Experimentally increased testosterone affects social rank and primary sex ratio in the spotless starling

It has been suggested that the amount of maternal testosterone allocated into the eggs might be implicated in the process of sex determination. However, recent findings on the effect that female social rank has on the level of egg testosterone suggest that reported associations between male-biased sex ratios and yolk testosterone may represent an indirect hormonal effect mediated by the interdependence among maternal hormones, female social rank, and sex ratio. Here, we report the results of a field experiment in which we manipulated the circulating levels of testosterone in female spotless starlings (Sturnus unicolor) before egg formation. Focal females were controlled in subsequent years to explore possible delayed effects of hormone manipulation on primary sex ratio and social status that could persist because of permanent hormonal change or through hormone-dominance interactions. The results indicate that testosterone-implanted females (T-females) produced significantly more sons than control females (C-females) in the year in which they were manipulated. These differences in offspring sex ratio between T- and C-females persisted in the next 3 years, although no additional hormone treatments were given. These results were not mediated by an eventual effect of testosterone treatment on the quality of the females' mates. A similar proportion of T- and C-females acquired a nest box and bred either in the manipulation year or in Year 1 after manipulation, but T-females tended to be more successful in acquiring a nest box than C-females in Years 2 and 3 after manipulation. These results suggest that added testosterone had a direct role on the acquisition and maintenance of high social rank. Delayed effects of testosterone on primary sex ratio might have been caused by altered endogenous production of T-females. Alternatively, the maintenance of sex ratio differences between T- and C-females long after having being implanted might be attributed to the positive effect that enhanced social rank of T-females has on their circulating testosterone levels.     

Consequences of elevating plasma testosterone in females of a socially monogamous songbird: evidence of constraints on male evolution?

To explore whether selection for testosterone-mediated traits in males might be constrained by costs of higher testosterone to females, we examined the effects of experimental elevation of plasma testosterone on physiological, reproductive, and behavioral parameters in a female songbird, the dark-eyed junco (Junco hyemalis). We used subcutaneous implants to elevate testosterone (T) in captive and free-living female juncos. In captive birds, we measured the effects of high T on body mass, feather molt, and brood patch formation. In the field, we monitored its effects on the timing of egg laying, clutch size, egg size, egg steroid levels, incubation, and nest-defense behavior. Females implanted with testosterone (T-females) had significantly higher circulating levels of testosterone than did control females (C-females). Captive T-females had lower body mass, were less likely to develop brood patches, and delayed feather molt relative to C-females. Among free-living females, the interval between nest completion and appearance of the first egg was longer for T-females than for C-females and egg yolk concentrations of testosterone were higher, but there were no significant differences in estradiol levels, clutch size, or egg size. Incubation and nest defense behavior were also similar between T- and C-females. Our results suggest that selection on males for higher testosterone might initially lead to a correlated response in females producing changes in body mass and feather molt, both of which could be detrimental. Other possible female responses would be delayed onset of reproduction, which might reduce reproductive success, and higher yolk testosterone, which might have either positive or negative effects on offspring development. We found no reason to expect reduced parental behavior by females as a negative fitness consequence of selection for higher testosterone in males.     

Cascading costs of reproduction in female house wrens induced to lay larger clutches

In many species, females produce fewer offspring than they are capable of rearing, possibly because increases in current reproductive effort come at the expense of a female's own survival and future reproduction. To test this, we induced female house wrens (Troglodytes aedon) to lay more eggs than they normally would and assessed the potential costs of increasing cumulative investment in the three main components of the avian breeding cycle – egg laying, incubation and nestling provisioning. Females with increased clutch sizes reared more offspring in the first brood than controls, but fledged a lower proportion of nestlings. Moreover, nestlings of experimental females were lighter than those of control females as brood size and prefledging mass were negatively correlated. In second broods of the season, when females were not manipulated, experimental females laid the same number of eggs as controls, but experienced an intraseasonal cost through reduced hatchling survival and a lower number of young fledged. Offspring of control and experimental females were equally likely to recruit to the breeding population, although control females produced more recruits per egg laid. The reproductive success of recruits from broods of experimental and control females did not differ. The manipulation also induced interseasonal costs to future reproduction, as experimental females had lower fecundity than controls when breeding at least 2 years after having their reproductive effort experimentally increased. Finally, females producing the modal clutch size of seven eggs in their first broods had the highest lifetime number of fledglings.     

