Tawny Owls Strix aluco with reliable food supply produce male-biased broods

Tawny Owls Strix aluco have been reported to skew the sex ratio of their offspring towards males when facing food shortage during the nestling period (and vice versa), because female fitness is more compromised by food shortage during development than male fitness. To test the generality of these results we used a DNA marker technique to determine the sex ratio in broods of Tawny Owls in Danish deciduous woodland during two years of ample food supply (rodent population outbreak) and two years of poor food supply. Of 268 nestlings, 59% were males (95% CI: 53–65%). This proportion was higher than previously reported for the species (49% in Northumberland, UK, and 52% in Hungary), but consistent with Fisherian sex allocation, which predicts a male bias of c . 57% based on inferred differences in energy requirements of male and female chicks. Contrary to previous results, brood sex ratios were not correlated with the resource abundance during the breeding seasons, despite considerable variation in breeding frequency, brood size or hatching date across years. Brood sex ratios were unaffected by brood reduction prior to DNA sampling, and nestling mortality rates after DNA sampling were not related to gender. The inconsistency between the sex ratio allocation patterns in our study and previous investigations suggests that adaptive sex allocation strategies differ across populations. These differences may relate to reproductive constraints in our population, where reproductive decisions seem primarily to concern whether to lay eggs at all, rather than adjust the sex ratio to differences in starvation risk of nestlings.     

Scale and seasonal sex-ratio trends in northern goshawk Accipiter gentilis broods

In many populations of size dimorphic birds, brood sex ratios change with advancing laying date. The slopes of these trends, however, vary in time and space, both between and within species. We studied brood sex ratios (proportion of males) of northern goshawks Accipiter gentilis in Finland in relation to laying date using ringing data from 1989 to 1998. At the nationwide scale, i.e. the whole of Finland, between-year variation in offspring sex ratio was moderate, and the sex ratio did not change with later laying date. At a regional scale, the sex ratio was seasonally constant in one region but decreased in another, although the laying-date/brood-size relationship was identical. Hence, the size and sex composition of goshawk broods are locally two uncoupled facets of reproduction. Both the national and regional patterns differ drastically from the pattern of a Dutch population, where sex ratio increased seasonally. We suggest that spatial variation in inter-annual seasonal sex-ratio trends might be indicative of the scale at which sex-ratio feedback functions. The sex ratio of breeders is a factor that could add to the understanding of the observed geographical differences in seasonal sex-ratio patterns.     

Female body condition and brood sex ratio in Yellow-legged Gulls Larus cachinnans

In the Yellow-legged Gull Larus cachinnans , males are the larger sex, and show more reproductive variance than females. We predicted that the proportion of male chicks in a brood should increase with female body condition. We investigated brood sex ratio by using DNA markers taken from samples of hatchlings or dead embryos, and female body condition using plasma cholesterol concentration as a reliable indicator. The brood sex ratio of females in good condition was male biased and the sex ratio of females in poor condition was female biased. This relationship was also significant in those nests where all the eggs laid were sexed. Thus, manipulation of embryo mortality cannot explain the biases reported in this study, suggesting that the sex ratio of the eggs was biased prior to laying. These results confirm that sex-ratio manipulation in gulls operates under natural conditions, and supports earlier experimental findings.     

Costs of rearing and sex‐ratio variation in southern grey shrike Lanius meridionalis broods

Several non‐mutually exclusive hypotheses predict adaptive variation in the offspring sex ratio. When conditions for breeding are adverse, parents are predicted to produce more offspring of the less costly sex to rear (‘the cost‐of‐reproduction hypothesis’). Moreover, they also should produce the more dispersing sex in order to diminish future competition (‘the local‐resource‐competition hypothesis’). Here, we analyse brood sex ratio according to rearing conditions in the southern shrike Lanius meridionalis, a species with moderately reversed sexual dimorphism. Our results suggest that females are more costly to rear than males in this species. Adult females proved heavier than males, and female nestling tended to be heavier than male nestlings. Moreover, the greater brood reduction, the more male‐biased was the brood, suggesting that brood reduction implied higher mortality in female nestlings. Consistent with these findings, the brood sex ratio was biased to the less costly sex (males) when breeding conditions were adverse (bad years or low‐quality male parents), supporting the cost‐of‐reproduction hypothesis. By contrast, these findings did not support the local‐resource‐competition hypothesis, which predicted female‐biased brood sex ratio under adverse conditions. As a whole, our results support the idea that birds adaptively modulate sex ratio in order to minimize reproduction costs.     

