Brood Sex Ratio Research Articles

Effect of Sampling Blood of Nestlings on Success of Fledging in White-eyed Vireos (Vireo griseus)

Abstract Because the white-eyed vireo (Vireo griseus) is a less sensitive species than the endangered black-capped vireo (V. atricapilla), it served as a proxy to quantify potentially adverse effects of sampling blood. Success of fledging was quantified using split-sampled broods with non-bled nestlings serving as a control. Sex ratio of broods was determined, and sampling and storage methods for blood were assessed. Obtaining blood from nestlings did not affect success of fledging. The male:female ratio of nestlings was 65:35.

Resource transfers and evolution: Helpful offspring and sex allocation

In some vertebrates, offspring help their parents produce additional offspring. Often individuals of one sex are more likely to become “helpers at the nest”. We analyze how such sex-biased offspring helping can influence sex-ratio evolution. It is essential to account for age-structure because the sex ratios of early broods influence how much help is available for later broods; previous authors have not correctly accounted for this fact. When each female produces the same sex ratio in all broods (as assumed in all previous analyses of sex-biased helping), the optimal investment strategy is biased towards the more-helpful sex. When a female has facultative control over the sex ratio in each brood and each helper of a given sex increases the resource available for offspring production by a fixed amount, the optimal strategy is to produce only the more-helpful sex in early broods and only the less-helpful sex in later broods. When there are nonlinear returns from helping, i.e., each helper increases the amount of resource available for reproduction by an amount dependent upon the number of helpers, the optimal strategy is to maximize resource accrual from helping in early broods (which may involve the production of both sexes) and then switch to the exclusive production of the less-helpful sex in later broods. The population sex ratio is biased towards the more-helpful sex regardless of whether the sex ratio is fixed or age-dependent. When fitness returns from helping exhibit environmental patchiness, females are selected to produce only males on some patches and only females on others, and the population sex ratio may be biased in either direction. We discuss our results in light of empirical information on offspring helping, and we show via meta-analysis that there is no support for the claim of Griffin et al. [Griffin, A.S., Sheldon, B.C., West, S.A., 2005. Cooperative breeders adjust offspring sex ratios to produce helpful helpers. Amer. Nat. 166, 628–632] that parents produce more of the helpful sex when that sex is rare or absent.

Local offspring density and sex ratio affect sex allocation in the great tit

The expected fitness gain for offspring of a given sex may depend on local population sex ratio and density. Knowing the influence of such social factors on brood sex ratios may contribute considerably to the understanding of sex allocation in higher vertebrates. For 3 consecutive years, we manipulated sex ratios and densities of juvenile great tits (Parus major) within forest plots and investigated how next year’s brood sex ratios were affected. For yearling female breeders, we considered the treatment of the plot of fledging and settlement and for experienced female breeders the treatment of the breeding plot. Experienced females produced more female offspring at increased plot densities and more males at reduced densities. Yearling female’s brood sex ratio was not affected by the density treatment but was positively related to the natural density in the plot of fledging. The plot sex ratio treatment affected sex allocation differently for yearling and experienced females. Yearling females produced more females when fledged from male-biased plots and more males when fledged from female-biased plots. Experienced females produced more females in control plots and unbiased brood sex ratios in male- and female-biased plots. Our results show that experimental changes in local sex ratio and density generate social or environmental cues that are used in sex allocation. We cannot show that sex allocation was adaptive. However, natural and experimental plot density and experimental plot sex ratio related differently to sex allocation for yearling and experienced females, which suggest that several sex allocation scenarios might act simultaneously. Key words: frequency dependence, local competition, sex-biased dispersal, social environment, Trivers and Willard hypothesis.[Behav Ecol]

Agricultural intensification exacerbates female-biased primary brood sex-ratio in tree swallows

