Sex allocation according to multiple sexually dimorphic traits of both parents in the barn swallow (Hirundo rustica)

Parents should differentially invest in sons or daughters depending on the sex‐specific fitness returns from male and female offspring. In species with sexually selected heritable male characters, highly ornamented fathers should overproduce sons, which will be more sexually attractive than sons of less ornamented fathers. Because of genetic correlations between the sexes, females that express traits which are under selection in males should also overproduce sons. However, sex allocation strategies may consist in reaction norms leading to spatiotemporal variation in the association between offspring sex ratio (SR) and parental phenotype. We analysed offspring SR in barn swallows (Hirundo rustica) over 8 years in relation to two sexually dimorphic traits: tail length and melanin‐based ventral plumage coloration. The proportion of sons increased with maternal plumage darkness and paternal tail length, consistently with sexual dimorphism in these traits. The size of the effect of these parental traits on SR was large compared to other studies of offspring SR in birds. Barn swallows thus manipulate offspring SR to overproduce ‘sexy sons’ and potentially to mitigate the costs of intralocus sexually antagonistic selection. Interannual variation in the relationships between offspring SR and parental traits was observed which may suggest phenotypic plasticity in sex allocation and provides a proximate explanation for inconsistent results of studies of sex allocation in relation to sexual ornamentation in birds.     

Geographic variation in sexual dimorphism in the barn owl Tyto alba: a role for direct selection or genetic correlation?

Geographic variation in sexually selected traits is commonly attributed to geographic variation in the net benefit accrued from bearing such traits. Although natural and sexual selection are potentially important in shaping geographic variation, genetic constraints may also play a role. Although a genetic correlation between two traits may itself be the outcome of natural or sexual selection, it may indirectly reinforce the establishment and maintenance of cline variation with respect to one particular trait when across the cline different values of other traits are selected. Using the barn owl Tyto alba, a species in which the plumage of females is more reddish‐brown and more marked with black spots than that of males, I report results that are consistent with the hypothesis that both direct selection and genetic constraints may help establish and maintain cline variation in sexual dichromatism. In this species, inter‐individual variation in plumage coloration and spottiness has a genetic basis, and these traits are not sensitive to the environment. Data, based on the measurement of skin specimens, is consistent with the hypothesis that the stronger European cline variation in male spottiness than in female spottiness depends on the combined effects of (1) the similar cline variation in male and female plumage coloration and (2) the more intense phenotypic correlation between plumage coloration and spottiness in males (darker birds are more heavily spotted in the two sexes, but especially males) which is a general feature among the globally distributed barn owls. In northern Europe, male and female T. a. guttata are reddish‐brown and heavily spotted, and in southern Europe male and female T. a. alba are white, but only females display many spots. Here, I discuss the relative importance of direct selection, genetic correlation and the post‐ice age invasion of Europe by T. alba, in generating sex‐specific cline variation in plumage spottiness and non‐sex‐specific cline variation in plumage coloration.     

Plumage coloration, not length or symmetry of tail-streamers, is a sexually selected trait in North American barn swallows

Sexual adornments often vary markedly across a species' range,which presumably is owing to differences in local environmentalconditions and the associated selection pressures, such as naturalversus sexual selection or the relative signaling value of differentornamental traits. However, there are only a few reported examplesin which the information content of mating signals varies geographically,and even fewer in which a set of secondary sexual traits servesdifferent signaling functions in different populations. Classicstudies of sexual selection in the European barn swallow (Hirundorustica rustica) demonstrate that elongate tail-streamers provideseveral reproductive advantages to males and females and areused as reliable signals of mate quality. Here, we show thattail-streamers do not appear to confer these same benefits ina population of barn swallows from North America (Hirundo rusticaerythrogaster). Instead, ventral plumage coloration, which ismore exaggerated in North American swallows compared with theirEuropean counterparts, predicts patterns of assortative matingand annual reproductive success in H. r. erythrogaster. Theseobservations support the idea that ornamental traits can servedifferent functions among animal populations and suggest thatgeographic variation in different sexual signals may facilitatepopulation divergence, which may ultimately lead to speciation.     