Internest sex-ratio variation and male brood survival in the ant Pheidole pallidula

Sex allocation in social insects has become a general modelin tests of inclusive fitness theory, sex-ratio theory, andparent-offspring conflict. Several studies have shown that colonysex ratios are often bimodally distributed, with some coloniesproducing mainly females and others mainly males. Sex specializationmay result from workers assessing their relatedness to malebrood versus female brood, relative to the average worker-relatednessasymmetry in other colonies of their population. Workers thenadjust the sex ratio in their own interest This hypothesis assumesthat workers can recognize the sex of the brood in their colonyand selectively eliminate males. We compared the primary sexratio (at the egg stage) and secondary sex ratio (reproductivepupae and adults) of colonies in the ant Pheidole pallidula.There was a strong bimodal distribution of secondary sex ratios,with most colonies producing mainly reproductives of one sex.In contrast, there was no evidence of a bimodal distributionof primary sex ratios. The proportion of haploid eggs producedby queens was 0.35 in early spring and decreased to about 0.1in summer. Male eggs also were present in virtually all fieldcolonies sampled in July, although eggs laid at this time ofyear never give rise to males. All male brood is, therefore,selectively eliminated beginning in July and continue to beeliminated through the rest of the year. Finally, the populationsex-ratio investment was female-biased. Together, these resultsare consistent with the hypothesis that workers control thesecondary sex ratio by selectively eliminating male brood inabout half the colonies, perhaps those with high relatednessasymmetry.[Behav Ecol 7: 292–298 (1996)]     

A snail with unbiased population sex ratios but highly biased brood sex ratios

Extraordinary sex ratio patterns and the underlying sex-determining mechanisms in various organisms are worth investigating, particularly because they shed light on adaptive sex-ratio adjustment. Here, we report an extremely large variation in the brood sex ratio in the freshwater snail, Pomacea canaliculata. In eight rearing series originating from three wild populations, sex ratios were highly variable among broods, ranging continuously from almost exclusively males to almost exclusively females. However, sex ratios were similar between broods from the same mating pair, indicating that sex ratio is a family trait. Irrespective of the large variations, the average sex ratios in all rearing series were not significantly different from 0.5. We argue that Fisher's adaptive sex-ratio theory can explain the equal average sex ratios, and the results, in turn, directly support Fisher's theory. Polyfactorial sex determination (in which sex is determined by three or more genetic factors) is suggested as the most likely mechanism producing the variable brood sex ratio.     

Do females adjust brood sex ratio according to males’ genetic structures in a gregarious passerine,the vinous‐throated parrotbill Paradoxornis webbianus?

A growing number of bird species are known to have fine‐scale genetic structure during the breeding season, with relatives breeding in close vicinity. Such genetic structure often has fitness consequences for parents, and sex ratio theory predicts that females should respond adaptively when they determine offspring sex. We examined whether or not females allocate offspring sex adaptively in response to the local genetic structures as well as other biotic and abiotic factors in a population of the vinous‐throated parrotbill Paradoxornis webbianus, a small passerine with strong flocking habit and various genetic structures among neighbouring males during the breeding season. The average brood sex ratio of hatchlings (secondary sex ratio) did not deviate from parity. In addition, the observed brood sex ratio was independent of the fine‐scale genetic structure and other factors including breeding density, clutch size, laying date, parents’ quality, and the presence of extrapair paternity. Accordingly, we reject the hypothesis of adaptive sex allocation by female parrotbills in association with local genetic structure and other factors. Instead we conclude that despite the plausible benefits of biased sex allocation, this species determines brood sex ratio via random sex allocation with equal probability of male and female offspring.     