Impacts of agriculture practices are documented at every ecosystem level from landscape structure to biodiversity. Birds are especially affected by agricultural modifications as shown by the decline of farmland species in Europe and North America. Few studies have assessed the effects of such modifications on individual characteristics directly influencing population dynamics. Several bird studies showed that sex-ratio may be adaptive and that mother condition affects the production of sons and daughters. However, little is known about the connections between environmental and individual characteristics on sex allocation. Here we quantified the variation in primary sex-ratio in tree swallows (Tachycineta bicolor) nesting in contrasted environments associated with agricultural intensification in southern Quebec, Canada. We found that intensive agricultural practices affected female sex-ratio allocation in this area, resulting in more biased sex-ratio towards daughters throughout most of the hatching period. Yet, this bias towards daughters was reduced as the season progressed in the most intensively cultivated areas, suggesting that tree swallows have problems foreseeing the difficult growth and postfledging conditions that their nestlings will experience in such environments. Our results thereby support the hypothesis that intensive agricultural areas act as an ecological trap in our study system. We also found that effects of agricultural intensification on sex allocation differed among years and affected the relationships between sex-ratio allocation and hatching date. Our results suggest that agricultural intensification modifies female sex allocation in tree swallows, but the importance of the effects might vary among years and depend on timing of breeding.

Primary sex ratios vary with clutch size in the size-dimorphic White-throated Dipper Cinclus cinclus

Sex allocation theory explains how parents should allocate reproductive effort to the rearing of sons and daughters. A central premise is that there is room for individual optimization of brood sex ratios as a function of the relative rearing costs of sons and daughters, the fitness return curves for sons and daughters, and the total amount of reproductive resources available. When sons are more expensive to raise than daughters, and fitness returns per unit investment are equal for the two sexes, we show with a simple graphical model that large clutches should be female-biased. We tested this prediction in a field study of the White-throated Dipper Cinclus cinclus in Norway. In this species, males are nearly 20 % heavier than females, and the predominant mating system is social monogamy with little or no extrapair paternity, indicative of similar reproductive variances for the two sexes. We found a significant deficit of males (27 % males) for the largest clutch size (=6 eggs) in the population and no deviation from parity for five-egg clutches (47 % males). This contrast could not be explained by any sex-biased mortality rate at the egg or nestling stages, and thus suggests facultative adjustment of the primary sex ratio. We also verified a sexual dimorphism in growth rates at the nestling stage. Our results indicate that different rearing costs of sons and daughters may select for clutch-size dependent sex-ratio adjustment in size-dimorphic species.

Brood Sex Ratio of the Lilac-crowned Parrot (Amazona finschi)

Abstract Evolutionary theory predicts birds should adjust the sex ratio of their broods in response to external factors that differentially affect the reproductive value of each sex. We examined the brood sex ratio in the Lilac-crowned Parrot (Amazona finschi) in relation to climate, hatching date, and hatching order. We used polymerase chain reaction amplifications to identify the gender of 66 nestlings from 32 clutches spanning 7 years. There was a tendency to produce more female offspring in years of high nestling survival following high rainfall with a slight female-bias in third-hatched nestlings. We found no significant associations between brood sex ratio and rainfall, hatching date, or hatching order within clutches. Our results suggest the examined factors provide insufficient differential costs or benefits of offspring gender to promote sex ratio bias in this monomorphic species.

Hatchling sex ratio and female mating status in the great reed warbler, Acrocephalus arundinaceus (Aves, Passeriformes): further evidence for offspring sex ratio manipulation

Offspring sex in birds is maternally determined, thus females are expected to vary brood sex ratio adaptively in response to various environmental and/or genetic conditions preferring one sex to the other sex. However, recent studies provide controversial evidence showing mixed support for adaptive sex manipulation theory. One problem may lie in the fact that only sex ratios at fledging have been used in many of these studies, ignoring post-hatching sex-biased mortality. To validate these results, some authors call for replication of previous researches. Here we examined hatchling sex ratio variation in relation to female mating status in a local population of facultatively polygynous great reed warblers Acrocephalus arundinaceus in Central Europe. In agreement with results of previous works, we found that monogamous and primary females had a higher proportion of sons in their broods than secondary females. These results provide further evidence that great reed warbler females manipulate the sex ratio of their offspring in response to their social status.

