Sons do not take advantage of a head start: parity in herring gull offspring sex ratios despite greater initial investment in males

Skewed adult sex ratios sometimes occur in populations of free‐living animals yet the proximate mechanisms, timing of sex‐biases, and the selective agents contributing to skew remain a source of debate with contradictory evidence from different systems. We investigated potential mechanisms contributing to sex biases in a population of herring gulls with an apparent female skew in the adult population. Theory predicts that skewed adult sex ratios will adaptively lead to skewed offspring sex ratios to restore balance in the effective breeding population. Parents may also adaptively bias offspring sex ratios to increase their own fitness in response to environmental factors. Therefore, we expected to detect skewed sex ratios either at hatching or at fledging as parents invest differentially in offspring of different sexes. We sampled complete clutches (n = 336 chicks) at hatching to quantify potential skews in sex ratios by position in the hatch order, time of season, year, and nesting context (nest density), finding no departure from equal sex ratios at hatching related to any of these factors. Further, we sampled 258 chicks at near‐fledging to investigate potential sex biases in survival at the chick stage. Again, no biases in sex ratios were recorded. Male offspring were favored in this population via greater maternal investment in eggs carrying male embryos and greater parental provisioning of male offspring which reached greater sizes by fledging. Despite the advantages realized by male offspring, females were equally as likely to fledge as males. Thus, biased adult sex ratios apparently arise in the post‐fledging and pre‐recruitment stage in our population.     

Unusual pattern of sex‐specific mortality in relation to initial brood sex composition in the black‐billed magpie Pica pica

In sexually size‐dimorphic species, brood sex composition may exert differential effects on sex‐specific mortality. We investigated the sex‐specific mortality and body condition in relation to brood sex composition in nestlings of the black‐billed magpie Pica pica. Neither significantly sex‐biased production at hatching nor overall sex‐biased mortality during the nestling period was found. Sex‐specific mortality as a function of brood sex composition, however, differed between female and male nestlings. We found higher mortality for females in male‐biased broods and higher mortality for males in female‐biased broods, a phenomenon that we call ‘rarer‐sex disadvantage’. As a result, fledging sex ratios became more biased in the direction of bias at hatching, a phenomenon that cannot be readily explained by previous hypotheses for sex‐specific mortality. Two temporal variables, fledging date and laying date, were also correlated with sex‐specific mortality: female nestlings in earlier broods experienced higher mortality than male nestlings whereas male nestlings in later broods experienced higher mortality. We suggest that this unusual pattern of mortality may be explained by adaptive adjustments of brood sex composition by parents, either through the effects of a slight sex difference in offspring dispersal patterns on parental fitness, or owing to sex differences as regards the benefits of early fledging.     

Sexual dimorphism,survival, and parental investment in relation to offspring sex in a precocial bird

Differential growth rate between males and females, owing to a sexual size dimorphism, has been proposed as a mechanism driving sex‐biased survival. How parents respond to this selection pressure through sex ratio manipulation and sex‐biased parental investment can have a dramatic influence on fitness. We determined how differential growth rates during early life resulting from sexual size dimorphism affected survival of young and how parents may respond in a precocial bird, the black brant Branta bernicla nigricans. We hypothesized that more rapidly growing male goslings would suffer greater mortality than females during brood rearing and that parents would respond to this by manipulating their primary sex ratio and parental investment. Male brant goslings suffered a 19.5% reduction in survival relative to female goslings and, based on simulation, we determined that a female biased population sex ratio at fledging was never overcome even though previous work demonstrated a slight male‐biased post‐fledging survival rate. Contrary to the Fisherian sex ratio adjustment hypothesis we found that individual adult female brant did not manipulate their primary sex ratio (50.39% male, n = 645), in response to the sex‐biased population level sex ratio. However, female condition at the start of the parental care period was a good predictor of their primary sex ratio. Finally, we examined how females changed their behavior in response to primary sex ratio of their broods. We hypothesized that parents would take male biased broods to areas with increased growth rates. Parents with male biased primary sex ratios took broods to areas with higher growth rates. These factors together suggest that sex‐biased growth rates during early life can dramatically affect population dynamics through sex‐biased survival and recruitment which in turn affects decisions parents make about sex allocation and sex‐biased parental investment in offspring to maximize fitness.     

