Brood Sex Ratio Research Articles

The 'Goldilocks Grub': reproductive responses to leafroller host development in Goniozus jacintae, a parasitoid of the light brown apple moth.

Many parasitoids alter their reproductive behaviour in response to the quality of encountered hosts. They make adaptive decisions concerning whether to parasitise a potential host, the number of eggs laid on an accepted host, and the allocation of sex to their offspring. Here we present evidence that Goniozus jacintae Farrugia (Hymenoptera: Bethylidae), a gregarious ectoparasitoid of larval tortricids, adjusts its reproductive response to the size and developmental stage of larvae of the light brown apple moth (LBAM), Epiphyas postvittana (Walker) (Lepidoptera: Tortricidae). Goniozus jacintae parasitises instars 3-6 of LBAM, but most readily parasitises the later, larger, instars. Brood sizes were bigger on larger hosts and brood sex ratios were female biased (proportion of males = 0.23) with extremely low variance (never >1 male in a brood at emergence), perhaps the most precise of all studied bethylids. Host size did not influence brood development time, which averaged 19.64 days, or the body size of male offspring. However, the size of females was positively correlated with host size and negatively correlated with brood size. The sizes of individual males and females were positively related to the average amount of host resource available to individuals within each brood, suggesting that adult body size is affected by scramble competition among feeding larvae. Average brood sizes were: 3rd instar host, 1.3 (SE ± 0.075); 4th instar, 2.8 (SE ± 0.18); 5th instar, 4.7 (SE ± 0.23); 6th instar, 5.4 (SE ± 0.28). The largest brood size observed was 8 individuals (7 females, 1 male) on the 6th instar of LBAM. These results suggest that later instars would give the highest yield to optimise mass-rearing of G. jacintae if used for augmentative biological pest control.

Are brood sex ratios adaptive? – The effect of experimentally altered brood sex ratios on parental feeding behaviour

Correlations between brood sex ratios (BSRs) and parental or environmental quality have been found in many species. This phenomenon is called sex ratio adjustment, and is expected to evolve if certain factors affect the fitness return from the offspring in a sex-dependent way. However, it is seldom studied whether biased sex ratios are indeed adaptive. We manipulated BSRs in a cross-fostering experiment, and investigated parental costs in terms of feeding rate and survival in the collared flycatcher (Ficedula albicollis). In our population, male nestlings can grow faster under good conditions, but are more sensitive to adverse conditions. Assuming that the sensitivity of the males results from their larger energy requirement, we predicted increased costs in broods with male-biased experimental BSR. Assuming that BSR adjustment is adaptive and related to parental care giving capacity, we expected higher feeding and survival rate by parents that originally had more sons, and predicted that low quality parents are less able to adjust their feeding rates to the needs of their foster broods or pay higher survival cost. However, we found that the manipulated BSR and its interaction with original BSR affected neither the feeding rate nor the survival of the parents. Only male feeding rate was correlated with original BSR, however, contrary to our prediction: males with female-biased original BSR fed their foster chicks more frequently. Our results, with those of a previous report about the effects of the experiment on nestlings, do not support that the observed BSRs are adaptive in our population.Significance statementMany hypotheses propose that higher vertebrates adaptively adjust the primary sex ratio of their offspring to individual or environmental quality. While the potential adaptive value of the observed patterns is regularly discussed, studies that specifically test the adaptivity of sex ratio adjustment are very scarce and correlative. Using a special cross-fostering experiment, we investigated whether original brood sex ratios are related to the rearing capacity of the parents, and experimental sex ratios are related to the rearing costs in terms of feeding effort or survival. We found no effect of experimentally altered brood sex ratios on either parental feeding effort or survival. Furthermore, contrary to the adaptive scenario, males that had female-biased broods originally had higher feeding rates. So far, we have found no evidence that the sex ratio adjustment is adaptive in the collared flycatcher.

Sex allocation is color morph-specific and associated with fledging condition in a wild bird.

Melanin-based color polymorphism is predicted to evolve and maintain through differential fitness of morphs in different environments, and several empirical studies indicate that life history strategies, physiology, and behavior vary among color morphs. Sex allocation theory predicts that parents should adjust their sex allocation based on differential costs of raising sons and daughters, and therefore, color morphs are expected to modify their brood sex ratio decisions. In color polymorphic tawny owls (Strix aluco), the pheomelanistic brown morph is associated with higher energy requirements, faster growth, and higher parental effort than the gray morph. As hypothesized, we find that brown tawny owl mothers produced more daughters in early broods and more males in late broods, whereas gray mothers did the opposite. At fledging, daughters of early broods and of brown mothers were heavier than those of late broods or gray mothers. Hence, larger and more costly daughters appeared to benefit more than males from being born to brown mothers early in the season. Brown mothers breeding later in the season produced more cheap sons, while gray mothers face fewer challenges under limited resources and favor daughters. These findings suggest that environmental conditions influence brood sex allocation strategies of genetically determined color morphs differently.