Critical Rearing Parameters of <I>Tetrastichus planipennisi</I> (Hymenoptera: Eulophidae) as Affected by Host Plant Substrate and Host-Parasitoid Group Structure

In laboratory assays, we evaluated the potential impact of host plant substrate types, host-parasitoid group sizes (densities), and parasitoid-to-host ratios on select fitness parameters of the larval endoparasitoid Tetrastichus planipennisi Yang (Hymenoptera: Eulophidae), newly introduced for biological control of emerald ash borer, Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), in the United States. Results from our study showed that offspring production and critical fitness parameters (body size and sex ratio) of T. planipennisi from parasitized emerald ash borer larvae are significantly influenced by host plant substrate type, host-parasitoid group size, parasitoid-to-host ratio, or a combination in the primary exposure assay. The number of both female and male T. planipennisi progeny was significantly greater when emerald ash borer larvae were inserted into tropical ash [Fraxinus uhdei (Wenz.) Lingelsh.] logs rather than green ash (Fraxinus pensylvanica Marshall). When maintained at a constant 1:1 parasitoid-to-host ratio, assays with larger host-parasitoid group sizes (3:3-12:12) produced significantly greater numbers of both male and female offspring per parental wasp compared with those with the single host-parasitoid (1:1) group treatment. As the parasitoid-to-host ratio increased from 1:1 to 8:1 in the assay, the average brood size (number of offspring per parasitized emerald ash borer larva) increased significantly, whereas the average brood sex ratio (female to male) changed from being female-biased (6:1) to male-biased (1:2); body size of female offspring as measured by the length of ovipositor and left hind tibia also was reduced significantly. Based on these findings, we suggest that the current method of rearing T. planipennisi with artificially infested-emerald ash borer larvae use the tropical ash logs for emerald ash borer insertion, a larger (> or = 3:3) host-parasitoid group size and 1:1 parasitoid-to-host ratio in the primary parasitoid exposure assays.

Is there an expected relationship between parental expenditure and sex ratio of litters or broods?

Parents may be selected to adjust the sex ratio of their offspring when parental expenditure yields different fitness returns from sons and daughters. This prediction is clear when parents produce only one offspring per reproductive attempt, but more complicated when parental resources are shared by several offspring, and parents may potentially influence the resource allocation among offspring as well as their number and sex. Here we present an optimization model to make predictions on how total parental expenditure may relate to the number and sex of offspring at every rank position within the litter or brood as well as the sex ratio of the litter/brood for the case of a large population with stable Fisherian sex ratio. We show that selection for sex ratio adjustment should be stronger for offspring at higher-ranking positions, for which the amount of resources received can be more predictable. Also, the relationship between parental resources devoted to a litter/brood of a given size and the primary sex ratio (proportion of males) is not expected to be a monotonically increasing function but rather a J-shaped relationship, steeper for small litters/broods and more extreme sexual dimorphism. Parental expenditure relates to increased sex ratio only for small variations in parental expenditure and for a given brood/litter size. For variable litter/brood sizes, a general relationship between parental resources and litter/brood sex ratio is not expected, although in practice pooling litters or broods of different sizes may produce negative relationships between parental expenditure and sex ratio of the litter/brood.

Maternal Adjustment of the Sex Ratio in Broods of the Broad-Horned Flour Beetle, Gnathocerus cornutus

We report that females of the broad-horned flour beetle, Gnathocerus cornutus, can plastically adjust the sex ratio in their broods in response to environmental quality. Specifically, females reared in nutritionally poor environments produce broods that are 65% female, on average, with the degree of female-bias in some broods approaching 95%. In addition, females reared in nutritionally poor environments lay significantly more eggs than do females reared on standard medium, which produce broods with an even sex ratio. These effects of the mother's environment on size and sex ratio in broods are manifest even when oviposition occurs in the standard nutritional environment; indeed, the degree of female-bias increases with advancing female age despite the availability of nutritional resources to females at the time of egg laying. Our studies rule out sex-specific differences in viability early in larval development as the mechanism for the bias in sex-ratio of broods, since females reared in nutritionally poor environments have broods with hatchability and larval viability comparable to those of nonstressed females. Our studies also rule out an effect of the sire on the sex ratio in broods, since all male mates were reared on standard medium. We discuss our results in the context of theories for the evolution of plastic sex-ratios in the face of environmental deterioration and discuss how plasticity can resolve a long-standing question about the conditions underlying the evolution of biased sex ratios.