Brood sex ratio in the Kentish plover

How and why do the mating opportunities of males and femalesdiffer in natural population of animals? Previously we showedthat females have higher mating opportunities than males inthe Kentish plover Charadrius alexandrinus. Both parents incubatethe eggs, and males provide more brood care than females; thusit is not obvious why the females find new mates sooner thanthe males. In this study we investigated whether the sex-biasedmating opportunities stem from biased offspring sex ratios.We determined the sex of newly hatched, precocial chicks usingCHD gene markers. Among fully sexed broods, 0.461 ± 0.024(SE) of chicks (454 chicks in 158 broods) were male, and thissex ratio was not significantly different from unity. The proportionof males at hatching decreased significantly over the breedingseason, which occurred consistently in all 3 years of the study.Large chicks were more likely to be males than females. Neitherparental age nor body size of male and female parents was relatedto brood sex ratio. We also sexed a number of chicks that werecaught after they left their nest (range of estimated ages 0–17days) and found that the proportion of males increased withbrood age. This relationship remained highly significant whencontrolling statistically for hatching date. As brood size decreaseddue to mortality after the chicks left their nest, these resultssuggest that the mortality of daughters was higher than thatof the sons shortly after hatching. Taken together, our resultsshow that the female-biased mating opportunities in the Kentishplover are not due to biased brood sex ratio at hatching but,at least in part, are due to female-biased chick mortality soonafter hatching.     

Chick mortality leads to male‐biased sex ratios in endangered Great Lakes Piping Plovers

Few investigators have studied the offspring sex ratios of monomorphic shorebirds because visually determining the sex of juveniles is not possible. We investigated the ontogeny of an observed male‐biased adult sex ratio in the federally endangered Great Lakes population of Piping Plovers (Charadrius melodus). We determined sex ratios at hatching, banding ( = 9.0 d old), and fledging (23 d old) to determine if the bias arises during the pre‐fledging period and, if so, at what stage. For three consecutive years (2012–2014), we used a molecular technique to determine the sex of 307 chicks and followed individuals to a stage where survival to fledging could be inferred. Within fully‐sexed broods at hatching, the average proportions of male chicks (2012–2014) were 0.47, 0.58, and 0.54, respectively. At banding, the sex ratio remained unbiased in 2012 (0.51), but was male‐biased in 2013 (0.59) and 2014 (0.57). Overall, the sex ratio did not differ significantly from parity at fledging in 2012, but did differ during 2013 (P = 0.01) and 2014 (P = 0.03). Using logistic regression models fit using Bayesian inference, we found strong support for a sex effect on chick survival to fledging age, with higher male than female survival (μ_male = 0.83 [95% credible interval: 0.75–0.90]; μ_female = 0.71 [0.61–0.80]). These results suggest that the male‐biased adult sex ratio in Piping Plovers arises, in part, due to differential survival during the pre‐fledging period. This difference did not result from female chicks hatching later in the season or weighing less at banding than male chicks, factors that could potentially affect the likelihood of survival. Future investigations into possible behavioral‐ or weather‐related influences on sex‐specific survival are needed. Our results have important implications for (1) identifying management efforts needed to increase recruitment given female‐biased chick mortality, and (2) conducting population viability analyses, which frequently assume an unbiased fledgling sex ratio.     

