Male-Biased Sex Ratios in Offspring of Attractive Males in the Guppy

The attractiveness hypothesis predicts that females produce offspring with male-biased sex ratios when they mate with attractive males because their male offspring will inherit the paternal sexual attractiveness and may have high reproductive success. In this study, we examined the effect of the attractiveness of the male guppy Poecilia reticulata in terms of the conspicuousness of its orange spot patterns, important criteria affecting female choice in this species, on the offspring sex ratios. We found that food-manipulation treatment altered the conspicuousness of the orange spot patterns in a full-sibling male pair. When females were presented to these males, they showed a greater mate preference for males having brighter orange spots than for those having duller orange spots. Subsequently, half of the females were mated with the preferred males and the remaining females were mated with the less preferred males. When the females exhibited a greater preference for their mates, their offspring sex ratios were more male biased. These results appear to be consistent with the prediction of the attractiveness hypothesis. In the guppy, as male sexual attractiveness is heritable, the male-biased sex ratios of the broods of attractive males may be adaptive.     

Offspring sex ratio is related to paternal train elaboration and yolk corticosterone in peafowl

Several recent experimental studies have provided strong evidence for the ability of birds to manipulate the sex ratio of their offspring prior to laying. Using a captive population of peafowl (Pavo cristatus), we tested experimentally the effects of paternal attractiveness on offspring sex ratio, and related sex ratio deviations to egg-yolk concentrations of testosterone, 17beta-estradiol and corticosterone. When females were mated to males whose attractiveness had been experimentally reduced by removing prominent eyespot feathers from their trains, they produced significantly more female offspring, had significantly higher yolk corticosterone concentrations and tended to have lower levels of yolk testosterone than when mated to the same males with their full complement of feathers. Concentrations of 17beta-estradiol did not vary consistently with sex ratio biases. These findings add to the small number of studies providing experimental evidence that female birds can control the primary sex ratio of their offspring in response to paternal attractiveness, and highlight the possibility that corticosterone and perhaps testosterone are involved in the sex manipulation process in birds.     

Adaptive Offspring Sex Ratio Depends on Male Tail Length in the Guppy

A biased sex ratio in a brood is considered to be an adaptive strategy under certain circumstances. For example, if the expected reproductive success of one sex is greater than that of the other, parents should produce more offspring of the former sex than the latter. A previous study has documented that in the guppy, Poecilia reticulata, the female offspring of males possessing proportionally longer tails exhibit smaller body sizes and show decreased reproductive outputs than those of males having shorter tails. On the other hand, the total lengths of the male offspring of the long‐tailed males are larger because of their longer tails; consequently, they exhibit greater sexual attractiveness to females. Therefore, it has been hypothesized that this asymmetry in the expected reproductive success between the male and female offspring of long‐tailed males may result in a biased sex ratio that is dependent on the tail lengths of their fathers. This hypothesis was tested in the present study. The results showed that the females that mated with long‐tailed males produced more male offspring than those that mated with short‐tailed males. Logistic regression analysis showed that the ratio of tail length to the standard length of the fathers is a determinant factor of the sex of their offspring. These results suggest that the manipulation of the offspring sex ratios by parents enhances the overall fitness of the offspring.     

Evolutionarily stable oviposition and sex ratio in parasitoid wasps with single‐sex broods

Abstract 1. The flexibility of hymenopteran sex ratios is well documented, particularly in structured populations featuring sib mating. 2. Using game theoretic models, the present study examines species producing single‐sex broods in which sib mating is unlikely, and focuses on the role of population density in determining evolutionarily stable oviposition strategies. 3. Since only mated females can produce offspring of both sexes while unmated females produce only male offspring, mated females are under selection to produce more females overall to balance the primary sex ratio. 4. As the proportion of all females that are mated should increase with density, offspring sex ratio of mated females is strongly linked to density at low to moderate densities. The present study shows that when density becomes low enough for fewer than half of all females to have mated, then female offspring generate higher fitness. 5. In this low density situation, females may gain a fitness benefit from waiting at their emergence site or from using other costly means to find and mate with males before ovipositing. 6. The predicted correspondence between females waiting at the emergence site and fewer than half of females in the population containing sperm, can be tested empirically, as can the somewhat counter‐intuitive prediction that greater access to males should yield a more male‐biased sex ratio in the offspring of mated females. 7. The present study also indicates how measuring the variance in giving up times by females waiting for males at low density, can provide insight into mechanisms determining waiting times.     