Sex-specific growth rates in zebra finch nestlings: a possible mechanism for sex ratio adjustment

Wild and captive zebra finches (Taenopygia guttata), like severalother species, produce a male-biased sex ratio at fledging whenfood is scarce. This is due to primary sex-ratio adjustmentand female-biased nestling mortality. Given that young femalesfledging at low body masses have been shown to have low fecundityas adults, lower returns to parents from producing female offspringin conditions of restricted food has been raised as a functionalexplanation (Trivers and Willard's hypothesis of adaptive sexualinvestment; 1973). However, an alternative, mechanistic hypothesisis that under restricted conditions female chicks are more costlyto produce. In consequence, lower returns to parents under theseconditions would happen earlier in the life of female offspringrather than later. To test this hypothesis, I hand-reared chickson a food gradient. In the absence of parent-offspring and sib-sibinteractions, final body mass and growth rates for females werelower in conditions of restricted food. For males, final bodymass and growth rates did not differ with food condition. Lowfemale growth rates in food-restricted conditions might be onepotential mechanism causing female-biased mortality in birds.More importantly, this result is the strongest evidence yetof female offspring experiencing higher marginal fitness benefitsfrom additional food than males and it has implications forprimary and secondary sex-ratio adjustment. Also, as this mechanismhas been shown in the absence of parent-offspring interactions,significant questions can now be raised as to how parental andoffspring behavior interact in their effects on secondary sex-ratioadjustment.     

Sex allocation and sex‐specific parental investment in an endangered seabird

Biased offspring sex ratio is relatively rare in birds and sex allocation can vary with environmental conditions, with the larger and more costly sex, which can be either the male or female depending on species, favoured during high food availability. Sex‐specific parental investment may lead to biased mortality and, coupled with unequal production of one sex, may result in biased adult sex ratio, with potential grave consequences on population stability. The African Penguin Spheniscus demersus, endemic to southern Africa, is an endangered monogamous seabird with bi‐parental care. Female adult African Penguins are smaller, have a higher foraging effort when breeding and higher mortality compared with adult males. In 2015, a year in which environmental conditions were favourable for breeding, African Penguin chick production on Bird Island, Algoa Bay, South Africa, was skewed towards males (1.5 males to 1 female). Males also had higher growth rates and fledging mass than females, with potentially higher post‐fledging survival. Female, but not male, parents had higher foraging effort and lower body condition with increasing number of male chicks in their brood, thereby revealing flexibility in their parental strategy, but also the costs of their investment in their current brood. The combination of male‐biased chick production and higher female mortality, possibly at the juvenile stage as a result of lower parental investment in female chicks, and/or at the adult stage as a result of higher parental investment, may contribute to a biased adult sex ratio (ASR) in this species. While further research during years of contrasting food availability is needed to confirm this trend, populations with male‐skewed ASRs have higher extinction risks and conservation strategies aiming to benefit female African Penguin might need to be developed.     

Sex ratio variation among broods of great reed warblers Acrocephalus arundinaceus

The sex of 746 great reed warbler fledglings (from 175 broods) was determined by the use of single primer polymerase chain reaction. The reliability of the technique was confirmed as 104 of the fledglings were subsequently recorded as adults of known sex. The overall sex ratio did not differ from unity. Variation in sex ratios between broods was larger than expected from a binomial distribution. Female identity explained some of the variation of brood sex ratio indicating that certain females consistently produced sex ratios that departed from the average value in the population. The theory of sex allocation predicts that parents should adjust the sex ratio of their brood to the relative value of sons and daughters and this may vary in relation to the quality of the parents or to the time of breeding. In the great reed warbler, the proportion of sons was not related to time of breeding, or to any of five female variables. Of five male variables, males with early arrival date tended to produce more daughters. The sex ratio of fledglings that were a result of extra-pair fertilizations did not differ from that of legitimate fledglings. Hence, there is currently no evidence of that female great reed warblers invest in a higher proportion of sons when mated with attractive males.     

Late‐breeding Great Cormorants Phalacrocorax carbo sinensis produce fewer young of the more vulnerable sex

We examined the brood sex ratio and offspring body mass in relation to the timing of breeding and brood size in the Great Cormorant Phalacrocorax carbo sinensis. The brood sex ratio was not related to brood size but it was significantly related to the hatching date, with a decreasing proportion of males in the brood in the course of the season. Male chicks had significantly lower body mass if they hatched later in the season, whereas there was no such relationship for female offspring. Assuming that environmental conditions deteriorate with progress of the breeding season, and male offspring may be more vulnerable to poor environmental conditions, the observed decline in the proportion of male offspring late in the season may be adaptive.     