Male‐skewed adult sex ratio,survival, mating opportunity and annual productivity in the Snowy Plover Charadrius alexandrinus

Sex differences in adult mortality may be responsible for male‐skewed adult sex ratios and male‐skewed parental care in some birds. Because a surplus of breeding males has been reported in serially polyandrous populations of Snowy Plover Charadrius alexandrinus, we examined sex ratio, early‐season nesting opportunities, adult survival and annual reproductive success of a Snowy Plover population at Monterey Bay, California. We tested the hypotheses that male adult survival was greater than female survival and that a sex difference in adult survival led to a skewed adult sex ratio, different mating opportunities and different annual productivity between the sexes. Virtually all females left chicks from their first broods to the care of the male and re‐nested with a new mate. As a result, females had time to parent three successful nesting attempts during the lengthy breeding season, whereas males had time for only two successful attempts. Among years, the median population of nesting Plovers was 96 males and 84 females (median difference = 9), resulting in one extra male per eight pairs. The number of potential breeders without mates during the early nesting period each year was higher in males than in females. Adult male survival (0.734 ± 0.028 se) was higher than female survival (0.693 ± 0.030 se) in top‐ranked models. Annually, females parented more successful clutches and fledged more chicks than their first mates of the season. Our results suggest that in C. alexandrinus a sex difference in adult survival results in a male‐skewed sex ratio, which creates more nesting opportunities and greater annual productivity for females than for males.     

Parental investment, sexual selection and sex ratios

Conventional sex roles imply caring females and competitive males. The evolution of sex role divergence is widely attributed to anisogamy initiating a self‐reinforcing process. The initial asymmetry in pre‐mating parental investment (eggs vs. sperm) is assumed to promote even greater divergence in post‐mating parental investment (parental care). But do we really understand the process? Trivers [Sexual Selection and the Descent of Man 1871–1971 (1972), Aldine Press, Chicago] introduced two arguments with a female and male perspective on whether to care for offspring that try to link pre‐mating and post‐mating investment. Here we review their merits and subsequent theoretical developments. The first argument is that females are more committed than males to providing care because they stand to lose a greater initial investment. This, however, commits the ‘Concorde Fallacy’ as optimal decisions should depend on future pay‐offs not past costs. Although the argument can be rephrased in terms of residual reproductive value when past investment affects future pay‐offs, it remains weak. The factors likely to change future pay‐offs seem to work against females providing more care than males. The second argument takes the reasonable premise that anisogamy produces a male‐biased operational sex ratio (OSR) leading to males competing for mates. Male care is then predicted to be less likely to evolve as it consumes resources that could otherwise be used to increase competitiveness. However, given each offspring has precisely two genetic parents (the Fisher condition), a biased OSR generates frequency‐dependent selection, analogous to Fisherian sex ratio selection, that favours increased parental investment by whichever sex faces more intense competition. Sex role divergence is therefore still an evolutionary conundrum. Here we review some possible solutions. Factors that promote conventional sex roles are sexual selection on males (but non‐random variance in male mating success must be high to override the Fisher condition), loss of paternity because of female multiple mating or group spawning and patterns of mortality that generate female‐biased adult sex ratios (ASR). We present an integrative model that shows how these factors interact to generate sex roles. We emphasize the need to distinguish between the ASR and the operational sex ratio (OSR). If mortality is higher when caring than competing this diminishes the likelihood of sex role divergence because this strongly limits the mating success of the earlier deserting sex. We illustrate this in a model where a change in relative mortality rates while caring and competing generates a shift from a mammalian type breeding system (female‐only care, male‐biased OSR and female‐biased ASR) to an avian type system (biparental care and a male‐biased OSR and ASR).     