Elevated testosterone levels affect female breeding success and yolk androgen deposition in a passerine bird

Although it is well documented that testosterone (T) is an important mediator in the regulation of behaviour in male vertebrates, its functional significance in females is less understood. Experimentally increased T in adult female birds has been found to have both advantageous and detrimental effects on behaviour and fitness. In addition, T may also mediate maternal effects when it is deposited into the egg yolk, and variations in androgen concentration between eggs contribute to differences in offspring phenotype and fitness. In this study we examined the effects of experimentally elevated female T on reproductive success and yolk androgen deposition in the spotless starling. The administration of exogenous T in female spotless starlings before egg laying caused negative effects on reproductive performance: when compared to control females T-females laid fewer eggs and raised fewer chicks. We also found an effect of elevated female T on yolk androgen deposition: T-females laid eggs with greater amounts of yolk T than control females, whereas yolk androstenedione levels were not affected. Although some of these effects likely involved a direct interference of female T with female reproductive function, some of them could be due to effects operating in eggs through maladaptive high T levels.     

Consequences of prenatal androgen exposure for the reproductive performance of female pheasants (Phasianus colchicus)

Maternal hormones in vertebrate eggs can mediate important forms of maternal effects. However, the function of hormone transfer to the eggs is still debated, especially because long-term fitness consequences have been little studied. We investigated the effect of prenatal exposure to physiologically elevated yolk testosterone (T) levels on reproduction of female pheasants (Phasianus colchicus) in captivity. We found that females hatching from T-injected eggs (T-females) had a lower egg-laying rate than controls, and their eggs were more frequently infertile than those laid by control females. There were no effects of prenatal maternal treatment on egg size and yolk T concentration, but eggs carrying a female embryo laid by T-females had smaller yolks than eggs with a male embryo, while there was no sex difference in yolk size among the eggs laid by control females. Progeny sex ratio was unaffected by maternal treatment. These findings suggest that the transfer of high androgen levels to the eggs by the mother is constrained by complex trade-offs between direct effects on her daughters' reproduction and by trans-generational differential consequences on male and female descendants.     

Bare-Part Color in Female Budgerigars Changes from Brown to Structural Blue following Testosterone Treatment but Is Not Strongly Masculinized

Whereas several studies have shown that experimentally increased levels of the androgenic steroid testosterone can affect female behavior, fewer studies have focused on the activational effects of exogenous testosterone on female morphology. With respect to colorful displays in birds, almost exclusively the effects of testosterone manipulation on female carotenoid-based colorations have been studied. Other color types such as structural colors (i.e. UV, blue and violet colors that result from differential light reflection in the nanostructures of the tissue) remain largely unstudied. Here, we investigated the short- and long-term effects of exogenous testosterone on the expression of structural bare-part coloration in female budgerigars, Melopsittacus undulatus. In this parrot species, bare-part coloration is expressed in the cere, a structure over the beak which is brown in females and structural blue in males. We experimentally increased plasma testosterone levels in testosterone-treated females (T-females) compared to controls (C-females) and we performed weekly spectrophotometric measurements of the cere for five weeks after implantation and one measurement after ten weeks. We also estimated the extent to which testosterone masculinized female cere color by comparing the experimental females with untreated males. We found significant effects of testosterone on cere color from week four after implantation onwards. T-females expressed significantly bluer ceres than C-females with higher values for brightness and UV reflectance. T-female cere color, however, remained significantly less blue than in males, while values for brightness and UV reflectance were significantly higher in T-females than in males. Our quantitative results show that exogenous testosterone induces the expression of structural blue color in females but does not strongly masculinize female cere coloration. We provide several potential pathways for the action of testosterone on structural color.     