The maintenance of phenotypic divergence through sexual selection: An experimental study in barn swallows Hirundo rustica

Previous studies have shown that sexual signals can rapidly diverge among closely related species. However, we lack experimental studies to demonstrate that differences in trait‐associated reproductive performance maintain sexual trait differences between closely related populations, in support for a role of sexual selection in speciation. Populations of Northern Hemisphere distributed barn swallows Hirundo rustica are closely related, yet differ in two plumage‐based traits: ventral color and length of the outermost tail feathers (streamers). Here we provide experimental evidence that manipulations of these traits result in different reproductive consequences in two subspecies of barn swallow: (H. r. erythrogaster in North America and H. r. transitiva in the East Mediterranean). Experimental results in Colorado, USA, demonstrate that males with (1) darkened ventral coloration and (2) shortened streamers gained paternity between two successive reproductive bouts. In contrast, exaggeration of both traits improved reproductive performance within H. r. transitiva in Israel: males with a combination treatment of darkened ventral coloration and elongated streamers gained paternity between two successive reproductive bouts. Collectively, these experimental results fill an important gap in our understanding for how divergent sexual selection maintains phenotype differentiation in closely related populations, an important aspect of the speciation process.     

Evolutionary trade‐off between naturally‐ and sexually‐selected melanin‐based colour traits in worldwide barn owls and allies

Natural selection typically constrains the evolution of sexually‐selected characters. The evolution of naturally‐ and sexually‐selected traits can be intertwined if they share part of their genetic machinery or if sex traits impair foraging success or increase the risk of depredation. The present study investigated phenotypic correlations between naturally‐ and sexually‐selected plumage traits in the Tytonidae (barn owls, grass owls, and masked owls). Phenotypic correlations indicate the extent to which selection on one trait will indirectly influence the evolution of another trait. In this group of birds, the ventral body side varies from white to dark reddish, a naturally‐selected pheomelanin‐based colour trait with important roles in predator–prey interactions. Owls also exhibit eumelanin‐based black spots, for which number and size signal different aspects of individual quality and are used in mate choice. These three plumage traits are strongly heritable and sexually dimorphic, with females being on average darker reddish and more spotted than males. Phenotypic correlations were measured between these three plumage traits in 3958 free‐living barn owls in Switzerland and 10 670 skin specimens from 34 Tyto taxa preserved in museums. Across Tyto taxa, the sexually‐selected plumage spottiness was positively correlated with the naturally‐selected reddish coloration, with redder birds being more heavily spotted. This suggests that they are genetically constrained or that natural and sexual selection are not antagonistically exerted on plumage traits. In a large sample of Swiss nestlings and within 34 Tyto taxa, the three plumage traits were positively correlated. The production of melanin pigments for one plumage trait is therefore not traded off against the production of melanin pigments for another plumage trait. Only in the most heavily‐spotted Tyto taxa do larger‐spotted individuals display fewer spots. This indicates that, at some threshold value, the evolution of many spots constrains the evolution of large spots. These analyses raise the possibility that different combinations of melanin‐based plumage traits may not be selectively equivalent.     

Parasite Infestation and Parental Care in the Barn Swallow Hirundo rustical a Test of the Resource-provisioning Model of Parasite-mediated Sexual Selection

Barn swallows, Hirundo rustica, are commonly infested by the haematophagous tropical fowl mite Omithonyssus bursa (Macronyssidae, Gamasida), which severely reduces various measures of reproductive success among the barn swallow hosts. Food provisioning rate by parent barn swallows, measured in terms of absolute feeding rate by males and females and relative feeding rate by males (percentage of food provided by the male parent), was not significantly related to natural levels of infestation of nests. Experimental manipulation of mite loads in nests during the egg-laying period of the first clutch, which also affected mite loads of parent barn swallows, significantly affected food-provisioning rates of single-brooded, but not of double-brooded barn swallows. These results suggest that effects of mites on the parenting ability of barn swallow hosts depend on host resistance towards parasites. This is consistent with the resource-provisioning hypothesis of parasite-mediated sexual selection, suggesting that females prefer parasite-free males because they are efficient parents, but also with the hypothesis that females prefer males with traits signalling genetic resistance to parasites.     