Sex-ratio depression as a brood-pattern criterion of radiation damage inDrosophila melanogaster

The results of analyses of the brood variance in the relative frequency of females to males in the progeny ofD. melanogaster males having structurally-normal chromosomes, not only established that there is a brood-curve of sex-ratio depression in the progeny of irradiated males, but also revealed that there was a curve of sex-ratio change in successive broods of offspring from unirradiated males even though the ratio of females to males in the total progeny was 1 : 1. The brood curve of sex-ratio change in both series of control males tended to be bimodal, with an increasing frequency of female offspring during the first five or six days, followed by a progressive decrease until about the 12th day, than subsequently an increase until about the 15–16th days, with perhaps again a decrease in the latest (17–24d) broods. The bimodality of the curve appeared to be the consequence of a complex relationship between brood-size, brood-rank and sex-ratio change. Following irradiation of the males with 3000r of X-rays, the incidence of female offspring decreased linearly for the first eight days, with a low of 44.32% in the 6–8d broods. This was followed by an abrupt return to a 1 : 1 sex-ratio in the 9th day and subsequent (10–24d) broods. The recovery of 50% female offspring in the 9th-day brood was interpreted as supporting evidence for the thesis that this brood was derived from cells that had been spermatogonia at the time of irradiation of the parental males. Supported by Research Grant GM 15009-01 from the Public Health Service (U.S.).     

Factors affecting recruitment age and recruitment probability in the Western Gull Larus occidentalis

We studied recruitment age (i.e. the age that a gull bred for the first time) and the probability that an adult reproduced during its life in three cohorts of Western Gulls Larus occidentalis breeding on southeast Farallon Island. We examined the effects of sex, hatching date, hatching order, brood size and annual variation in food supply during the recruitment period. Modal recruitment ages of males and females were 4 and 5 years, respectively, a difference that was significant. None of the factors examined, including hatching date, hatching order and brood size, had a significant effect on recruitment age. The probability of breeding in males who experienced low food supply in their fourth and fifth years was lower than that of males who experienced favourable food supply in their fourth and fifth years. Recruitment probability (the proportion of the sample group that recruited among those available to recruit during a given year) of 4- and 5-year-old males was lower in the food-poor 1983 El Nino than in years of favourable food supply. The only variable affecting recruitment probability of females was age: recruitment probability was greater among 5-year-olds than 4-year-olds. Recruitment age in males was more sensitive to annual variation in food supply, probably because females have more difficulty recruiting due to an excess of adult females in the population and because of the different roles in reproduction in which males are primarily responsible for provisioning chicks and maintaining the territory.     

High interannual variation in the hatching sex ratio of Tengmalm’s owl broods during a vole cycle

The sex ratio at hatching in broods of Tengmalms owl (Aegolius funereus) in northern Sweden was investigated for 3 years characterized by different phases of the vole and owl cycle. Previous work showed the sex ratio in this species to be male-biased for 1 year with a favourable food supply, and that in feeding experiments male nestlings (but not females) suffered higher mortality when food was limited, but not otherwise. Here we present data from a complete 3-year owl cycle, showing that mean brood sex ratio varied significantly among years, being male-biased (65% males) in the first year of high owl breeding density, unbiased (49%) in the second year of high owl breeding density, and female-biased (33% males) in the owls low year. Brood sex ratio did not vary significantly within years with laying date or parental age. Vole availability, and therefore the owls food supply, declined during the 3 years studied. Tengmalms owl parents thus appear to adaptively adjust the sex ratio of their broods according to the expected annual mortality risk of sons.     

Inter-Annual Variability of Fledgling Sex Ratio in King Penguins

As the number of breeding pairs depends on the adult sex ratio in a monogamous species with biparental care, investigating sex-ratio variability in natural populations is essential to understand population dynamics. Using 10 years of data (2000–2009) in a seasonally monogamous seabird, the king penguin (Aptenodytes patagonicus), we investigated the annual sex ratio at fledging, and the potential environmental causes for its variation. Over more than 4000 birds, the annual sex ratio at fledging was highly variable (ranging from 44.4% to 58.3% of males), and on average slightly biased towards males (51.6%). Yearly variation in sex-ratio bias was neither related to density within the colony, nor to global or local oceanographic conditions known to affect both the productivity and accessibility of penguin foraging areas. However, rising sea surface temperature coincided with an increase in fledging sex-ratio variability. Fledging sex ratio was also correlated with difference in body condition between male and female fledglings. When more males were produced in a given year, their body condition was higher (and reciprocally), suggesting that parents might adopt a sex-biased allocation strategy depending on yearly environmental conditions and/or that the effect of environmental parameters on chick condition and survival may be sex-dependent. The initial bias in sex ratio observed at the juvenile stage tended to return to 1∶1 equilibrium upon first breeding attempts, as would be expected from Fisher’s classic theory of offspring sex-ratio variation.     