Non-Seasonal Changes in the Intensity of Female Mate Preference and Offspring Sex Ratio in the Wild GuppyPoecilia reticulata

It has been demonstrated that the exaggeration of male sexual ornaments and the intensity of female mate preferences of a wild guppy population change over a period of several months. However, the factors that determine the short-term changes in male ornaments and female preferences remained unclear. In this study, we examined the effect of season on these short-term changes by measuring these traits in the same seasons of different years for a wild guppy population in Okinawa, Japan. We also compared the characteristics of the offspring in each collection term, as female guppies are known to have the ability to control offspring characteristics, such as brood size and sex ratios, depending on their mates' attractiveness. Results showed that the total lengths of the males changed seasonally; males in the summer were larger than those in the spring. In contrast, the size of orange spots in males and the intensity of female mating preferences differed in the same seasons of different years. Brood size and offspring body size in each term showed seasonal changes. However, offspring sex ratios exhibited different patterns in the same seasons of different years. Females produced female-biased broods when attractive males with large orange spots were rare. These results suggest that short-term changes in some traits of adult male and female guppies as well as offspring sex ratios may be not determined by seasonal factors, and that these traits may be interrelated.

Brood sex ratio varies with diet composition in a generalist raptor

While sex allocation has been investigated productively at both population and family levels, as yet no general theory has been developed that is capable of linking processes at these two ecological scales, and very few empirical studies have examined cross-scale patterns. In Finnish northern goshawks (Accipiter gentilis), nestling sex ratio of local subpopulations is related to the spatial and temporal variation in the abundance of their principal avian prey, woodland grouse. Using data from an urban breeding population in Hamburg, Germany, I investigated: (1) whether brood sex ratio of goshawks varies with diet composition at the family level; (2) whether such variation could reflect adaptive adjustment; and (3) how family-level allocation can drive population-level patterns, such as those observed in Finland. Feral pigeons (Columba livia) were the most important prey species, with a pooled contribution to total diet of 36%. Brood sex ratio varied significantly with the proportion of pigeons in the breeding-season diet of pairs (increasing male bias). However, there was no evidence for sex-differential effects of diet composition, so it remains unclear whether the observed sex-ratio variation was an adaptive response. As all study pairs inhabited an (urban) environment where pigeons were unusually abundant, family-level sex-ratio adjustment caused a marked male bias in offspring sex ratio at the population level (male-biased nestling sex ratio in four of five years; pooled data: 60% males). This suggests that the large-scale variation observed in Finnish goshawk populations mirrors sex-ratio adjustment shown by individual families in response to small-scale environmental conditions. Apart from linking patterns empirically across ecological scales, this study is, to my knowledge, the first to demonstrate that family-level brood sex ratio varies with realized resource use (diet composition) in a raptor species. Previous studies either failed to find significant associations or, more commonly, violated theoretical assumptions by measuring environmental prey abundance (often integrated over large areas) rather than realized prey use of individual breeding pairs. I conducted a meta-analysis of offspring sex-ratio data from 17 goshawk populations across Europe to put my results into perspective. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105, 937‐951.