Constrained sex allocation in a parasitoid due to variation in male quality

The theory of constrained sex allocation posits that when a fraction of females in a haplodiploid population go unmated and thus produce only male offspring, mated females will evolve to lay a female-biased sex ratio. I examined evidence for constrained sex ratio evolution in the parasitic hymenopteran Uscana semifumipennis. Mated females in the laboratory produced more female-biased sex ratios than the sex ratio of adults hatching from field-collected eggs, consistent with constrained sex allocation theory. However, the male with whom a female mated affected her offspring sex ratio, even when sperm was successfully transferred, suggesting that constrained sex ratios can occur even in populations where all females succeed in mating. A positive relationship between sex ratio and fecundity indicates that females may become sperm-limited. Variation among males occurred even at low fecundity, however, suggesting that other factors may also be involved. Further, a quantitative genetic experiment found significant additive genetic variance in the population for the sex ratio of offspring produced by females. This has only rarely been demonstrated in a natural population of parasitoids, but is a necessary condition for sex ratio evolution. Finally, matings with larger males produced more female-biased offspring sex-ratios, suggesting positive selection on male size. Because the great majority of parasitic hymenoptera are monandrous, the finding of natural variation among males in their capacity to fertilize offspring, even after mating successfully, suggests that females may often be constrained in the sex allocation by inadequate number or quality of sperm transferred.     

Sex ratio bias in the dung beetle <Emphasis Type="Italic">Onthophagus taurus</Emphasis>: adaptive allocation or sex-specific offspring mortality?

Sex allocation theory predicts that females should adjust the sex of their offspring when the fitness returns of one sex are higher than the other. However, biased sex ratios may also arise if mortality differs between the sexes. Here, we examine whether offspring sex ratio bias in the dung beetle, Onthophagus taurus, represents adaptive sex allocation by females or is due to sex-specific mortality. First, we re-analyze an existing data set to show that females produce an excess of daughters when mating to smaller, less attractive males and near equal sex ratio with large, more attractive males. We show, that this results from females adjusting larval provisions after mating to males of variable attractiveness which in turn influences the likelihood that sons die during development. Second, we conduct a manipulative experiment varying the quantity and quality of larval provisions and show that the mortality of sons increased when larval provisions were reduced. Collectively, our work demonstrates that offspring mortality is contingent on the amount of resources provisioned by females and that sons have greater nutritional demands than daughters during development, leading to higher mortality. Our results therefore demonstrate the importance of considering sex-specific offspring mortality in studies of sex ratio evolution.     

Offspring sex ratios correlate with pair-male condition in a cooperatively breeding fairy-wren

We examined sex allocation patterns in island and mainland populationsof cooperatively breeding white-winged fairy-wrens. The markeddifferences in social structure between island and mainlandpopulations, in addition to dramatic plumage variation amongmales both within and between populations, provided a uniquesituation in which we could investigate different predictionsfrom sex allocation theory in a single species. First, we testthe repayment (local resource enhancement) hypothesis by askingwhether females biased offspring sex ratios in relation to theassistance they derived from helpers. Second, we test the malequality (attractiveness) hypothesis, which suggests that femalesmated to attractive high-quality males should bias offspringsex ratios in favor of males. Finally, we test the idea thatfemales in good condition should bias offspring sex ratios towardmales because they are able to allocate more resources to offspring,whereas females in poor condition should have increased benefitsfrom producing more female offspring (Trivers-Willard hypothesis).We used molecular sexing techniques to assess total offspringsex ratios of 86 breeding pairs over 2 years. Both offspringand first brood sex ratios were correlated with the pair-male'sbody condition such that females increased the proportion ofmales in their brood in relation to the body condition (masscorrected for body size) of their social partner. This relationwas both significant and remarkably similar in both years ofour study and in both island and mainland populations. Althoughconfidence of paternity can be low in this and other fairy-wrenspecies, we show how this finding might be consistent with themale quality (attractiveness) hypothesis with respect to malecondition. There was no support for the repayment hypothesis;the presence of helpers had no effect on offspring sex ratios.There was weak support for both the male quality (attractiveness)hypothesis with respect to plumage color and the maternal conditionhypothesis, but their influence on offspring sex ratios wasnegligible after controlling for the effects of pair-male condition.     