On the issue of the primary sex ratio in the pied flycatcher <i>(Ficedula hypoleuca)</i>

An assessment of the primary sex ratio in the local pied flycatcher population in South Karelia (Russia) is given and its variations in broods depending on breeding characteristics and characteristics of the parents are analyzed. The proportion of males in 30 clutches examined in 2012 did not differ from the proportion of females and amounted to 50.8% of the total number of eggs laid (n = 199), although it varied from 14.3% to 85.7% in individual nests. A weak positive correlation between the proportion of males in the brood and the timing of breeding and the ornamentation of the feathers that form the white spot on the male’s wing (the first tertiary and the 5–6th upper greater coverts of the secondary feathers), and a negative correlation with the length of the wing and the third primary feather of the female were shown. Analysis using generalized linear models showed a significant relationship of primary sex ratio in the brood only with the length of the female’s third primary feather. Possible mechanisms of changes in the sex ratio in broods are discussed.

A possible case of offspring sex manipulation as result of a biased adult sex ratio

Although random meiosis should prevent the facultative adjustment of offspring sex ratio, theory predicts that females should produce more of the sex with the higher reproductive value. We reported a case of offspring sex ratio manipulation in grass wrens Cistothorus platensis. Males in better body condition would have higher reproductive value than females due to the potential for social polygyny and extra-pair fertilizations. On the other hand, local demography influences reproductive strategies in grass wrens as male abundance affects both social polygyny and extra-pair paternity frequencies. We evaluated whether females bias their brood sex ratio in response to adult sex ratio and nestling body condition (a proxy for female’s prospects of producing high-quality males). Females raised more male offspring when males were less abundant in the population (female-biased adult sex ratio). However, we found no relationship between nestling body condition and brood sex ratio, suggesting that females did not bias the brood sex ratio towards males when able to raise nestlings in better body condition. Taken together, our results provide the first suggestive evidence that female birds can manipulate their offspring sex ratio in response to the adult sex ratio.

Whoever their partner, female blue tits breed the same

Abstract Do females modify their reproductive investment if they do not succeed in pairing with a male that matches their preference? In a two-year experiment, we asked female blue tits (Cyanistes caeruleus) to select among six males, and then successively paired them with their preferred and their most avoided male. We monitored female reproductive investment through nest building activity, timing of breeding, size and number of eggs, number of fertilized eggs, and brood sex ratio. We found that females preferred males with a chromatic coloration (blue UV chroma of the head crown) that matched their own, but also that they preferred males with a lower achromatic coloration (less bright color of the cheeks and head crown). Although females paired with their preferred males tended to build heavier nests during the breeding season, we found no evidence of an effect of the pairing treatment on timing of breeding, or any other aspects of female reproductive investment. We however found that laying dates, clutch sizes, egg sizes, and brood sex ratio were significantly repeatable within females between the two years, despite the opposite pairing treatments. These findings show that in female blue tits, the males with which they are paired do not substantially alter their reproductive decisions.

Male’s age and plumage coloration predicts brood sex ratio in the Common Kingfisher (Alcedo atthis)

Plumage ornaments of birds, both pigment-based and structural, are considered age or condition-dependent, honestly signalling male quality, sexually selected traits, influencing the variation in breeding performance as well as adaptive sex allocation. In the present study, we examine the effect of age of males on the plumage structural colour in Common Kingfisher, and evaluate the breeding output (laying date, total number of young per breeding season, mean number of young per nest, total number of broods) and sex ratio in relation to two ornamental traits (forehead and rump structural colour). During the three years 2016–2018 in the Danube river system (south-western Slovakia), we collected data from 49 males, 102 broods and 645 nestlings. Our data demonstrate that structural colour is a condition-related and age-sensitive signal, while males with increased weight expressed less saturated blue forehead feathers, and old males displayed more saturated cyan rump feathers than young ones. Moreover, the brood sex ratio varies with male coloration in an age-dependent manner. The plumage colour of young males did not affect the brood sex ratio, whereas old males with increasing intensity of cyan rump had more sons than duller ones. Neither single ornamental trait nor age predicted breeding output of kingfisher males. Nevertheless, our results indicate that the blue structural plumage of kingfisher males may serve as an indicator of age and a certain kind of quality with a consequence on adaptive sex allocation in this species.