Sex ratio and maternal investment in the multivoltine large carpenter bee Xylocopa sulcatipes (Apoideh: Anthophoridae)

Abstract. 1. Females of the multivoltine carpenter bee Xylocopa sulcutipes (Maa) (Hymenoptera: Anthophoridae) usually excavate a straight tunnel in dead twigs and mass provision a linear array of up to ten brood cells with pollen and nectar. An egg is deposited upon each food mass within one cell.
2. Female offspring generally receive a higher provisioning mass (0.180 ± 0.048 g) than males, a significant difference ( P > 0.001). There are, however, male larvae that receive as much food or more as their sisters or female larvae reared in another nest.
3. There is a close positive association between the size of a mother and the weight of provisions for individual daughters, but not for sons.
4. Female offspring are positioned in the innermost brood cells (Gositions 1, 2 and 3). The sex ratio of the outer cells is either significantly male biased (positions 4–6) or skewed towards males (positions 8 and 9). Positions 7 and 10 are in equilibrium.
5. Solitary females produce a significantly female biased sex ratio ( P < 0.01). Sex ratio in social nests is skewed toward females, but not significantly so ( P < 0.2). There is no significant difference between the sex ratio of solitary and social nests ( P = 0.361). The population sex ratio (pooled sex ratio of all broods produced) is significantly female biased ( P = 0.003).
6. Females kept in the laboratory produced female biased sex ratios whilst unmated females produced all-male broods indicating that insemination and ovarian development are not causally related.
7. The expected sex ratio (ESR) under equal investment, calculated as 1/CR (CR = mean male provision weight/mean female provision weight), is 137.5:117.5 (males:females), and differs significantly from that observed, 104:151 (males:females) ( P < 0.001). The 'Local Resource Enhlancement' hypothesis best explains the female biased sex ratio found in X.sulcatipes and its maintenance in the population.     

Sex differences in parental care: Gametic investment,sexual selection,and social environment

Male and female parents often provide different type and amount of care to their offspring. Three major drivers have been proposed to explain parental sex roles: (1) differential gametic investment by males and females that precipitates into sex difference in care, (2) different intensity of sexual selection acting on males and females, and (3) biased social environment that facilitates the more common sex to provide more care. Here, we provide the most comprehensive assessment of these hypotheses using detailed parental care data from 792 bird species covering 126 families. We found no evidence for the gametic investment hypothesis: neither gamete sizes nor gamete production by males relative to females was related to sex difference in parental care. However, sexual selection correlated with parental sex roles, because the male share in care relative to female decreased with both extra‐pair paternity and frequency of male polygamy. Parental sex roles were also related to social environment, because male parental care increased with male‐biased adult sex ratios (ASRs). Taken together, our results are consistent with recent theories suggesting that gametic investment is not tied to parental sex roles, and highlight the importance of both sexual selection and ASR in influencing parental sex roles.     

Sex ratio in relation to timing of breeding, and laying sequence in a dimorphic seabird

When the cost of rearing sons and daughters differs and the subsequent survival and reproductive success of one sex is more dependent than the other, on the amount of parental investment, adult females tend to produce more chicks of the more dependent sex if the females are in good condition themselves. One method of varying the total investment in each sex is through modifying the sex ratio of offspring produced. This study shows that in broods of European Shags Phalacrocorax aristotelis , the sex ratio varied with laying date. Presumably in this species, the lifetime reproductive success of males is more dependent on the level of parental investment. Early breeders are in better condition, the brood sex ratio of early broods was male biased (0.63), while that of late broods was female biased (0.36). The overall difference in sex ratio found between early and late nests could be attributed to manipulation of sex in the first laid egg. In early broods, 77% of the first hatched chicks were male but only 30% of the first hatched chicks in late broods were male. The sex combination of the first two chicks in a brood significantly affected growth as measured by asymptotic mass.     