Is egg carrying attractive? Mate choice in the golden egg bug (Coreidae,Heteroptera)

In several species of fish, females select males that are already guarding eggs in their nests. It is a matter of debate as to whether a female selects a good nest site for her offspring (natural selection) or a male for his attractiveness (sexual selection). The golden egg bug, Phyllomorpha laciniata Vill, resembles fish in the sense that mating males carry more eggs than single males, but in the bugs, female mate choice is decoupled from egg site choice. The sexual selection hypothesis predicts that if females select males using male egg load as a cue for male quality, they should not mate with a male when eggs are removed, regardless of his mating attempts. When individual females were enclosed with an egg-loaded male and an unloaded male, they mated equally often with both males, although the loaded males courted more. In addition, when only successful males were used, females mated equally often with the loaded male and the unloaded male irrespective of sex ratio. Male choice rather than female choice affected mating frequency when sex ratio was equal. Therefore, females do not select the male by the eggs he carries, but successful males may receive many eggs due to egg dumping by alien females while they mate or as a consequence of mate guarding.     

No evidence for adjustment of sex allocation in relation to paternal ornamentation and paternity in barn swallows

Sex allocation theory predicts that parents should adjust investment in sons and daughters according to relative fitness of differently sexed offspring. In species with female preference for highly ornamented males, one advantage potentially accruing to parents from investing more in sons of the most ornamented males is that male offspring will inherit characters ensuring sexual attractiveness or high-quality genes, if ornaments honestly reveal male genetic quality. Furthermore, in species where extra-pair fertilizations occur, offspring sired by an extra-pair male are expected to more frequently be male than those of the legitimate male if the latter is of lower quality than the extra-pair male. We investigated adjustment of sex ratio of offspring in relation to ornamentation of the extra-pair and the social mate of females by direct manipulation of tails of male barn swallows Hirundo rustica . Molecular sexing of the offspring was performed using the W chromosome-linked avian chromo-helicase-DNA-binding protein (CHD) gene while paternity assessment was conducted by typing of hypervariable microsatellite loci. Extra-pair offspring sex ratio was not affected by ornamentation of their biological fathers relative to the experimental ornamentation of the parental male. Experimental ornamentation of the parental males did not affect the sex ratio of nestlings in their broods. Female barn swallows might be unable to bias offspring sex ratio at hatching according to the quality of the biological father. Alternatively, fitness benefits in terms of sexual attractiveness of sons might be balanced by the cost of compensating for little parental care provided by highly ornamented parental males, if sons are more costly to rear than daughters, or the advantage of producing more daughters, if males with large ornaments contribute differentially more to the viability of daughters than sons.     

Genetic benefits of mate choice separated from differential maternal investment in red junglefowl (Gallus gallus)

Abstract Females may choose more attractive mates to obtain better viability or attractiveness genes for their offspring. A number of studies have demonstrated a positive relationship between paternal attractiveness and offspring quality. However, this pattern could be due to inheritance of paternal genes and/or it could be due to increased maternal investment in the offspring of more attractive males. To isolate female responses to male appearance from paternal genetic effects, I housed female red junglefowl ( Gallus gallus ) with vasectomized (sterile) males and artificially inseminated them. Male junglefowl with larger combs are more attractive to females. Females laid more eggs when housed with a large-combed, as opposed to a small-combed, vasectomized mate. Neither egg volume nor offspring body condition was associated with comb size of the mother's vasectomized mate. Paternal genetics appeared important. Body condition and comb size were greater for the sons of large-combed sperm donor males. This is consistent with the hypothesis that genetic benefits to offspring maintain female preference for the most ornate males. It is possible that greater body condition and comb size in sons of large-combed sires was not caused by genetic differences, but instead was due to compounds in the ejaculate of large-combed sperm donors inducing greater reproductive investment from females. However, females artificially inseminated by large-combed males did not produce more or larger eggs than females artificially inseminated by small-combed males, and thus there is no other evidence consistent with ejaculate-induced differential investment. Furthermore, only in older chicks was body condition significantly related to sire comb size, suggesting genetic rather than differential investment mechanisms.     

Do female Drosophila melanogaster adaptively bias offspring sex ratios in relation to the age of their mate?