Among us: How to find an optimal clutch sex ratio when the clutch sex ratios of co-foundress wasps are not known beforehand?

A female-biased clutch sex ratio is selected under local mate competition since it reduces competition between male sibs and provides extra mating opportunities for sons. Theoretical models predict that the brood sex ratio of fig wasps becomes less female-biased as the number of wasps laying eggs in the same fig increases. Empirical data have demonstrated that pollinator females lay most of their male eggs first, followed by the female ones in an oviposition bout (males first rule). As oviposition sites are limited in the fig, the brood sex ratio is positively correlated with the number of wasps laying eggs. To understand the strategy of sex ratio adjustment of Sycobia hodites, a non-pollinating fig wasp associated with Ficus benjamina, we compared observed and simulated data. The female wasp starts oviposition shortly after landing on the fig. This process involves the insertion of the ovipositor through the fig surface and into the flower's ovary via the stigma/style. The brood sex ratio of S. hodites did not significantly correlate with the fig brood size. Brood simulations supported the notion that the sex ratio of S. hodites is automatically determined by the male-first rule, involving no control by the mothers. The simulation model best fitting empirical data considered that: (1) females lay few eggs per fig (six or less) and distribute their eggs among several figs, and (2) the probability of an egg producing a male wasp decreases with its position in the egg sequence. Empirical and simulated data showed a wide variation in both brood size and sex ratio, suggesting that selection for an optimal clutch sex ratio is possibly weak. We argue that when selective pressure for sex ratio adjustment is weak, evolution favors male polymorphism as a strategy to cope with mate competition.

Variance components of sex determination in the copepod Tigriopus californicus estimated from a pedigree analysis.

Extensive theory exists regarding population sex ratio evolution that predicts equal sex ratio (when parental investment is equal). In most animals, sex chromosomes determine the sex of offspring, and this fixed genotype for sex has made theory difficult to test since genotypic variance for the trait (sex) is lacking. It has long been argued that the genotype has become fixed in most animals due to the strong selection for equal sex ratios. The marine copepod Tigriopus californicus has no sex chromosomes, multiple genes affecting female brood sex ratio, and a brood sex ratio that responds to selection. The species thus provides an opportune system in which to test established sex ratio theory. In this paper, we further our exploration of polygenic sex determination in T. californicus using an incomplete diallel crossing design for analysis of the variance components of sex determination in the species. Our data confirm the presence of extra-binomial variance for sex, further confirming that sex is not determined through simple Mendelian trait inheritance. In addition, our crosses and backcrosses of isofemale lines selected for biased brood sex ratios show intermediate phenotypic means, as expected if sex is a threshold trait determined by an underlying "liability" trait controlled by many genes of small effects. Furthermore, crosses between families from the same selection line had similar increases in phenotypic variance as crosses between families from different selection lines, suggesting families from artificial selection lines responded to selection pressure through different underlying genetic bases. Finally, we estimate heritability of an individual to be male or female on the observed binary scale as 0.09 (95% CI: 0.034-0.14). This work furthers our accumulating evidence for polygenic sex determination in T. californicus laying the foundation for this as a model species in future studies of sex ratio evolution theory.

Identifying factors affecting captive breeding success in a critically endangered species

ABSTRACT Captive breeding programs are an increasingly important tool for species’ conservation efforts, but not all species reproduce well in captivity. Identifying factors that affect the reproductive success of captive populations is crucial to improving the performance and management of conservation-breeding programs, both by providing individuals for release and informing decision making. We examined breeding records collected from the long-running conservation-breeding program for the critically endangered Orange-bellied Parrot Neophema chrysogaster over an 11-year period. We examined egg hatching rate, nestling survival rate, and offspring sex ratio in response to a wide range of variables related to characteristics of individual birds, breeding events, and the captive environment. The hatch rate of eggs was higher in first clutches compared to second clutches and was lower than the wild population. The survival rate of nestlings through to fledging was variable between years but became higher and more consistent over the last five years of the study period. Variation in brood sex ratio was not related to any of the potential explanatory variables that we examined. This is one of the first studies to examine reproductive data in a long-running conservation-breeding program and shows that many common metrics do not explain reproductive variation. Our approach provides a framework for managers to investigate factors affecting reproductive success in conservation breeding programs more broadly.