Reproductive correlates of plumage coloration of female Mountain Bluebirds

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

Age‐dependent relationships between coloration and reproduction in a species exhibiting delayed plumage maturation in females

Males of many bird species exhibit delayed plumage maturation (DPM), a condition in which young individuals display an immature plumage. Several adaptive hypotheses have been suggested for the signaling utility of DPM in males. Tree swallows Tachycineta bicolor, however, are one of the few bird species to exhibit DPM in females, but not in males. Few studies have focused on the age‐dependent signaling function of female plumage traits due to the uncommon nature of DPM in females. Therefore, we used reflectance spectrometry and scanning electron microscopy of sub‐adult (melanin‐based brown) and adult (iridescent‐blue structural) female tree swallows to characterize plumage coloration. Next, we asked whether variation in plumage coloration in females reflects condition and reproductive performance between and within age classes. We found that older females were in better body condition and laid eggs earlier in the season compared to young females; however, average egg mass and reproductive success (number of offspring fledged and offspring condition) did not differ between age classes. There were significant relationships indicating that young females with more‐ornamented (darker brown melanin) plumage laid smaller eggs, but hatched eggs earlier in the season leading to nestlings in better condition compared to less‐ornamented young females. Older females that were more ornamented (brighter, greater blue chroma, and lower hues in iridescent plumage) laid heavier eggs, but ornamentation was negatively associated with immune function, health, and reproductive success. Together, these data suggest that female ornamentation reflects reproductive performance and that there are complicated relationships between plumage coloration, condition, and reproductive performance that ultimately influence reproductive success.     

Lifetime reproductive success,selection on lifespan,and multiple sexual ornaments in male European barn swallows

Natural and sexual selection arise when individual fitness varies according to focal traits. Extra‐pair paternities (EPPs) can affect the intensity of selection by influencing variance in fitness among individuals. Studies of selection require that individual fitness is estimated using proxies of lifetime reproductive success (LRS). However, estimating LRS is difficult in large, open populations where EPPs cause reallocation of biological paternity. Here, we used extensive field sampling to estimate LRS in a population of barn swallows (Hirundo rustica) to estimate selection on lifespan and ornamental traits of males. We found selection on lifespan mediated both by within‐ and extra‐pair fertilization success and selection on tail length mediated by within‐ but not extra‐pair fertilization success. In addition, we found selection on tail white spots via extra‐pair fertilization success after controlling for selection on other traits. These results were not confounded by factors that hamper studies of LRS, including nonexhaustive sampling of offspring and biased sampling of males. Hence, natural and sexual selection mediated by LRS operates on lifespan, tail length, and size of the tail white spots in barn swallows.     

Female‐ and male‐specific signals of quality in the barn owl

Most bird studies of female signalling have been confined to species in which females display a male‐ornament in a vestigial form. However, a great deal of benefit may be gained from considering phenotypic traits that are specific to females. This is because (1) sex‐specific traits may signal sex‐specific qualities and (2) females may develop a male‐ornament not because they are selected to do so, but because fathers transmit to daughters the underlying genes for its expression (genetic correlation between the sexes). We investigated these two propositions in the barn owl Tyto alba, a species in which male plumage is lighter in colour and less marked with black spots than that of females. Firstly, we present published evidence that female plumage spottiness reflects parasite resistance ability. We also show that male plumage coloration is correlated with reproductive success, male feeding rate and heart mass. Secondly, cross‐fostering experiments demonstrate that plumage coloration and spottiness are genetically correlated between the sexes. This implies that if a given trait value is selected in one sex, the other sex will indirectly evolve towards a similar value. This prediction is supported by the observation that female plumage coloration and spottiness resembled that of males, in comparisons at the level of Tyto alba alba populations, Tyto alba subspecies and Tyto species. Our results therefore support the hypothesis that sex‐specific traits signal sex‐specific qualities and that a gene for a sex‐specific trait can be expressed in the other sex as the consequence of a genetic correlation between the sexes.     

Geographical and seasonal variation in the intensity of sexual selection in the barn swallow Hirundo rustica: a meta‐analysis