Modification of offspring sex ratios in response to parental quality is predicted when the long-term fitness returns of sons and daughters differ. One factor that may influence a mother's sex allocation decision is the quality (or attractiveness) of her mate. We investigated whether the sex ratios of offspring produced by female Drosophila melanogaster are biased with respect to the age of the males to which they are mated, and whether there is an adaptive basis for this phenomenon. We found that females mated to old males (13 d post-eclosion) initially produced a greater proportion of daughters than did females mated to young males (1 d post-eclosion). This pattern does not appear to be due to a systematic difference in the numbers or mortality of the X- and Y-bearing sperm originating from old and young fathers, as the overall sex ratios of all offspring produced from a single copulation did not differ between broods fathered by the two types of males. The sons of older males fared worse in competitive mating assays than did the sons of younger males, while daughters of old and young males were of comparable fitness. These results suggest that there is an adaptive basis for the observed sex ratio modification.     

Offspring sex ratio produced by female guppies in the wild correlates with sexual ornaments of their sons

The attractiveness hypothesis predicts that females produce broods with male-biased sex ratios when they mate with attractive males. This hypothesis presumes that sons in broods with male-biased sex ratios sired by attractive males have high reproductive success, whereas the reproductive success of daughters is relatively constant, regardless of the attractiveness of their sires. However, there is little direct evidence for this assumption. We have examined the relationships between offspring sex ratios and (1) sexual ornamentation of sons and (2) body size of daughters in broods from wild female guppies Poecilia reticulata. Wild pregnant females were collected and allowed to give birth in the laboratory. Body size and sexual ornamentation of offspring were measured at maturity. Our analysis revealed a significant positive correlation between offspring sex ratios (the proportion of sons per brood) and the total length as well as the area of orange spots of sons, two attributes that influence female mating preferences in guppies. The sex ratio was not associated with the body size of daughters. These results suggest that by performing adaptive sex allocation according to the expected reproductive success of sons and daughters, female guppies can enhance the overall fitness of their offspring.     

Cross‐fostering eggs reveals that female collared flycatchers adjust clutch sex ratios according to parental ability to invest in offspring

Across animal taxa, reproductive success is generally more variable and more strongly dependent upon body condition for males than for females; in such cases, parents able to produce offspring in above‐average condition are predicted to produce sons, whereas parents unable to produce offspring in good condition should produce daughters. We tested this hypothesis in the collared flycatcher (Ficedula albicollis) by cross‐fostering eggs among nests and using the condition of foster young that parents raised to fledging as a functional measure of their ability to produce fit offspring. As predicted, females raising heavier‐than‐average foster fledglings with their social mate initially produced male‐biased primary sex ratios, whereas those raising lighter‐than‐average foster fledglings produced female‐biased primary sex ratios. Females also produced male‐biased clutches when mated to males with large secondary sexual characters (wing patches), and tended to produce male‐biased clutches earlier within breeding seasons relative to females breeding later. However, females did not adjust the sex of individuals within their clutches; sex was distributed randomly with respect to egg size, laying order and paternity. Future research investigating the proximate mechanisms linking ecological contexts and the quality of offspring parents are able to produce with primary sex‐ratio variation could provide fundamental insight into the evolution of context‐dependent sex‐ratio adjustment.     

Female mate choice across mating stages and between sequential mates in flour beetles

Few studies have examined how female premating choice correlates with the outcome of copulatory and post-copulatory processes. It has been shown that polyandrous Tribolium castaneum females discriminate among males before mating based on olfactory cues, and also exert cryptic choice during mating through several mechanisms. This study tested whether a male's relative attractiveness predicted his insemination success during copulation. Bioassays with male olfactory cues were used to rank two males as more and less attractive to females; each female was then mated to either her more attractive male followed by less attractive male, or vice versa. Dissections immediately after second copulations revealed a significantly higher percent of successful inseminations for females that remated with more attractive males compared with those that remated with less attractive males. These results indicate that cryptic female choice during copulation reinforces precopulatory female choice in T. castaneum, and suggest that females could use cryptic choice to trade up to more attractive males, possibly gaining better phenotypic or genetic quality of sires.     