Integrative assessment of immunity, health-state, growth and survival of Magellanic penguin chicks in a colony exposed to ecotourism

Accumulating reports of negative impacts of tourist activities on wildlife emphasize the importance of closely monitoring focal populations. Although some effects are readily noticed, more subtle ones such as changes in physiological functions of individuals might go overlooked. Based on evidence of altered physiology associated with ecotourism on Magellanic penguins Spheniscus magellanicus, here we performed an integrated assessment using a diverse physiological toolkit together with more traditional fitness-related measures to better understand mechanisms and potential consequences. Chicks exposed to tourism showed altered immune parameters and elevated flea prevalence, reinforcing previous findings. Tourism-exposed female, but not male, chicks also showed relatively lower hematocrit and plasma protein levels, providing evidence consistent with a sex-specific response to tourist visitation. Physiological alterations detected in tourism-exposed young chicks (week 1–2) were maintained and the effect on flea infestation increased during the study period (week 4–5 of post-hatch). Despite the effects on physiology, these did not seem to translate into immediate fitness costs. No detectable tourism effects were found on brood sex ratios, chick growth and body condition, and survival until week 5–6 post-hatch. We detected no effects on reproductive output and only a marginal effect on nest survival during incubation despite previous reports of tourism-associated alterations in stress indices of adults. This disconnection could result if the physiological changes are not strong enough to impact fitness, if effects balance each other out, or if changes are part of a copying strategy. Alternatively, the physiological alterations might only show impacts later in the brooding cycle or even after chick emancipation from their parents. Our results suggest that integrative monitoring of potential anthropogenic impacts on wildlife should include evaluation of physiological mechanisms and individual-level responses in populations exposed to human activities.

Technical comment on "sex ratios when helpers stay at the nest".

I contributed a paper to volume 60 of the journal. The paper reported on my study of sex-ratio evolution when one sex (females) is helpful but the other sex (males) suffers less from kin competition. I had based my study on a kin-selection model, and so I was dismayed to discover an error in the relatedness calculations therein. Specifically, relatedness coefficients that should have been calculated using a sampling-without-replacement scheme were instead calculated using sampling with replacement. Here, I correct my error and show how it impacts my original findings. I argue that my main conclusions are unchanged. Furthermore, only two new findings contrast with those I presented earlier. First, changing those model details unrelated to the marginal fitness benefits of help does not, in turn, impact substantially the conflict that occurs between mates over the brood sex ratio (I had previously reported some noteworthy impact was possible). Second, help can reduce sex-ratio conflict between mates more effectively when breeders occur in smaller groups (previously, I had said this occurred in larger groups).

A two‐sex integrated population model reveals intersexual differences in life history strategies in Cooper's hawks

AbstractIntegrated population models (IPMs) combine population counts with demographic data to improve the precision of estimates of population size and demographic rates. IPMs can also provide information on demographic parameters for which data are lacking (e.g., immigration) if other sufficient information is available. IPMs often use demographic data from one sex, usually females, implicitly assuming the chosen sex adequately represents the species' life history; for highly size‐dimorphic raptors, that assumption may not hold true. We developed a two‐sex IPM for a New Mexico population of the highly size‐dimorphic Cooper's hawk (Accipiter cooperii) from 2011 to 2020 and compared estimates of life history characteristics between sexes. Because we had data to directly estimate sex‐ and age‐specific probabilities of breeding, fecundity, survival, brood sex ratios, and emigration rates, we could indirectly estimate age‐specific immigration rates for both sexes. Our two‐sex IPM revealed that population growth was most strongly associated with increased immigration and decreased first‐year (FY) emigration in females and with after‐first‐year (AFY) survival in males. Most males that were recruited as new breeders in our study area were AFY residents, whereas most female recruits were AFY immigrants. All females that fledged in our study area and survived until their first breeding season bred in their first year, whereas only 3% of surviving FY males bred. We found evidence of density dependence in the survival of AFY males (r = −0.11, 95% credible interval = −0.36 to −0.03) and in AFY female immigration (r = −0.17, 95% credible interval = −0.27 to −0.12). Our findings reveal that male and female Cooper's hawks differ in their response to population density, and in how they contribute to population growth. Thus, sexual differences in vital rates can be an important consideration in raptor population models.

