength of sexual selection increases,as predicted. However, males may still invest more, relativeto females, into immunocompetence if the impact of parasiteson condition differs for the sexes and/or if the relationshipbetween condition and reproduction differs for the sexes. Weargue that these previously overlooked assumptions may be criticalfor predicting sex-specific patterns of immunocompetence.