Sex of the first hatched chick influences survival of the brood in the herring gull (Larus argentatus)

Differences in the growth rate of male and female offspring can result in different parental rearing costs for sons and daughters. Such differences may also influence the survival chances of male and female offspring when conditions are unfavourable. In birds, hatching asynchrony leads to hierarchical competition for food between siblings. Therefore, the sex of the chick in the first hatched position in the brood may influence breeding success by affecting the extent to which the later hatched chicks can compete for resources. The interaction between brood sex composition and chick performance in the herring gull Larus argentatus was examined under different environmental conditions. When environmental conditions were relatively good, chick survival within broods was better when a female was first to hatch, an effect that was most obvious later in the season. When conditions were poorer however, sex of the first hatched chicks was not related to brood survival. In neither situation did the overall primary sex ratio differ from equality. However in the year of relatively good food availability, the first chick in the brood was more likely to be male early in the season, which was when the disadvantageous effects on brood survival of males being in this position are weakest.     

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.     

The influence of sex and body size on nestling survival and recruitment in the house sparrow

Differences in the survival rates of males and females over the period from hatching to recruitment can have important impacts on individual fitness and population demographics. However, whilst the influence of an individual's sex on nestling growth and survival has been well studied, less is known about sex‐specific survival over the period between fledging and recruitment. Here, we analyse nestling survival and recruitment in an isolated, island population of house sparrows (Passer domesticus), using data collected over a 4‐year period. Nestlings that had a greater mass at 1 day old were more likely to fledge. Recruitment was also positively associated with day 11 mass. The positive influence of nestling mass on survival to fledging also increased as brood size increased. There was no difference in the survival of male and female individuals prior to fledging. In contrast, over the period from fledging to recruitment, females had significantly less mortality than males. Recruitment was also positively associated with 11‐day‐old mass. Neither the nestling sex ratio nor the fledging sex ratio deviated from 0.5, but the sex ratio amongst recruits was female biased. Our study shows that sex can influence juvenile survival, but also shows that its effect varies between different life‐history stages; therefore, these stages should be considered separately if we want to understand at what point sex‐specific differences in juvenile survival occur. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101 , 680–688.     

Offspring desertion and parental care in the Whiskered Tern Chlidonias hybrida

In species with biparental care, a conflict of interest can arise if one mate tries to maximize its own reproductive success at the expense of the other's. One of the mates can desert the brood to accrue a number of benefits to enhance its own fitness, leaving parental care to the remaining parent. This study is the first to describe the desertion pattern in a tern species (Sternidae). We investigated offspring desertion in the Whiskered Tern Chlidonias hybrida, a species with semi‐precocial chicks. Offspring desertion was recorded in 52% of nests prior to fledging (n = 131 nests). Females also deserted during the post‐fledging period. Of the deserters, 97% were females. Desertions started when chicks were 5 days old and no longer required intense brooding. Desertions before fledging did not affect fledging success. Provisioning rates between pair members differed, and females supplied much less food than males. Female provisioning rate affected the chances of nest desertion significantly: daily desertion rates were lower when females supplied more food. After females had deserted, males increased their provisioning rates but compensated for the loss of female care only partly in two‐ and three‐chick broods. Only in small (one‐chick) broods was compensation full. We conclude that male and female Whiskered Terns adopt different reproductive strategies in the population studied here. Females invest much less in parental care than males, providing less food and deserting more frequently. Given the ready availability of food and low predation pressure, benefits appear to accrue to females that desert; selection forces may therefore not be acting against female desertion.     

Sex‐biased breeding dispersal is predicted by social environment in birds

Sex‐biased dispersal is common in vertebrates, although the ecological and evolutionary causes of sex differences in dispersal are debated. Here, we investigate sex differences in both natal and breeding dispersal distances using a large dataset on birds including 86 species from 41 families. Using phylogenetic comparative analyses, we investigate whether sex‐biased natal and breeding dispersal are associated with sexual selection, parental sex roles, adult sex ratio (ASR), or adult mortality. We show that neither the intensity of sexual selection, nor the extent of sex bias in parental care was associated with sex‐biased natal or breeding dispersal. However, breeding dispersal was related to the social environment since male‐biased ASRs were associated with female‐biased breeding dispersal. Male‐biased ASRs were associated with female‐biased breeding dispersal. Sex bias in adult mortality was not consistently related to sex‐biased breeding dispersal. These results may indicate that the rare sex has a stronger tendency to disperse in order to find new mating opportunities. Alternatively, higher mortality of the more dispersive sex could account for biased ASRs, although our results do not give a strong support to this explanation. Whichever is the case, our findings improve our understanding of the causes and consequences of sex‐biased dispersal. Since the direction of causality is not yet known, we call for future studies to identify the causal relationships linking mortality, dispersal, and ASR.     