Brood sex ratio variation in a colonial raptor, the Eleonora's falcon, Falco eleonorae

Brood sex ratio variation in a colonial raptor, the Eleonora's falcon, Falco eleonorae

Are brood sex ratios adaptive?-The effect of experimentally altered brood sex ratio on nestling growth, mortality and recruitment.

Brood sex ratios (BSRs) have often been found to be nonrandom in respect of parental and environmental quality, and many hypotheses suggest that nonrandom sex ratios can be adaptive. To specifically test the adaptive value of biased BSRs, it is crucial to disentangle the consequences of BSR and maternal effects. In multiparous species, this requires cross-fostering experiments where foster parents rear offspring originating from multiple broods, and where the interactive effect of original and manipulated BSR on fitness components is tested. To our knowledge, our study on collared flycatchers (Ficedula albicollis) is the first that meets these requirements. In this species, where BSRs had previously been shown to be related to parental characteristics, we altered the original BSR of the parents shortly after hatching by cross-fostering nestlings among trios of broods and examined the effects on growth, mortality and recruitment of the nestlings. We found that original and experimental BSR, as well as the interaction of the two, were unrelated to the fitness components considered. Nestling growth was related only to background variables, namely brood size and hatching rank. Nestling mortality was related only to hatching asynchrony. Our results therefore do not support that the observed BSRs are adaptive in our study population. However, we cannot exclude the possibility of direct effects of experimentally altered BSRs on parental fitness, which should be evaluated in the future. In addition, studies similar to ours are required on various species to get a clearer picture of the adaptive value of nonrandom BSRs.

Brood sex ratios in Merlins reflect characteristics of the associated breeding male and population density

Population‐level estimates of offspring sex ratios in birds typically approximate parity whereas biased ratios within nests are not uncommon. In sexually dimorphic raptors, the costs and relative fitness benefits of rearing male and female progeny vary with changing environmental circumstances. This may lead to substantial deviations from balanced investment in offspring of a particular sex by individual parents. Based on a 13‐year dataset for breeding Merlins Falco columbarius in Saskatoon, Canada, we used a model selection approach to assess the influence of parents, nest‐mates and nesting area on brood sex ratio during the nestling phase. The best model for predicting brood sex ratio included age of the breeding male and brood size for each nest (n = 127); nests with older male breeders and smaller brood sizes had more female young. The population‐level annualized average proportion of male offspring was 0.472 ± 0.017 (mean ± standard error), but tended towards greater production of female young during an initial period of population growth (8 years, 10–21 pairs; proportion male 0.435 ± 0.031) versus a period when the population fluctuated around a presumed carrying capacity (11 years, 24–33 pairs; proportion male 0.500 ± 0.017). Energetics appears to be a finely tuned mechanism driving sex ratio allocation in Merlins at both brood and population levels. Provisioning food for young in the nest represents the male's ability to successfully capture prey, reflecting his age and/or experience, as well as the availability of prey to the male. Confounding this mechanism to determine sex ratio allocation are the pressures created by population dynamics that dictate competition for resources both within the nest (brood size) and external to the nest (population density).

Male survivorship and the evolution of eusociality in partially bivoltine sweat bees.

Eusociality, where workers typically forfeit their own reproduction to assist their mothers in raising siblings, is a fundamental paradox in evolutionary biology. By sacrificing personal reproduction, helpers pay a significant cost, which must be outweighed by indirect fitness benefits of helping to raise siblings. In 1983, Jon Seger developed a model showing how in the haplodiploid Hymenoptera (ants, wasps and bees), a partially bivoltine life cycle with alternating sex ratios may have promoted the evolution of eusociality. Seger predicted that eusociality would be more likely to evolve in hymenopterans where a foundress produces a male-biased first brood sex ratio and a female-biased second brood. This allows first brood females to capitalize on super-sister relatedness through helping to produce the female-biased second brood. In Seger's model, the key factor driving alternating sex ratios was that first brood males survive to mate with females of both the second and the first brood, reducing the reproductive value of second brood males. Despite being potentially critical in the evolution of eusociality, however, male survivorship has received little empirical attention. Here, we tested whether first brood males survive across broods in the facultatively eusocial sweat bee Halictus rubicundus. We obtained high estimates of survival and, while recapture rates were low, at least 10% of first brood males survived until the second brood. We provide empirical evidence supporting Seger's model. Further work, measuring brood sex ratios and comparing abilities of first and second brood males to compete for fertilizations, is required to fully parameterize the model.