Female survival,lifetime reproductive success and mating status in a passerine bird

In facultatively polygynous birds, secondary females of polygynously mated males typically have reduced annual reproductive success, because polygynous males provide less paternal care than monogamous males. Life history theory predicts that, as a result of increased reproductive investment, secondary females should suffer from reduced survival and lifetime reproductive success, but previous studies provided only weak support for this hypothesis. We used 7 years of data to study the fitness of female collared flycatchers Ficedula albicollis in relation to mating status by estimating survival and lifetime reproductive success. Taking differences in recapture probability into account, a mark-recapture analysis revealed that females observed at least once to breed as secondary female had higher survival than other females. This relationship was not confounded by laying date, because when we assessed the impact of laying date on survival, we found similar survival patterns. Females of polygynous males had reduced breeding success in terms of number of young fledged during the current reproductive event. However, during their lifetime females found at least once in primary or secondary mating status produced significantly more eggs, and at least the same number of fledglings and recruits as monogamous females. Thus, in the collared flycatcher, females of polygynously mated males seem to suffer from mating status during the most recent reproductive event, but considering survival and lifetime reproductive success, the apparently disadvantageous mating event is not necessarily associated with reduced residual reproductive value.     

Hot or Not: The Effects of Exogenous Testosterone on Female Attractiveness to Male Conspecifics in the Budgerigar

An increasing number of studies indicate that not only females but also males can be selective when choosing a mate. In species exhibiting male or mutual mate choice, females may benefit from being attractive. While male attractiveness is often positively influenced by higher plasma levels of the androgenic hormone testosterone, it has been shown that testosterone can masculinise female behavior and morphology in several bird species, potentially rendering them less attractive. In this study, we investigated whether female budgerigars, Melopsittacus undulatus , suffer from increased plasma testosterone levels through a negative effect on their attractiveness to males. We experimentally increased plasma testosterone levels in testosterone-treated females (T-females) compared to controls (C-females) and allowed males to choose between a T- and a C-female in a two-way choice situation. Although testosterone treatment significantly affected female behavioral and morphological characteristics, males did not show a significant difference in preference between T- and C-females. These results suggest that experimentally increasing testosterone levels in females does not appear to influence male preference during initial mate choice. Our findings indicate that selection for higher levels of testosterone in male budgerigars is probably not constrained by a correlated response to selection causing negative effects on female attractiveness during initial mate choice. Evaluating whether or not a potential constraint may arise from negative testosterone-induced effects on other fitness related traits in females requires further work.     

Anti-social and social behaviour of adolescent yellow-rumped caciques (Icterinae: Cacicus cela)

In an individually marked population in southeastern Peru, males and females of the polygynous, colonial yellow-rumped cacique delayed successful breeding until they were 3 years old. Adolescent (2-year-old) females competed for nest sites, built nests, laid fertile eggs, incubated, and sometimes fed nestlings, but always abandoned nests before they fledged young. Females that completed nests during their adolescent years were also more successful in establishing nest sites the following year. Adolescent females that failed to establish nest sites and those that lost nests to predators tended to leave the study area altogether. By attempting to nest early, adolescent females may facilitate future nesting attempts by forming coalitions with other females and assessing nest site quality in an area. Adolescent males, on the other hand, avoided aggressive interactions with other males, but were very aggressive towards adult females and fledglings. Adolescent males attacked females in nests, chased females to and from the colony, pecked and mounted recent fledglings, and, rarely, pulled nestlings out of nests. This harassment resulted in the death of nestlings and fledglings and was energetically costly to females, which often fought adolescent males while defending their young. Adolescent males preferentially harassed nestling-feeding females, avoided egg-laying females and were never observed attempting to mate with females. Adult males rarely interfered during any of these interactions. Such anti-social behaviour may function as a way of practising adult behaviour, as a means of eliminating future competitors, or may be an epiphenomenon of rising hormone levels and therefore of no adaptive significance.     

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.     

Elevated testosterone reduces choosiness in female dark-eyed juncos (Junco hyemalis): evidence for a hormonal constraint on sexual selection?

