Female blue tits adjust parental effort to manipulated male UV attractiveness

The differential allocation hypothesis predicts that parents should adjust their current investment in relation to perceived mate attractiveness if this affects offspring fitness. It should be selectively advantageous to risk more of their future reproductive success by investing heavily in current offspring of high reproductive value but to decrease investment if offspring value is low. If the benefits of mate attractiveness are limited to a particular offspring sex we would instead expect relative investment in male versus female offspring to vary with mate attractiveness, referred to as 'differential sex allocation'. We present strong evidence for differential allocation of parental feeding effort in the wild and show an immediate effect on a component of offspring fitness. By experimentally reducing male UV crown coloration, a trait known to indicate attractiveness and viability in wild-breeding blue tits (Parus caeruleus), we show that females, but not males, reduce parental feeding rates and that this reduces the skeletal growth of offspring. However, differential sex allocation does not occur. We conclude that blue tit females use male UV coloration as an indicator of expected offspring fitness and adjust their investment accordingly.     

Covariance of paternity and sex with laying order explains male bias in extra-pair offspring in a wild bird population

It has been hypothesized that parents increase their fitness by biasing the sex ratio of extra-pair offspring (EPO) towards males. Here, we report a male bias among EPO in a wild population of blue tits (Cyanistes caeruleus). This resulted from a decline in both the proportion of males and EPO over the laying order of eggs in the clutch. However, previous studies suggest that, unlike the decline in EPO with laying order, the relationship between offspring sex ratio and laying order is not consistent between years and populations in this species. Hence, we caution against treating the decline in proportion of males with laying order, and the resulting male bias among EPO, as support for the above hypothesis. Variable patterns of offspring sex and paternity over the laying order may explain inconsistent associations between offspring sex and paternity, between and within species.     

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.     

Female choice for good genes and sex-biased broods in guppies

In a population of guppies Poecilia reticulata females were found to prefer large males and the offspring of these males had a higher survival rate than those sired by smaller males, suggesting that females were selecting their mates on the basis of their good genes. The possibility that females differentially invested in male or female offspring depending on the size or attractiveness of their mate was also investigated, but no relationship was found between a male's attractiveness or body size and the sex ratio of the offspring he sired. However, a strong female-biased sex ratio was detected in the broods and the proportion of males produced decreased with the amount of time that a female was confined with a male. The possible reasons for this are discussed.     

Correlations between ultraviolet coloration, overwinter survival and offspring sex ratio in the blue tit

We studied the correlations between offspring sex ratio, UV coloration and overwinter survival in a population of blue tits, breeding in Gotland, Sweden, over three consecutive breeding seasons. In 2 of 3 years, we found that females paired to males with relatively brighter UV-coloration produced a greater proportion of sons in their broods, and that this effect was significant with all 3 years combined, despite a significant year by male UV interaction. In addition, we found other correlates of sex ratio (breeding time, female age and clutch size) in some, but not all years, and some of these showed significantly different relationships with sex ratio between years. In both years for which data were available, there were indications that males with relatively brighter UV coloration, and that paired with females that produced male-biased clutches, were more likely to survive to the next year. In addition, we also found that in both males and females, individuals produced similar sex ratios in consecutive years. Because correlations with the sex ratio may be expected to be weak, variation in results between years within the same population may be explained by low statistical power or genuine biological differences. Our results suggest that conclusions about sex ratio variation in birds should be based on multiple years. The correlations that we found in some years of this study are consistent with models of adaptive sex ratio adjustment in response to mate quality. However, careful experimental work is required to provide tests of the assumptions of these models, and should be a priority for future work.     

Manipulation of male attractiveness induces rapid changes in avian maternal yolk androgen deposition

Avian eggs contain maternal androgens that may adjust offspringdevelopment to environmental conditions. We review evidenceand functional explanations for the relationship between androgenconcentrations in avian eggs and male attractiveness. Experimentalstudies in captive birds show generally positive relationships,but results from correlational and experimental field studiesare less consistent, perhaps because they lack a within-femaledesign to control for confounding between-female variation.We analyzed the effect of male attractiveness on yolk levelsof maternal androgens in a wild bird, using a correlationaland experimental approach with a within-female design. We manipulatedthe sexually selected UV coloration of the crown feathers ofmale blue tits (Cyanistes caeruleus) after their female hadlaid the second egg and measured the subsequent effect on androgenconcentrations (testosterone and androstenedione) in the fifth,seventh, and ninth eggs relative to that in the second egg.Levels of testosterone, but not androstenedione, in eggs 5 and7 were higher for control (attractive) than for UV-reduced (unattractive)males. This effect disappeared in the ninth egg, coincidingwith the recovery of UV coloration after manipulation. Thissuggests that females are capable of rapid adjustments of testosteronedeposition in response to changes in their mate's ornamentalplumage. However, androgen concentrations in the second eggand pretreatment male crown coloration were not correlated.Possibly, the combination of relatively small variation in UVcoloration before treatment and the influence of unknown confoundingvariables in the correlative approach resulted in insufficientstatistical power to detect such a correlation.     