Sexual selection arises from competition among individuals for access to mates, resulting in the evolution of conspicuous sexually selected traits, especially when inter‐sexual competition is mediated by mate choice. Different sexual selection regimes may occur among populations/subspecies within the same species. This is particularly the case when mate choice is based on multiple sexually selected traits. However, empirical evidence supporting this hypothesis at the among‐populations level is scarce. We conducted a meta‐analysis of the intensity of sexual selection on the largest database to date for a single species, the barn swallow (Hirundo rustica), relying on quantitative estimates of sexual selection. The intensity of sexual selection was expressed as the strength (effect size) of the relationships between six plumage ornaments (tail length, tail asymmetry, size of white spots on tail, ventral plumage colour, throat plumage colour and throat patch size) and several fitness proxies related to reproduction, parental care, offspring quality, arrival date from spring migration, and survival. The data were gathered for four geographically separated subspecies (H. r. rustica, H. r. erythrogaster, H. r. gutturalis, H. r. transitiva). The overall mean effect size (Z_r = 0.214; 95% confidence interval = 0.175–0.254; N = 329) was of intermediate magnitude, with intensity of sexual selection being stronger in males than in females. Effect sizes varied during the breeding cycle, being larger before egg deposition, when competition for access to mates reaches its maximum (i.e. in the promiscuous part of the breeding cycle), and decreasing thereafter. In addition, effect sizes from experiments were not significantly larger than those from correlative studies. Finally, sexual selection on different sexually dimorphic traits varied among subspecies. This last result suggests that morphological divergence among populations has partly arisen from divergent sexual selection, which may eventually lead to speciation.     

Quantitative genetics of plumage color: lifetime effects of early nest environment on a colorful sexual signal

Phenotypic differences among individuals are often linked to differential survival and mating success. Quantifying the relative influence of genetic and environmental variation on phenotype allows evolutionary biologists to make predictions about the potential for a given trait to respond to selection and various aspects of environmental variation. In particular, the environment individuals experience during early development can have lasting effects on phenotype later in life. Here, we used a natural full‐sib/half‐sib design as well as within‐individual longitudinal analyses to examine genetic and various environmental influences on plumage color. We find that variation in melanin‐based plumage color – a trait known to influence mating success in adult North American barn swallows (Hirundo rustica erythrogaster) – is influenced by both genetics and aspects of the developmental environment, including variation due to the maternal phenotype and the nest environment. Within individuals, nestling color is predictive of adult color. Accordingly, these early environmental influences are relevant to the sexually selected plumage color variation in adults. Early environmental conditions appear to have important lifelong implications for individual reproductive performance through sexual signal development in barn swallows. Our results indicate that feather color variation conveys information about developmental conditions and maternal care alleles to potential mates in North American barn swallows. Melanin‐based colors are used for sexual signaling in many organisms, and our study suggests that these signals may be more sensitive to environmental variation than previously thought.     

Selection on multiple sexual signals in two Central and Eastern European populations of the barn swallow

Variation in intensity and targets of sexual selection on multiple traits has been suggested to play a major role in promoting phenotypic differentiation between populations, although the divergence in selection may depend on year, local conditions or age. In this study, we quantified sexual selection for two putative sexual signals across two Central and East European barn swallow (Hirundo rustica rustica) populations from Czech Republic and Romania over multiple years. We then related these differences in selection to variation in sexual characters among barn swallow populations. Our results show that tail length and ventral coloration vary between populations, sexes, and age classes (first‐time breeders vs. experienced birds). We found that selection on tail length was stronger in first‐time breeders than in experienced birds and in males than in females in the Romanian population, while these differences between age groups and sexes were weak in Czech birds. We suggest that the populational difference in selection on tail length might be related to the differences in breeding conditions. Our results show that ventral coloration is darker (i.e., has lower brightness) in the Romanian than in the Czech population, and in experienced birds and males compared with first‐time breeders and females, respectively. The sexual difference in ventral coloration may suggest sexual selection on this trait, which is supported by the significant directional selection of ventral coloration in first‐time breeding males on laying date. However, after controlling for the confounding effect of wing length and tarsus length, the partial directional selection gradient on this trait turned nonsignificant, suggesting that the advantage of dark ventral coloration in early breeding birds is determined by the correlated traits of body size. These findings show that ventral coloration may be advantageous over the breeding season, but the underlying mechanism of this relationship is not clarified.     

The relationship between plumage coloration and aggression in female tree swallows

Intrasexual competition is an important selective force that can favor the evolution of honest signals of fighting ability. Research has focused predominantly on male birds, but many female birds also possess plumage ornaments that could mediate the outcome of competitive interactions. We examined the relationship between blue and white structural coloration and aggression in female tree swallows Tachycineta bicolor. Tree swallows are secondary cavity nesters and females show delayed plumage maturation which may be related to intense competition for nest sites. We compared plumage reflectance of second‐year (SY) and after‐second year (ASY) females, and within‐individual changes in plumage reflectance of ASY females in two successive years. We assessed aggression by placing a caged SY female 2 m from the nest box of an ASY female and quantified the ASY female's response in relation to her own plumage coloration. We found substantial differences in plumage reflectance of SY and ASY females, but found that ASY females became greener with duller white coloration in the second year. This may be due to poor weather that made reproduction particularly costly in the first year of the study and suggests coloration is influenced by condition. We found no relationship between dorsal coloration and aggressiveness. Rather, brighter white females spent more time on their nest box and within 2 m of the intruder than females with dull breasts. Our findings suggest that brighter white ASY females may perceive a SY female as less of a threat or that there may be a trade‐off associated with aggression and parental care that is related to white brightness.     