Brood parasitism causes female‐biased host nestling mortality regardless of parasite species

Inequality in male and female numbers may affect population dynamics and extinction probabilities and so has significant conservation implications. We previously demonstrated that Brown‐headed Cowbird Molothrus ater brood parasitism of Song Sparrows Melospiza melodia results in a 50% reduction in the proportion of female host offspring by day 6 post‐hatch and at fledging, which modelling demonstrated is as significant as nest predation in affecting demography. Many avian brood parasites possess special adaptations to parasitize specific hosts so this sex‐ratio effect could be specific to the interaction between these two species. Alternatively, perturbations associated with brood parasitism per se (e.g. the addition of an extra, larger, unrelated nestling), rather than a Cowbird nestling specifically, may be responsible. We experimentally eliminated the effects of Cowbird‐specific traits by parasitizing nests with a conspecific nestling rather than a Cowbird, while otherwise emulating the circumstances of Cowbird parasitism by adding an extra, larger (2‐day‐older), unrelated Song Sparrow nestling to Song Sparrow nests. Our parasitism treatment led to few host offspring deaths and no evidence of male‐biased sex ratios by day 6 post‐hatch. However, after day 6, female nestling mortality rates increased significantly in experimentally parasitized nests, resulting in a 60% reduction in the proportion of females fledging. Cowbird‐specific traits are thus not necessary to cause female‐biased host nestling mortality and far more general features associated with Cowbird parasitism instead appear responsible. Our results suggest, however, that Cowbird‐specific traits may help accelerate the pace of female host deaths. The conservation implications of our results could be wide reaching. Cowbirds are unrelated to all their hosts, are larger than the great majority, and a Cowbird nestling's presence can mean there is an extra mouth to feed. Thus, sex‐biased mortality in parasitized nests could be occurring across a range of host species.     

Facultative and non‐facultative sex ratio adjustments in a dimorphic bird species

If parental allocation to each offspring sex has the same cost/benefit ratio, Fisher's hypothesis predicts a sex ratio biased towards the cheaper sex. However, in dimorphic birds there is little evidence for this, especially at hatching. We investigated the pre‐fledgling 1) sex ratio, 2) body condition and 3) sex‐differential mortality in a population of the glossy ibis Plegadis falcinellus, in southern Spain between 2001 and 2011. We defined two age groups for the period between hatching and fledging. We also compared pre‐fledgling with the autumn sex ratio. Metabolic rates were estimated by the doubly labeled water (DLW) technique to establish that sons (the bigger sex) were 18% more energy demanding than daughters, and to compute the predicted Fisher's sex ratio (0.465). As population size increased between years, body condition decreased in both sexes, and mortality increased more for daughters than sons prior to fledging. At the same time, the proportion of males among chicks close to fledging increased (average sex ratio: 0.606) while the proportion close to hatching decreased (average sex ratio: 0.434, in line with Fisher's prediction). Furthermore, the proportions of males at fledging and the following autumn were negatively correlated across years. We suggest that, as population density increased and conditions worsened the larger sex had relatively higher survival. These differences in survival produce a shift from a facultative female‐biased sex ratio at hatching into a non‐facultative male‐biased sex ratio of fledglings. Additionally, the excess of males at fledging was counterbalanced by sex‐related dispersal during the autumn. Overall, glossy ibis sex ratio is a product of a combination of facultative and non‐facultative adjustments triggered by environmental conditions, driven by rapid population growth, and mediated by highly interrelated life‐history traits such as body condition, mortality, and dispersal.     