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.     

Sex allocation in response to paternal attractiveness in the zebra finch

Females mated to attractive males are predicted to produce male-biasedbroods. Previous studies on zebra finches, Taeniopygia guttata,in which colored leg rings were used to alter male attractiveness,support this hypothesis. However, because molecular sexing techniqueswere not available, it was not known when during developmentthis bias arose. Also, because both attractive (red-ringed)and unattractive (green-ringed) males were within the same aviary,assortative mating between treatments may have confounded theresults. Using two different experimental designs, we testedwhether the sex ratio of zebra finch eggs and chicks differedin response to paternal ring color whilst controlling for assortativemating between treatments. In the aviary experiment, birds couldinteract socially, but all males in an aviary had the same legring color. In the cage experiment, each female was randomlyassigned a red- or green-ringed mate, thus also eliminatingassortative mating within treatments. Offspring were sexed basedon plumage or using a molecular method. The sex ratio at layingdid not differ between treatments in either the aviary (n =313 eggs) or cage (n = 151 eggs) experiments, suggesting thatfemale zebra finches do not manipulate the primary sex ratioin response to their mate's ring color. However, in the cageexperiment we found greater male embryonic mortality in theattractive group, which resulted in a female-biased sex ratioat sexual maturity, that is, in the opposite direction to thatfound in previous studies. Possible explanations for the disparitybetween our results and those of previous studies are considered.     

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.     

Local resource competition and sex ratio in the ant Cataglyphis cursor

The local resource competition (LRC) hypothesis predicts thatwherever philopatric offspring compete for resources with theirmothers, offspring sex ratios should be biased in favor of thedispersing sex. In ants, LRC is typically found in polygynous(multiple queen) species where foundation of new nests occursby budding, which results in a strong population structure anda male-biased population-wide sex ratio. However, under polygyny,the effect of LRC on sex allocation is often blurred by theeffect of lowered relatedness asymmetries among colony members.Moreover, environmental factors, such as the availability ofresources, have also been shown to deeply influence sex ratioin ants. We investigated sex allocation in the monogynous (singlequeen) ant Cataglyphis cursor, a species where colonies reproduceby budding and both male and female sexuals are produced throughparthenogenesis, so that between-colony variations in relatednessasymmetries should be reduced. Our results show that sex allocationin C. cursor is highly male biased both at the colony and populationlevels. Genetic analyses indicate a significant isolation-by-distancein the study population, consistent with limited dispersal offemales. As expected from asexual reproduction, only weak variationsin relatedness asymmetry of workers toward sexual offspringoccur across colonies, and they are not associated with colonysex ratio. Inconsistent with the predictions of the resourceavailability hypothesis, the male bias significantly increaseswith colony size, and investment in males, but not in females,is positively correlated with total investment in sexuals. Overall,our results are consistent with the predictions of the LRC hypothesisto account for sex ratio variation in this species.     

Experimental manipulation of maternal effort produces differential effects in sons and daughters: implications for adaptive sex ratios in the blue-footed booby

Sex allocation theory predicts that mothers in good conditionshould bias their brood sex ratio in response to the differentialbenefits obtained from increased maternal expenditure in sonsand daughters. Although there is well-documented variationof offspring sex ratios in several bird species according tomaternal condition, the assumption that maternal condition has different fitness consequences for male and for female offspringremains unclear. The blue-footed booby (Sula nebouxii) is asexually size-dimorphic seabird, with females approximately31% heavier than males. It has been reported that the sex ratiois male biased in years with poor feeding conditions, whichsuggests that either females adjust their sex ratio in accordancewith their condition or that they suffer differential brood mortality before their sex can be determined. In this studyI tested whether the condition of mothers affected their daughters'fitness more than their sons' fitness. I manipulated maternalinvestment by trimming the flight feathers and thereby handicappingfemales during the chick-rearing period. Adult females in thehandicapped group had a poorer physical condition at end ofchick growth, as measured by mass and by the residuals of masson wing length compared to control birds. Female chicks wereaffected by the handicapping experiment, showing a lower massand shorter wing length (reduced approximately 8% in both measures)than controls. However, this effect was not found in male chicks.Hatching sex ratios were also related to female body conditionat hatching. The brood sex ratio of females in poor conditionwas male biased but was female biased for females in good condition.Overall, these results suggest that the variation in the sexratio in blue-footed boobies is an adaptive response to thedisadvantage daughters face from being reared under poor conditions.     