Facultative adjustment of the offspring sex ratio and male attractiveness: a systematic review and meta‐analysis

Females can benefit from mate choice for male traits (e.g. sexual ornaments or body condition) that reliably signal the effect that mating will have on mean offspring fitness. These male‐derived benefits can be due to material and/or genetic effects. The latter include an increase in the attractiveness, hence likely mating success, of sons. Females can potentially enhance any sex‐biased benefits of mating with certain males by adjusting the offspring sex ratio depending on their mate's phenotype. One hypothesis is that females should produce mainly sons when mating with more attractive or higher quality males. Here we perform a meta‐analysis of the empirical literature that has accumulated to test this hypothesis. The mean effect size was small (r = 0.064–0.095; i.e. explaining <1% of variation in offspring sex ratios) but statistically significant in the predicted direction. It was, however, not robust to correction for an apparent publication bias towards significantly positive results. We also examined the strength of the relationship using different indices of male attractiveness/quality that have been invoked by researchers (ornaments, behavioural displays, female preference scores, body condition, male age, body size, and whether a male is a within‐pair or extra‐pair mate). Only ornamentation and body size significantly predicted the proportion of sons produced. We obtained similar results regardless of whether we ran a standard random‐effects meta‐analysis, or a multi‐level, Bayesian model that included a correction for phylogenetic non‐independence. A moderate proportion of the variance in effect sizes (51.6–56.2%) was due to variation that was not attributable to sampling error (i.e. sample size). Much of this non‐sampling error variance was not attributable to phylogenetic effects or high repeatability of effect sizes among species. It was approximately equally attributable to differences (occurring for unknown reasons) in effect sizes among and within studies (25.3, 22.9% of the total variance). There were no significant effects of year of publication or two aspects of study design (experimental/observational or field/laboratory) on reported effect sizes. We discuss various practical reasons and theoretical arguments as to why small effect sizes should be expected, and why there might be relatively high variation among studies. Currently, there are no species where replicated, experimental studies show that mothers adjust the offspring sex ratio in response to a generally preferred male phenotype. Ultimately, we need more experimental studies that test directly whether females produce more sons when mated to relatively more attractive males, and that provide the requisite evidence that their sons have higher mean fitness than their daughters.     

The effect of interspecific mating on sex ratios in the twospotted spider mite and the Banks grass mite (Acarina: Tetranychidae)

Mixed populations of the twospotted spider mite (TSM),Tetranychus urticae (Koch), and the Banks grass mite (BGM),Oligonychus pratensis (Banks), occur on corn and sorghum plants in late summer in the Great Plains. Interspecific matings between these arrhenotokous species occur readily in the laboratory but yield no female offspring. The effect of interspecific mating on female: male sex ratios was measured by examining the F₁ progeny of females that mated with both heterospecific and conspecific males in no-choice situations. TSM females that mated first with BGM males and then with TSM males produced a smaller percentage of female offspring than TSM females that mated only with TSM males (43.1±5.8 and 78.9±2.8% females, respectively). Similarly, BGM females mated with heterospecific males and then with conspecific males produced fewer female offspring than females mated only with BGM males (55.7±5.2 and 77.5±2.5%, respectively). Lower female: male sex ratios were produced also by BGM females that mated with TSM males after first mating with conspecifics (62.4±3.4%). In mixed populations containing males of both species, females also produced lower female: male sex ratios, but these ratios were not as low as expected based on mating propensities and progeny sex ratios observed in no-choice tests. These data suggest that interspecific mating may substantially reduce female fitness in both mite species by reducing the output of female offspring, but in mixed populations this effect is mitigated by unidentified behavioral mechanisms.     

Multiple mating and sequential mate choice in guppies: females trade up

The trade-up hypothesis outlines a behavioural strategy that females could use to maximize the genetic benefits to their offspring. The hypothesis proposes that females should be more willing to accept a mate when the new male encountered is a superior genetic source to previous mates. We provide a direct test of the trade-up hypothesis using guppies (Poecilia reticulata), and evaluate both behavioural and paternity data. Virgin female guppies were presented sequentially with two males of varying attractiveness, and their responsiveness to each male was quantified. Male attractiveness (ornamentation) was scored as the amount of orange coloration on their body. Females were generally less responsive to second-encountered males, yet responsiveness to second males was an increasing function of male ornamentation. These attractive second males also sired a greater proportion of the offspring. There was an overall tendency for last-male advantage in paternity, and this advantage was most exaggerated when the second male was more ornamented than the first. Finally, we found that our estimate of relative sperm number did not account for any significant variation in paternity. Our results suggest that female guppies may use pre-copulatory mechanisms to maximize the genetic quality of their offspring.     