Because testosterone (T) often mediates the expression of attractive displays and ornaments, in the absence of constraints sexual selection should lead to an evolutionary increase in male T levels. One candidate constraint would be a genetic correlation between the sexes that leads to a correlated response in females. If increased T in females were to have deleterious effects on mate choice, the effect of sexual selection on male T would be weakened. Using female dark-eyed juncos (Junco hyemalis), we tested whether experimentally enhancing female T would lead to a decrease in discrimination between two classes of males, one treated with T (T-males) and one control (C-males). The two female treatments (T-implanted and C-females) spent equal amounts of time with both classes of males, but T-treated females failed to show a preference for either male treatment, whereas C-females showed a significant preference, albeit in an unexpected direction (for C-males). T-females were less discriminating than C-females, irrespective of the direction of their preference. To our knowledge, this is the first study to show that circulating hormones can alter female choosiness without reducing sexual motivation. Our results suggest that hormonal correlations between the sexes have the potential to constrain sexual selection on males.     

Yearly variation in factors associated with local recruitment of Tree Swallows

Identifying factors that influence the chances of fledglings becoming local recruits can inform us about the ecology of population dynamics and factors influencing parental fitness. We studied Tree Swallows (Tachycineta bicolor) nesting in boxes in western Michigan from 1994 to 2005 and found that 145 of 2405 (6.0%) nestlings that fledged from 840 nests became local recruits. Most local recruits returned to breed at the study site the year after they fledged. The sex ratio of local recruits (76 males and 69 females) did not differ from 1:1. Analysis of data from all years combined revealed that the likelihood of fledglings becoming local recruits was not significantly influenced by the year they fledged, clutch initiation date, clutch size, brood size, age class and physical characteristics of female parents, or the physical characteristics of putative male parents. However, analysis of data from each year revealed that some variables had a significant effect on the likelihood of fledglings becoming local recruits in some years, including female age class in two years, female parent right tail fork length in one year, female parent right wing length in one year, clutch initiation date in two years, clutch size in two years, and brood size in one year. The proportion of fledglings produced each year that became local recruits was not correlated with either mean air temperature or total rainfall during June and July, the immediate post-fledgling period. Taken together, these analyses suggest that chance or yearly variation in some other factor(s) with the potential to affect post-fledging survival that we did not measure (e.g., predation post-fledging or climatic conditions during migration or in migratory stopover sites and wintering areas) had an important effect on local recruitment of fledgling Tree Swallows in our study. We did find that local recruits were more likely to be from nests where egg-laying began earlier rather than later in the season, with 137 of 145 (94%) of local recruits coming from clutches initiated between 5 and 30 May. Because swallows that fledged earlier in the season had more time to prepare for migration than those that fledged later, we hypothesize that the “relative age effect” had an important effect on local recruitment of Tree Swallows at our study site.     

Aspects of the breeding biology of the Chinspot Batis Batis molitor in Acacia savanna in Swaziland

Some aspects of the breeding ecology of the Chinspot Batis (Batis molitor) were studied in Mlawula Nature Reserve, north-eastern Swaziland. Nests were predominantly built in thorny bushes or trees. Eggs were laid between 20 September and 2 January. However there was a definite peak in November, during which the majority of eggs were laid. Nesting success in the Chinspot Batis was over 30%, while fecundity was 0.65 fledglings/pair/annum. Except for a single occasion, pairs did not double brood unless breeding failed or fledged chicks disappeared. Replacement nests, however, were the norm where a previous nest had failed. Adult batises were observed to feed predominantly on caterpillars and moths. Observations at the nest confirmed that batises fed predominantly moths and caterpillars to their nestlings. The rate at which nestlings were fed depended on their age, older chicks being fed more frequently than younger ones. The correlation between feeding rate and nestling age, and between feeding rate and fledgling age, was significant. In contrast to nestlings, younger fledglings were fed at a higher rate than older ones.     