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.     

Parental care and UV coloration in blue tits: opposite correlations in males and females between provisioning rate and mate’s coloration

Parental investment and sexually‐selected signals can be intimately related, either because the signals indicate the amount of investment that an individual is prepared to make, and hence its value as a mate (the ‘good parent process’), or because individuals are selected to vary their own investment in relation to their mate’s signals (‘differential allocation’ or ‘reproductive compensation’). Correlations between parental investment and the sexually selected signals of both an individual and its mate are therefore of central interest in sexual selection. Blue tits Cyanistes caeruleus are an ideal study species to investigate such correlations because they provide substantial amounts of biparental care and possess sexually‐selected structural UV coloration that seems to signal attractiveness in both sexes. We investigated whether feeding rates of male and female blue tits were correlated with either their own or their mate’s UV coloration, and whether any such correlation was affected by the sex ratio of the brood. We also investigated whether any such correlations were reflected in offspring phenotype. Feeding rates were not correlated with either sex of parent’s own UV coloration. However, they were correlated with the mate’s UV coloration, but in opposite directions in males and females: females had higher feeding rates when mated to bright UV males, implying differential allocation, while males had lower feeding rates when mated to bright UV females, implying reproductive compensation. These relationships were unaffected by the sex ratio of the brood. In addition, fledgling tarsus length, but not mass, was related to male UV coloration, and to female UV coloration in interaction with male age. These results suggest that both male and female attractiveness influence parental investment of the mate, and that this in turn affects offspring phenotype. We found no evidence for differential sex allocation.     

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.     

Female Control of Offspring Sex Ratios Based on Male Attractiveness in the Guppy

The sex allocation hypothesis predicts that females manipulate the offspring sex ratios according to mate attractiveness. Although there is increasing evidence to support this prediction, it is possible that paternal effects may often obscure the relationship between female control of offspring sex ratios and male attractiveness. In the present study, we examined whether females played a primary role in the manipulation their offspring sex ratios based on male attractiveness, in the guppy Poecilia reticulata, a live‐bearing fish. We excluded the paternal effects by controlling the relative sexual attractiveness of the male by presenting them to the females along with a more attractive or less attractive stimulus male. The test male was perceived to be relatively more attractive by females when it was presented along with a less attractive stimulus male, or vice versa. Subsequently, test male was mated in two different roles (relatively more and less attractive) with two females. If females were responsible for offspring sex ratio manipulation, the sex ratio of the brood would be altered on the basis of the relative attractiveness of the test male. On the other hand, if males play a primary role in offspring sex ratio manipulation, the sex ratios would not differ with the relative attractiveness of the test male. We found that females gave birth to more male‐biased broods when they mated with test males in the attractive role than when they mated with males in the less attractive role. This finding suggests that females are responsible for the manipulation of offspring sex ratios based on the attractiveness of their mates.     

Sex ratio schedules in a dynamic game: the effect of competitive asymmetry by male emergence order

Studies of sex allocation have provided some of the most successfultests of theory in behavioral and evolutionary ecology. Forinstance, local mate competition (LMC) theory has explainedvariation in sex allocation across numerous species. However,some patterns of sex ratio variation remain unexplained by existingtheory. Most existing models have ignored variation in malecompetitive ability and assumed all males have equal opportunitiesto mate within a patch. However, in some species experiencingLMC, males often fight fiercely for mates, such that male matingsuccess varies with male fighting ability. Here, we examinethe effect of competitive ability on optimal sex allocationschedules using a dynamic programming approach. This model assumesan asymmetric competitive ability derived from different mortalitiesaccording to the timing of male emergence. If the mortalityof newly emerging males is larger than that of already emergedmales, our model predicts a more female-biased sex ratio thanexpected under traditional LMC models. In addition, femalesare predicted to produce new males constantly at a low rateover the offspring emergence period. We show that our modelsuccessfully predicts the sex ratios produced by females ofthe parasitoid wasp Melittobia, a genus renowned for its vigorouslyfighting males and lower than expected sex ratios.     