Maternal influences on brood sex ratios: an experimental study in tree swallows

When the reproductive value of sons and daughters differ, parents are expected to adjust the sex ratio of their offspring to produce more of the sex that provides greater fitness returns. The body condition of females or environmental factors, such as food abundance and mate quality, may influence these expected fitness returns. In a previous study of tree swallows (Tachycineta bicolor), we found that females produced more sons in their broods when they were in better body condition (mass corrected for size). We tested this relationship by experimentally clipping some flight feathers to reduce female body condition. As predicted, we found that females with clipped feathers had a lower proportion of sons in their broods and poorer body condition. However, female body condition alone was not a significant predictor of brood sex ratio in our experiment. We suggest that brood sex ratio is causally related to some other factor that covaries with body condition, most likely the foraging ability of females. The hypothesis that brood sex ratios are influenced by individual differences in female foraging ability is supported by a high repeatability of brood sex ratio for individual females. Thus, maternal effects may have a strong influence on the sex ratios of offspring.     

Sex-ratio optimization with helpers at the nest

In many cooperatively breeding animals, offspring produced earlier in life assist their parents in raising subsequent broods. Such helping behaviour is often confined to offspring of one sex. Sex-allocation theory predicts that parents overproduce offspring of the helping sex, but the expected degree of sex-ratio bias was thought to depend on specific details of female and male life histories, hampering empirical tests of the theory. Here we demonstrate the following two theories. (i) If all parents produce the same sex ratio, the evolutionarily stable sex ratio obeys a very simple rule that is valid for a general class of life histories. The rule predicts that the expected sex-ratio bias depends on the product of only two parameters which are relatively easily measured: the average number of helping offspring per nest and the relative contribution to offspring production per helper. (ii) If the benefit of helping varies between parents, and parents facultatively adjust the sex ratio accordingly, then the population sex ratio is not necessarily biased towards the helping sex. For example, in line with empirical evidence, if helpers are produced under favourable conditions and parents do not adjust their clutch size to the number of helpers, then a surplus of the non-helping sex is expected.     

Food availability and the male's role in parental care in double-brooded Treecreepers Certhia familiaris

The aim of this work was to examine differences in paternal and maternal care in a double-brooded, monogamous species, the Treecreeper Certhia familiaris, in relation to food availability. As a measure of parental care, we recorded the hourly feeding activity of parents when the nestlings from their first and second breeding attempts were 7 and 12 days old. Feeding frequency of the first brood increased with the age of the nestlings and also with the brood size when 12 days old. While the feeding activities of the females were similar with respect to the first and second broods, the males were less active and failed to provide any food to their nestlings in 15 cases out of 28 second broods. In spite of this, the fledglings from the second broods were heavier than those in the first. Such a pattern of male behaviour was possible without being a disadvantage to the chicks because the food supply increased during the breeding season and the female could provide food for the young alone. Thus paternal care was particularly important in times of poor food supply, i.e. during the first brood, where the extent of these males' activity in feeding the 7-day-old nestlings was positively correlated with the average mass of the nestlings. Our results support the idea that the male of monogamous, altricial bird species often makes important contributions to raising the young, especially during periods when it is difficult for the female to do so alone. Males show flexibility in their pattern of parental care, and male Treecreepers change their contribution to the first and second broods within the same season.     

Sex allocation in the ant Camponotus (Colobopsis) nipponicus (Wheeler): II. The effect of resource availability on sex-ratio variability

Sex-ratio studies have played a prominent role in tests of kin selection theory in the eusocial Hymenoptera. The winner in sex-ratio conflict between queens and workers must control the ratio through proximate mechanisms. To determine how a colony adjusts its sex ratio, the mechanism of sex-ratio determination was analyzed in the field in colonies of the ant Camponotus (Colobopsis) nipponicus. A path model including five colony characteristics showed that the resource availability of the colony (quantified as the amount of stored fat in the bodies of the workers) has a large positive effect on the proportion of new queens in the female larvae, but has little effect on male production. The results indicated that a colony adjusts the sex ratio by altering the proportion of new queens obtained from a diploid brood in response to resource availability rather than by eliminating male larvae.     

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.