Within‐female plasticity in sex allocation is associated with a behavioural polyphenism in house wrens

Sex allocation theory assumes individual plasticity in maternal strategies, but few studies have investigated within‐individual changes across environments. In house wrens, differences between nests in the degree of hatching synchrony of eggs represent a behavioural polyphenism in females, and its expression varies with seasonal changes in the environment. Between‐nest differences in hatching asynchrony also create different environments for offspring, and sons are more strongly affected than daughters by sibling competition when hatching occurs asynchronously over several days. Here, we examined variation in hatching asynchrony and sex allocation, and its consequences for offspring fitness. The number and condition of fledglings declined seasonally, and the frequency of asynchronous hatching increased. In broods hatched asynchronously, sons, which are over‐represented in the earlier‐laid eggs, were in better condition than daughters, which are over‐represented in the later‐laid eggs. Nonetheless, asynchronous broods were more productive later within seasons. The proportion of sons in asynchronous broods increased seasonally, whereas there was a seasonal increase in the production of daughters by mothers hatching their eggs synchronously, which was characterized by within‐female changes in offspring sex and not by sex‐biased mortality. As adults, sons from asynchronous broods were in better condition and produced more broods of their own than males from synchronous broods, and both males and females from asynchronous broods had higher lifetime reproductive success than those from synchronous broods. In conclusion, hatching patterns are under maternal control, representing distinct strategies for allocating offspring within broods, and are associated with offspring sex ratios and differences in offspring reproductive success.     

Female size and age-specific fecundity in the small white butterfly,Pieris rapae crucivora Boisduval (Lepidoptera: Pieridae)

Individual differences in several reproductive parameters of female Pieris rapae were investigated in a controlled laboratory condition. Lifetime and age-specific fecundity showed considerable variability between individuals. Larger females began oviposition at an earlier age than smaller ones, and larger females were more fecund than smaller ones. Larger females laid a larger proportion of their eggs in the early stages of their reproductive lifetime, whilst smaller females laid the larger proportion of their eggs later in their reproductive lifetime. The significance of the variance in age-specific fecundity associated with female size is discussed with respect to the seasonal change in size and habitat utilization of this species.     

Postnatal developmental consequences of altered insulin sensitivity in female sheep treated prenatally with testosterone

Prenatally testosterone (T)-treated female sheep exhibit ovarian and endocrinological features that resemble those of women with polycystic ovarian syndrome (PCOS), which include luteinizing hormone excess, polyfollicular ovaries, functional hyperandrogenism, and anovulation. In this study, we determined the developmental impact of prenatal T treatment on insulin sensitivity indexes (ISI), a variable that is affected in a majority of PCOS women. Pregnant ewes were treated with 60 mg testosterone propionate intramuscularly in cottonseed oil two times a week or vehicle [control (C)] from days 30 to 90 of gestation. T-females weighed less than C-females or males (P < 0.05) at birth and at 5 wk of age. T-females had an increased anogenital ratio. An intravenous glucose tolerance test followed by an insulin tolerance test conducted after an overnight fast at 5, 20, and 30 wk of age (n = 7-8/group) revealed that ISI were higher at 5 than 30 wk of age in C-females, reflective of a developing insulin resistance associated with puberty. T-females had higher basal insulin levels, higher fasting insulin-to-glucose ratio, and higher incremental area under the insulin curve to the glucose challenge. The ISI of T-females was similar to that of males. No differences in ISI were evident between groups at 20 and 30 wk of age. Mean basal plasma glucose concentrations and glucose disappearance and uptake did not differ between groups at any age. Our findings suggest that prenatal T treatment leads to offspring with reduced birth weight and impaired insulin sensitivity in early postnatal life.     

Female differential allocation in response to extrapair offspring and social mate attractiveness

Renewed debate over what benefits females might gain from producing extra‐pair offspring emphasizes the possibility that apparent differences in quality between within‐pair and extra‐pair offspring are confounded by greater maternal investment in extra‐pair offspring. Moreover, the attractiveness of a female''s social mate can also influence contributions of both partners to a reproductive attempt. Here, we explore the complexities involved in parental investment decisions in response to extra‐pair offspring and mate attractiveness with a focus on the female point of view. Adult zebra finches paired and reproduced in a colony setting. A male''s early‐life diet quality and his extra‐pair reproductive success were used as metrics of his mating attractiveness. Females paired with males that achieved extra‐pair success laid heavier eggs than other females and spent less time attending their nests than their mates or other females. Extra‐pair nestlings were fed more protein‐rich hen''s egg than within‐pair nestlings. Females producing extra‐pair offspring had more surviving sons than females producing only within‐pair offspring. Collectively, results show that females differentially allocate resources in response to offspring extra‐pair status and their social mate''s attractiveness. Females may also obtain fitness benefits through the production of extra‐pair offspring.     