Lethal combat and sex ratio evolution in a parasitoid wasp

Sex allocation theory provides excellent opportunities for testinghow behavior and life histories are adjusted in response toenvironmental variation. One of the most successful areas fromthis respect is Hamilton's local mate competition theory. Aspredicted by theory, a large number of animal species have beenshown to adjust their offspring sex ratios (proportion male)conditionally, laying less female-biased sex ratios as the numberof females that lay eggs on a patch increases. However, recentstudies have shown that this predicted pattern is not followedby 2 parasitoid species in the genus Melittobia, which alwaysproduce extremely female-biased sex ratios. A possible explanationfor this is that males fight fatally and that males producedby the first female to lay eggs on a patch have a competitiveadvantage over later emerging males. This scenario would negatethe advantage of later females producing a less female-biasedsex ratio. Here we examine fatal fighting and sex ratio evolutionin another species, Melittobia acasta. We show that femalesof this species also fail to adjust their offspring sex ratioin response to the number of females laying eggs on a patch.We then show that although earlier emerging males do have anadvantage in winning fights, this advantage 1) can be reducedby an interaction with body size, with larger males more likelyto win fights and 2) only holds for a brief period around thetime at which the younger males emerge from their pupae. Thissuggests that lethal male combat cannot fully explain the lackof sex ratio shift observed in Melittobia species. We discussalternative explanations.     

Age differences in blue tit Parus caeruleus plumage colour: within‐individual changes or colour‐biased survival?

In many species of passerine birds yearlings display a less elaborate version of the adult secondary sexual traits, but the causes of such differences in ornamentation are not always well understood. We studied age-related changes in blue tit Parus caeruleus UV/blue structural crown coloration, a sexually selected trait. In our Austrian study population, older blue tits, irrespective of sex, displayed on average a more ultraviolet (lower hue, higher UV chroma), more chromatic and brighter crown coloration than yearlings. This age dichromatism was caused by within-individual changes in the expression of crown coloration between years since males and females became more UV, more chromatic and brighter as they aged. Colour biased survival did not contribute to the observed pattern of age dichromatism since crown coloration was largely unrelated to overwinter survival. Between-year repeatability of crown colour was significant for most colour variables but low in general, and lower for females than for males. In the blue tit, yearling males might benefit from being less ornamented by avoiding adult aggression but at the expense of sexual attractiveness. Adaptive explanations of blue tit age dichromatism should however take into account that age effects were of similar magnitude in males and females. This suggests that both male and female yearlings could benefit from being less ornamented and hence that sexual selection might be acting on both sexes simultaneously in this species.     

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.     

Offspring sex ratio allocation in the parasitic jaeger: selection for pale females and melanic males?

The maintenance of plumage color polymorphism in the parasiticjaeger (Stercorarius parasiticus) is still not well understood.Earlier studies indicated that selection may favor pale femalesand melanic males. If so, females would maximize their fitness,producing pale female and melanic male offspring. We thereforepredicted that females might bias their offspring sex ratiotoward daughters in pale pairs and toward sons in melanic pairs.Females might also choose to mate assortatively in relationto plumage color, thereby maximizing the probability of producingeither pale or melanic offspring. Because females are largerthan males, differential rearing costs may affect the offspringsex ratio independent of parental plumage color. We examinedoffspring sex ratio allocation, breeding variables indicativeof parental quality, and mating pattern in relation to plumagecolor in a colony of parasitic jaegers in northern Norway. Jaegerstended to mate assortatively in relation to plumage color. Thereproductive performance declined with season, and matched pairsappeared to be of lower quality than mixed pairs. The proportionof male offspring increased with hatching date in matched paleand mixed pairs, whereas the situation was reversed in matchedmelanic pairs. Matched pale pairs produced an overall surplusof favorable pale but costly daughters despite their lower quality,while melanic pairs produced a surplus of favorable melanicsons. However, differential offspring rearing costs and parentalrearing capacity may have additionally affected the realizedoffspring sex ratio. Mixed pairs producing an overall surplusof pale and melanic daughters allocated their resources accordingto differential rearing costs and parental quality only. Wesuggest that both strategies of sex ratio allocation togetherwith differences in reproductive success in matched versus mixedpairs may have a balancing effect on the mating pattern betweenplumage morphs and may contribute to the maintenance of thecolor polymorphism in this species.