Structural coloration and sexual selection in the barn swallow Hirundo rustica

Structural coloration has been hypothesized to play a role insexual selection, and we tested whether this was the case ina field study of the barn swallow Hirundo rustica. The dorsaliridescent plumage of barn swallows has a strong reflectancein the ultraviolet (UV) region, with adult males on averagereflecting 8-9% more than adult females, as revealed by a 2-yearstudy in southwestern Spain. The correlation between structural coloration (described by the reflectance in the UV part of thespectrum, UV chroma and blue chroma) and three other secondarysexual characters significantly associated with male matingsuccess (tail length, tail asymmetry, and red facial coloration)was weak and generally nonsignificant. Nor was there a significantrelationship between color parameters and body condition. Wetested for an association between structural coloration of the dorsal plumage and sexual selection in a number of independenttests. Arrival date of males was not significantly relatedto color; there was no significant relationship between colorationand probability of survival or age; mated males did not havestronger reflectance than unmated males; and the duration ofthe premating period was not significantly related to color.Reproductive success was not significantly correlated withplumage coloration in males, nor was the feeding rate of offspringby brightly colored males higher than that of males with lessbright plumage. Given that sample sizes were large, and the power of statistical tests high, we conclude that current sexualselection on the coloration of the dorsal plumage in the barnswallow is, at best, weak.     

Chronic malaria infections increase family inequalities and reduce parental fitness: experimental evidence from a wild bird population

Avian malaria parasites (Plasmodium) occur commonly in wild birds and are an increasingly popular model system for understanding host–parasite co‐evolution. However, whether these parasites have fitness consequences for hosts in endemic areas is much debated, particularly since wild‐caught individuals almost always harbour chronic infections of very low parasite density. We used the anti‐malarial drug Malarone^TM to test experimentally for fitness effects of chronic malaria infection in a wild population of breeding blue tits (Cyanistes caeruleus). Medication caused a pronounced reduction in Plasmodium infection intensity, usually resulting in complete clearance of these parasites from the blood, as revealed by quantitative PCR. Positive effects of medication on malaria‐infected birds were found at multiple stages during breeding, with medicated females showing higher hatching success, provisioning rates and fledging success compared to controls. Most strikingly, we found that treatment of maternal malaria infections strongly altered within‐family differences, with reduced inequality in hatching probability and fledging mass within broods reared by medicated females. These within‐brood effects appear to explain higher fledging success among medicated females and are consistent with a model of parental optimism in which smaller (marginal) offspring can be successfully raised to independence if additional resources become available during the breeding attempt. Overall, these results demonstrate that chronic avian malaria infections, far from being benign, can have significant effects on host fitness and may thus constitute an important selection pressure in wild bird populations.     

Structural (UV) and carotenoid‐based plumage coloration – signals for parental investment?

Parental care increases parental fitness through improved offspring condition and survival but comes at a cost for the caretaker(s). To increase life‐time fitness, caring parents are, therefore, expected to adjust their reproductive investment to current environmental conditions and parental capacities. The latter is thought to be signaled via ornamental traits of the bearer. We here investigated whether pre‐ and/or posthatching investment of blue tit (Cyanistes caeruleus) parents was related to ornamental plumage traits (UV crown coloration and carotenoid‐based plumage coloration) expressed by either the individual itself (i.e. “good parent hypothesis”) or its partner (i.e. “differential allocation hypothesis”). Our results show that neither prehatching (that is clutch size and offspring begging intensity) nor posthatching parental investment (provisioning rate, offspring body condition at fledging) was related to an individual's UV crown coloration or to that of its partner. Similar observations were made for carotenoid‐based plumage coloration, except for a consistent positive relationship between offspring begging intensity and maternal carotenoid‐based plumage coloration. This sex‐specific pattern likely reflects a maternal effect mediated via maternally derived egg substances, given that the relationship persisted when offspring were cross‐fostered. This suggests that females adjust their offspring's phenotype toward own phenotype, which may facilitate in particular mother‐offspring co‐adaptation. Overall, our results contribute to the current state of evidence that structural or pigment‐based plumage coloration of blue tits are inconsistently correlated with central life‐history traits.     