Do female spotless starlings Sturnus unicolor adjust maternal investment according to male attractiveness?

In birds, female egg allocation patterns have a strong influence in offspring development and differential investment in egg size and composition has been shown to respond to male attractiveness. In this study we experimentally manipulated the perceived attractiveness of male starlings Sturnus unicolor by increasing the amount of green material in some nests (a male courtship display in this species). We predicted that, if female investment before laying is related to male attractiveness, experimental females would increase their reproductive investment in response to the addition of plants in their nests when compared to control females. We found that our manipulation caused variations in female reproductive investment in a way that seems to influence offspring quantity but not offspring quality: Females laid larger clutch sizes but not larger eggs when green plant material was added. However, yolk androgens contents were not related to the experimental manipulation. Contrary to expectations, females breeding in experimental nests laid eggs with smaller amounts of eggshell pigments. Interestingly, we found that eggs laid later in the sequence had higher testosterone levels and showed more intense egg colouration than eggs laid earlier in the sequence. These differences at the within-clutch level suggest that selection has favoured compensatory strategies for hatching asynchrony. Alternatively, since nest sabotages by other females are most common at the beginning of laying, this could be seen as female strategy to minimise losses due to nest sabotages. As far as we know, this is the first study to show that an external egg characteristic such as blue-green colouration reflects yolk androgen concentration.     

Breeding success of the Great Bustard Otis tarda in Zamora Province, Spain, in 1984

Great Bustards are still fairly abundant (5-2 nests/1000 ha) in Zamora. Overall fecundity was high (92% of adult females attempt to nest; 2–47 eggs/clutch; 89% egg fertility), but pre- and post-hatching mortality were also high (50% of eggs laid; 57% of nestlings hatched), due mainly to agricultural machinery but also to natural predators. Calculated annual yields per 100 adult females were: 227 eggs, 102 hatchlings, 44 fledglings. Production of 44 fledglings/y could sustain a stable adult population (mortality approx. 8%/y) if mortality of immatures is not more than 18–22%/y.     

Growth of yellow-headed blackbird Xanthocephalus xanthocephalus nestlings in relation to maternal body condition, egg mass, and yolk carotenoids concentrations

Condition‐dependent resource allocation to eggs can affect offspring growth and survival, with potentially different effects on male and female offspring, particularly in sexually dimorphic species. We investigated the influence of maternal body condition (i.e., mass‐tarsus residuals) and two measures of female resource allocation (i.e., egg mass, yolk carotenoid concentrations) on nestling mass and growth rates in the polygynous and highly size dimorphic yellow‐headed blackbird Xanthocephalus xanthocephalus. Egg characteristics and carotenoid concentrations were obtained from the third‐laid egg of each clutch and were correlated with the mass and growth rates of the first two asynchronously hatched nestlings. Maternal body condition was associated with the growth of first‐hatched, but not second‐hatched nestlings. Specifically, females in better body condition produced larger and faster growing first‐hatched nestlings than females in poorer body condition. As predicted for a polygynous, size‐dimorphic species, females that fledged first‐hatched sons were in better body condition than females that fledged first‐hatched daughters. Associations between egg mass, yolk carotenoid content, and nestling growth were also specific to hatching‐order. Egg mass was positively correlated with the mass and growth rates of second‐hatched nestlings, and yolk concentrations of β‐carotene were positively correlated with second‐hatched nestling mass. Surprisingly, the relationship between yolk lutein and hatchling growth differed between the sexes. Females with high concentrations of yolk lutein produced larger and faster growing first‐hatched sons, but smaller first‐hatched daughters than females with lower lutein concentrations. Mass and growth rates did not differ between first‐ and second‐hatched nestlings of the same sex, despite asynchronous hatching in the species. Results from this study suggest that maternal body condition and the allocation of resources to eggs have carotenoid‐, sex‐, and/or hatch‐order‐specific effects on yellow‐headed blackbird nestlings.