Breeding ecology of ground tits in northeastern Tibetan plateau, with special reference to cooperative breeding system

Abstract Data on breeding ecology of a color-band marked population of the ground tit Parus humilis were collected in north Qinghai on the Tibetan plateau, during 2008 and 2009. In spring the birds excavated 0.8-3.2 m long nesting burrows under the ground. First-egg laying occurred between late April and late June during which a pair produced one brood. Incubation was done by female alone for 15-16 days and nestling-feeding by both sexes and helpers in any for 23-25 days. Average brood size at fledging was 5.8 (± 1.4 SD, 3-8) and all the 27 observed nesting attempts fledged at least one young. At the population level, brood sex ratio did not differ from 1:1. The birds are a territory-living resident, with annual resight rates being 48% (22 of 46) in adult breeders and 10% (7 of 67) in yearlings. Pairs were socially monogamous, of which 23% (9 of 40) contained one and sometimes two male helpers, most likely being philopatric sons of the breeders. The formation of cooperative groups is similar to the population in central Tibet but differs from that in south Guansu where breeding ground tits exhibit a high level of annual turnover.

Secondary sex ratio covaries with demographic trends and ecological conditions in the barn swallow

Parents are expected to invest more in the sex that benefits most from the local environment. When the quality of breeding sites varies spatially and natal dispersal of males and females differs, parents in high-quality habitats should skew their progeny sex ratio in favor of the less dispersing sex. We tested this prediction in the barn swallow (Hirundo rustica L.), by relating the proportion of male offspring around fledging (secondary sex ratio) of first and second broods to the ecological quality (presence of livestock farming and relative surface of hayfields in the foraging range) and local demographic trends of the farms where the colonies were located. Consistent with our predictions, the proportion of male offspring, which are more philopatric than females, increased with the extent of hayfields, which are high quality, preferred foraging habitats. Moreover, the proportion of male offspring in second broods was smaller in colonies with positive demographic trends, possibly indicating density-dependent effects on sex ratio. Independent of the mechanism generating uneven sex ratio (zygote sex ratio adjustment or sex-related pre-fledging mortality), barn swallows breeding under favorable conditions overproduced the sex that is more likely to benefit from such conditions.

Reduced Polymorphism Associated with X Chromosome Meiotic Drive in the Stalk-Eyed Fly Teleopsis dalmanni

Sex chromosome meiotic drive has been suggested as a cause of several evolutionary genetic phenomena, including genomic conflicts that give rise to reproductive isolation between new species. In this paper we present a population genetic analysis of X chromosome drive in the stalk-eyed fly, Teleopsis dalmanni, to determine how this natural polymorphism influences genetic diversity. We analyzed patterns of DNA sequence variation at two X-linked regions (comprising 1325 bp) approximately 50 cM apart and one autosomal region (comprising 921 bp) for 50 males, half of which were collected in the field from one of two allopatric locations and the other half were derived from lab-reared individuals with known brood sex ratios. These two populations are recently diverged but exhibit partial postzygotic reproductive isolation, i.e. crosses produce sterile hybrid males and fertile females. We find no nucleotide or microsatellite variation on the drive X chromosome, whereas the same individuals show levels of variation at autosomal regions that are similar to field-collected flies. Furthermore, one field-caught individual collected 10 years previously had a nearly identical X haplotype to the drive X, and is over 2% divergent from other haplotypes sampled from the field. These results are consistent with a selective sweep that has removed genetic variation from much of the drive X chromosome. We discuss how this finding may relate to the rapid evolution of postzygotic reproductive isolation that has been documented for these flies.

Hatching asynchrony and offspring sex: an experiment on maternal effects in the yellow-legged gull