Brood Sex Ratio in European Honey Buzzards Pernis apivorus is Related to Spring Phenology

In sexually size-dimorphic bird species, rearing costs of sons and daughters usually differ and may be important in the evolution of offspring sex ratio adjustment. Raptors have reversed sexual size dimorphism and the smaller males are sometimes found to be overrepresented in food-poor territories or years. As a raptor with small reversed sexual dimorphism (6% in body mass), the European Honey Buzzard Pernis apivorus is expected to show little or no brood sex ratio bias in relation to environmental conditions. We molecularly sexed 311 chicks of 195 broods in and around The Netherlands, during 1996–2014. We examined which environmental factors explained brood sex ratio variation best. Overall, sex ratio was not biased (all nests pooled: 50.8% females) but more females were produced in years when on average Honey Buzzards bred earlier (32% sex ratio change over a ten-day range in annual mean laying date). Within-year laying date variation, hatching order, abundance of wasp (Vespinae) nests (main food source) and summer weather did not explain sex ratio variation. In the Veluwe and Drenthe (1974–2014), Honey Buzzards laid eggs earlier when the spring was warmer, which resulted in a c. 9-day advance in laying date over 40 years. As warm spring weather was also a predictor of a higher density of wasp colonies, we expected female chicks to benefit more from growing up in wasp-rich years than males, if the sex ratio biases were adaptive. However, this differential growth benefit was not noticeable in chick body mass; chick body mass was best explained by negative effects of relative laying date (within a year) and hatching order. The potential benefit for female nestlings (compared to males) of growing up in years with warm springs, when egg laying occurs early and wasp colonies are more abundant, remains unknown.

Sex ratios and the city: Secondary offspring sex ratios, parental corticosterone, and parental body condition in an urban-adapted bird

The Trivers–Willard hypothesis states that mothers should adjust their offspring sex ratio according to their own condition and the environment they face during breeding. Past tests of this hypothesis have focused on how natural variation in weather, food availability, or predation pressure shapes sex allocation trade-offs. However, anthropogenic activities, such as urbanization, can alter all of the above characteristics presenting animals with novel challenges in optimizing their brood sex ratio. Previous research has examined how urban living influences individual body condition in several bird taxa, but few have explored subsequent impacts on secondary offspring sex ratio. One likely mediator of the link between environmental conditions, parental condition, and sex ratios is corticosterone (CORT), the primary glucocorticoid in birds. Research on CORT’s influence on sex ratios has focused solely on maternal CORT. However, for species with biparental care, paternal CORT or the similarity of maternal and paternal phenotypes may also help ensure that offspring demand matches parental care quality. To test these hypotheses, we explore offspring secondary sex ratios in European starlings (Sturnus vulgaris). We did not find an effect of site or parental body condition on the production of the more costly sex (males). Instead, we found preliminary evidence suggesting that the similarity of maternal and paternal CORT levels within a breeding pair may increase the likelihood of successfully fledging sons. Maternal and paternal CORT were not significant predictors of secondary sex ratio, suggesting that parental similarity, rather than parental CORT alone, could play a role in shaping secondary offspring sex ratios, but additional work is needed to support this pattern. Starlings are considered an urban-adapted species, making them a compelling model for future studies of the relationship between urbanization, parental body condition, and sex ratios.

Density dependence of clutch size and offspring sex ratio in starling colonies

Optimal life‐history theory predicts that individuals should adjust both the number and the sex of their offspring to maximize fitness in response to environmental and social factors such as breeding density. While reductions in optimal clutch size are well‐studied in birds, the evidence for sex ratio adjustments is still equivocal and, so far, we lack a thorough understanding of how these strategies interact to maximize fitness. Here, we investigate how breeding density simultaneously affects brood sex ratio and clutch size in a sexually dimorphic and polygynous bird. We tested the prediction that mothers breeding at a higher density lay smaller clutches and overproduce daughters, the sex with less variable fitness returns and that disperses further away from their natal territory. We distributed nest boxes at either a high (HD) or a low density (LD) and monitored clutch sizes and sex ratios during five years in a wild breeding colony of spotless starlings. While mothers breeding in HD nests produced more daughters than those breeding in LD nests, the density dependence of clutch size varied among years, with a tendency to lay smaller clutches in HD nests. Our results suggest that mothers consistently adjust offspring sex ratio in response to breeding density, whereas adjustments in clutch size varied in a more complex way. These results support the role of sex allocation strategies in response to density and show that further theoretical and empirical research is required to understand the interaction between clutch size and sex ratio adjustments in animals.