Nestling sex ratio is associated with both male and female attractiveness in rock sparrows

According to theory, in species in which male variance in reproductive success exceeds that of the females, sons are more costly to produce; females mated with high quality males or those in better condition should produce more sons. In monogamous species, however, the variance in the reproductive success of the two sexes is often similar and mate choice is often mutual, making predictions regarding sex allocation more difficult. In the rock sparrow Petronia petronia, both males and females have a sexually selected yellow patch on the breast, whose size correlates with individual body condition. We investigated whether the brood sex ratio co‐varies with the size of the yellow patch of the father and the mother in a sample of 173 broods (818 chicks) over 8 breeding seasons. While the size of the yellow patch of the mother and the father did not predict per se a deviation from the expected 1:1 sex ratio, brood sex ratios were predicted by the interaction of male and female yellow patch size. This result is surprising, as the ornament is sexually selected by both males and females as an indicator of quality in both sexes and should therefore be inherited by all offspring irrespective of their sex. It indirectly suggests that other sex‐specific traits associated with patch size (e.g. polygyny in males and fecundity in females) may explain the sex allocation bias observed in rock sparrows. Thus, female individual quality alone, as expressed through the size of the yellow patch, was not associated with the biases in sex ratios reported in this study. Our results rather suggest that sex allocation occurs in response to male attractiveness in interaction with female attractiveness. In other words, females tend to preferentially allocate towards the sex of the parent with more developed ornament within the pair.     

Acute corticosterone treatment prior to ovulation biases offspring sex ratios towards males in zebra finches Taeniopygia guttata

Researchers have documented significant skews in the primary sex ratios of avian offspring in relation to a variety of environmental and social cues. Zebra finches Taeniopygia guttata, in particular, adjust offspring sex ratio according to both the quality and quantity of available food, as well as male attractiveness. The mechanisms behind such manipulation of offspring sex remain elusive. Recent studies suggest that females with chronically elevated corticosterone levels (both naturally and artificially) produce significantly female biased offspring sex ratios. We tested the effects of a pharmacological dose of corticosterone or progesterone administered at the time of sex chromosome segregation on the primary sex ratio of zebra finch offspring to determine whether one or both hormones act on offspring sex at this critical period. Females were injected with 20 μg of corticosterone or 20 μg of progesterone five hours prior to the predicted time of ovulation of the 3^rd or 4th ovulating follicle. A third group of females were unmanipulated. The corticosterone treated group produced 72% males while the control group produced 37.5% in the 3rd or 4th ovulation of the sequence. Progesterone injections disrupted ovulation and oviposition in 90% of females. Corticosterone administration did not adversely affect oviposition or ovulation. Females injected with corticosterone had significantly elevated levels of corticosterone 20 min, 1 h and 2.5 h post‐injection and produced significantly more males compared to untreated females. Our results suggest that offspring sex ratios may be influenced at the time of meiotic division by acute exposure to corticosterone and provides evidence for the timing of this effect.     

Egg sex ratio and paternal traits: using within-individual comparisons

Empirical studies of sex ratios in birds have been limited dueto difficulties in determining offspring sex. Since molecularsexing techniques removed this constraint, the last 5 yearshas seen a great increase in studies of clutch sex ratio manipulationby female birds. Typically these studies investigate variationin clutch sex ratios across individuals in relation to environmentalcharacteristics or parental traits, and often they find no relationships. In this study we also found that clutch sex ratiosdid not vary in relation to a number of biological and environmentalfactors for 238 great tit Parus major nests. However, interestingsex ratio biases were revealed when variation in clutch sexratios was analyzed within individual females breeding in successiveyears. There was a significant positive relationship betweenthe change in sex ratio of a female's clutch from one yearto the next and the relative body condition of her partner.Females mating with males of higher body condition in yearx + 1 produced relatively male-biased sex ratios, and the oppositewas true for females mated with lower condition males. Within-individualanalysis also allowed investigations of sex ratio in relationto partner change. There was no change in sex ratios of femalespairing with the same male; however, females pairing with anew male produced clutches significantly more female biased. Comparisons of clutch sex ratios within individuals may be apowerful method for detecting sex ratio variation, and perhapsfemale birds may indeed manipulate egg sex but require personalcontextual experience for such decisions.