Maternal effects may allow parents to increase their own fitness by adjusting progeny phenotype to pre- and post-natal conditions depending on the sex of individual offspring. In birds, sex-specific maternal effects can be mediated by predictable gradients of rearing conditions due to hatching asynchrony, egg laying date, order and mass, and brood sex ratio. In this study, we analysed the growth and survival consequences of experimentally reduced hatching asynchrony and order per se and in conjuction with other maternal effects (laying date, egg mass and sibling sex ratio) in yellow-legged gull (Larus michahellis) chicks of either sex. Survival of male chicks in synchronised broods was reduced compared to that of their sisters and that of male chicks in control broods, whereas survival of females but not that of males declined with hatching order independently of hatching synchronisation treatment. Survival of female but not male chicks declined with egg mass. In addition, survival of individual chicks, independently of their sex, was positively predicted by the proportion of females among their siblings. Hatching synchronisation resulted in larger average body size and in a weaker dependence of body mass on hatching order, suggesting that by modulating hatching asynchrony parents can effectively tune the size hierarchy among the offspring. Thus, survival of yellow-legged gull chicks of either sex was differentially affected by maternal effects as mediated by hatching asynchrony, hatching order as well as egg mass, which constitute predictable gradients of pre- or post-natal environmental conditions potentially under parental control.

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.

Do parents and helpers adjust their provisioning effort in relation to nestling sex in a cooperatively breeding bird?

In cooperatively breeding vertebrates, investment strategies of breeders and helpers may be affected by the sex of offspring that they provision because the fitness benefits gained from caring for sons or daughters will depend upon the fitness impacts of positive (i.e. local resource enhancement) and negative (i.e. local resource competition) social interactions between relatives. We investigated the relationship between provisioning effort by breeders and helpers and the sex of nestlings that they provision, using long-term data over 16 years and detailed behavioural observation of interactions between carers and offspring in a cooperatively breeding species, the long-tailed tit, Aegithalos caudatus . We predicted preferential care for male nestlings because males are the more helpful sex in the kin-selected social system of long-tailed tits. However, we found that despite the significant size difference between male and female nestlings, there was no evidence of facultative adjustment of provisioning effort by either breeders or helpers in relation to brood sex ratio. In addition, we found no evidence for sex-biased provisioning by carers, and no bias in the feeds received by nestlings according to their sex. Thus, we conclude that the provisioning strategy of long-tailed tits does not incorporate differential investment according to sex. We suggest that the unpredictable nature of helping in the atypical cooperative breeding system of long-tailed tits makes random provisioning effort in relation to nestling sex the best option for carers.

Brood sex ratio variation in a cooperatively breeding bird

In cooperatively breeding species, the fitness consequences of producing sons or daughters depend upon the fitness impacts of positive (repayment hypothesis) and negative (local competition hypothesis) social interactions among relatives. In this study, we examine brood sex allocation in relation to the predictions of both the repayment and the local competition hypotheses in the cooperatively breeding long-tailed tit Aegithalos caudatus. At the population level, we found that annual brood sex ratio was negatively related to the number of male survivors across years, as predicted by the local competition hypothesis. At an individual level, in contrast to predictions of the repayment hypothesis, there was no evidence for facultative control of brood sex ratio. However, immigrant females produced a greater proportion of sons than resident females, a result consistent with both hypotheses. We conclude that female long-tailed tits make adaptive decisions about brood sex allocation.

Relationship between brood sex ratio and male ornaments depends on male age in a warbler

Sex allocation theory predicts that females should bias the sex ratio of their offspring in response to differences in the reproductive value of sons and daughters. For example, paternal traits (ornaments) that are associated with male quality and reproductive success may result in male-biased sex ratios when inheritance of these traits is more beneficial to sons than to daughters. However, the information content of male ornaments, as well as the ornaments themselves, may change with male experience. If females are responsive to changes in the information content of signals, the result may be context-specific patterns of sex allocation. In a 3-year study of 50 common yellowthroat, Geothlypis trichas, females and their 287 young, we found that biased sex ratios were associated with male ornamentation, but that these deviations were not consistent across male experience classes. Females produced clutches with male-biased sex ratios when they were mated to young males with elaborate ornaments, but not when they were mated to older males. Male-biased broods were also associated with differences in offspring quality, as measured by average nestling mass. At our field site in New York, U.S.A., ornaments are generally more indicative of viability and condition among young, inexperienced males than among older males. We suggest that the relationship between male traits and the relative value of sons and daughters may change across male experience classes. Such changes may obscure patterns of adaptive sex-ratio bias and contribute to mixed results in the sex allocation literature.