Testing the predictions of sex allocation hypotheses in dimorphic,cooperatively breeding riflemen

Evolutionary theory predicts that parents should invest equally in the two sexes. If one sex is more costly, a production bias is predicted in favour of the other. Two well‐studied causes of differential costs are size dimorphism, in which the larger sex should be more costly, and sex‐biased helping in cooperative breeders, in which the more helpful sex should be less costly because future helping “repays” some of its parents’ investment. We studied a bird species in which both processes should favor production of males. Female riflemen Acanthisitta chloris are larger than males, and we documented greater provisioning effort in more female‐biased broods indicating they are likely costlier to raise. Riflemen are also cooperative breeders, and males provide more help than females. Contrary to expectations, we observed no male bias in brood sex ratios, which did not differ significantly from parity. We tested whether the lack of a population‐wide pattern was a result of facultative sex allocation by individual females, but this hypothesis was not supported either. Our results show an absence of adaptive patterns despite a clear directional hypothesis derived from theory. This appears to be associated with a suboptimal female‐biased investment ratio. We conclude that predictions of adaptive sex allocation may falter because of mechanistic constraint, unrecognized costs and benefits, or weak selection.     

Male‐specific mortality biases secondary sex ratio in Eurasian tree sparrows Passer montanus

Sex allocation theory predicts that parents bias the offspring sex ratio strategically. In avian species, the offspring sex ratio can be biased at multiple growth stages, although the mechanisms are not well known. It is crucial to reveal a cause and timing of biased offspring sex ratio. We investigated (i) offspring sex ratio at multiple growth stages, from laying to fledging; and (ii) the stage at which offspring sex ratio became biased; and (iii) the cause of biased offspring sex ratio in Eurasian tree sparrows Passer montanus. Sex determination of 218 offspring, including hatchlings and unhatched eggs from 41 clutches, suggested that the offspring sex ratio was not biased at the egg‐laying stage but was significantly female‐biased after the laying stage due to higher mortality of male embryos. Half of the unhatched eggs showed no sign of embryo development (37/74, 50.00%), and most undeveloped eggs were male (36/37, 97.30%). Additional experiments using an incubator suggested that the cause of embryo developmental failure was a lack of developmental ability within the egg, rather than a failure of incubation. This study highlights the importance of clarifying offspring sex ratio at multiple stages and suggests that offspring sex ratio is adjusted after fertilization.     

Mortality costs of sexual selection and parental care in natural populations of birds

Abstract Is the cost of reproduction different between males and females? On the one hand, males typically compete intensely for mates, thus sexual selection theory predicts higher cost of reproduction for males in species with intense male‐male competition. On the other hand, care provisioning such as incubating the eggs and raising young may also be costly, thus parental care theory predicts higher mortality for the care‐giving sex, which is often the female. We tested both hypotheses of reproductive costs using phylogenetic comparative analyses of sex‐specific adult mortality rates of 194 bird species across 41 families. First, we show that evolutionary increases in male‐male competition were associated with male‐biased mortalities. This relationship is consistent between two measures of mating competition: social mating system and testis size. Second, as predicted by the parental cost hypothesis, females have significantly higher adult mortalities (mean ± SE, 0.364 ± 0.01) than males (0.328 ± 0.01). However, the mortality cost of parental care was only detectable in males, when the influence of mating competition was statistically controlled. Taken together, our results challenge the traditional explanation of female‐biased avian mortalities, because evolutionary changes in female care were unrelated to changes in mortality bias. The interspecific variation in avian mortality bias, as we show here, is driven by males, specifically via the costs of both mating competition and parental care. We also discuss alternative hypotheses for why most birds exhibit female‐biased mortalities, whereas in mammals male‐biased mortalities predominate.