Cohabitation with farm animals rather than breeding effort increases the infection with feather‐associated bacteria in the barn swallow Hirundo rustica

Feather‐associated bacteria are widespread inhabitants of avian plumage. However, the determinants of the between‐individual variation in plumage bacterial loads are less well understood. Infection intensities can be determined by ecological factors, such as breeding habitat, and can be actively regulated by hosts via preening. Preening, yet, is a resource intensive activity, and thus might be traded‐off against reproductive investment in breeding birds. Here, we studied barn swallows Hirundo rustica to assess the bacterial cost of reproduction in relation to nesting site micro‐habitats. Barn swallows prefer to breed in the company of large‐sized farm animals, although the presence of mammalian livestock in barns assures a warm and humid micro‐climate that favours bacterial proliferation. Thus, we experimentally manipulated brood sizes of birds breeding in barns with, or without, farm animals and measured total cultivable bacteria (TCB) and feather‐degrading bacteria (FDB) from the plumage. We found that the abundance of feather‐associated bacteria (i.e. both TCB and FDB) in females, but not males, breeding in barns with livestock were significantly higher than in conspecifics breeding in empty barns. Plumage bacterial loads, however, were not affected by brood size manipulations in either sex. In addition, we report a negative relationship between both TCB and FDB and hatching date in females, and several sex and seasonal differences in plumage bacterial abundances. Our study is the first to show that breeding micro‐habitat (i.e. livestock co‐tenancy) has consequences for the abundance of feather‐associated bacteria.     

Sex‐linked inheritance,genetic correlations and sexual dimorphism in three melanin‐based colour traits in the barn owl

Theory states that genes on the sex chromosomes have stronger effects on sexual dimorphism than genes on the autosomes. Although empirical data are not necessarily consistent with this theory, this situation may prevail because the relative role of sex‐linked and autosomally inherited genes on sexual dimorphism has rarely been evaluated. We estimated the quantitative genetics of three sexually dimorphic melanin‐based traits in the barn owl (Tyto alba), in which females are on average darker reddish pheomelanic and display more and larger black eumelanic feather spots than males. The plumage traits with higher sex‐linked inheritance showed lower heritability and genetic correlations, but contrary to prediction, these traits showed less pronounced sexual dimorphism. Strong offspring sexual dimorphism primarily resulted from daughters not expressing malelike melanin‐based traits and from sons expressing femalelike traits to similar degrees as their sisters. We conclude that in the barn owl, polymorphism at autosomal genes rather than at sex‐linked genes generate variation in sexual dimorphism in melanin‐based traits.     

Developmental plasticity varied with sex and position in hatching hierarchy in nestlings of the asynchronous European roller,Coracias garrulus

Allocation rules between ornamental and other functional traits of birds may differ among individuals and vary with environmental conditions. We supplemented roller (Coracias garrulus) nestlings with methionine in a between‐nest design to investigate the way in which the sex and position in the hatching hierarchy affect the allocation of resources among growth, immunity, and plumage coloration. Methionine induces the production of lymphocytes at expense of growth; thus, we used it to manipulate growth and immunity, which are two traits likely to compromise plumage coloration. We predicted that late‐hatched chicks within a brood (juniors) compared to early‐hatched chicks (seniors) should allocate more to traits directly providing fitness than to ornamental traits because juniors are more affected than seniors by sibling competition. The methionine treatment effectively enhanced the production of lymphocytes in experimental broods. This appeared to be at the expense of plumage coloration in junior nestlings because, in supplemented nests, junior males showed a trend to display less greenish bellies than junior males from control nests. However, juniors from supplemented nests maintained wing growth as in control juniors. The plumage coloration of seniors was unaffected by the methionine supplementation, although they paid the costs of lymphocyte production at a level of growth that was reduced compared to senior nestlings in control nests. Hence, sex, and hatching order affected resource allocation among growth, immunity, and plumage coloration of roller nestlings. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99 , 500–511.