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Cooperative breeding is a form of breeding system where in addition to a core breeding pair, one or more usually non-breeding individuals provide offspring care. Cooperative breeding is widespread in birds, but its origin and maintenance in contemporary populations are debated. Although deviations in adult sex ratio (ASR, the proportion of males in the adult population) have been hypothesized to influence the occurrence of cooperative breeding because of the resulting surplus of one sex and limited availability of breeding partners, this hypothesis has not been tested across a wide range of taxa. By using data from 188 bird species and phylogenetically controlled analyses, we show that cooperatively breeding species have more male-biased ASRs than non-cooperative species. Importantly, ASR predicts helper sex ratio: in species with more male-biased ASR, helper sex ratio is also more male biased. We also show that offspring sex ratios do not predict ASRs, so that the skewed ASRs emerge during the period when individuals aim to obtain a breeding position or later during adulthood. In line with this result, we found that ASR (among both cooperatively and non-cooperatively breeding species) is inversely related to sex bias in dispersal distance, suggesting that the cost of dispersal is more severe for the further-dispersing sex. As females usually disperse further in birds, this explains the generally male-biased ASR, and in combination with benefits of philopatry for males, this probably explains why ASR is more biased in cooperatively breeding species. Taken together, our results suggest that a sex bias in helping in cooperatively breeding species relates to biased ASRs. We propose that this relationship is driven by sex-specific costs and benefits of dispersal and helping, as well as other demographic factors. Future phylogenetic comparative and experimental work is needed to establish how this relationship emerges.This article is part of the themed issue 'Adult sex ratios and reproductive decisions: a critical re-examination of sex differences in human and animal societies'., (© 2017 The Author(s).)

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

1. An increase of competition among adults or nestlings usually negatively affects breeding output. Yet little is known about the differential effects that competition has on the offspring sexes. This could be important because it may influence parental reproductive decisions. 2. In sexual size dimorphic species, two main contradictory mechanisms are proposed regarding sex-specific effects of competition on nestling performance assuming that parents do not feed their chicks differentially: (i) the larger sex requires more resources to grow and is more sensitive to a deterioration of the rearing conditions ('costly sex hypothesis'); (ii) the larger sex has a competitive advantage in intra-brood competition and performs better under adverse conditions ('competitive advantage hypothesis'). 3. In the present study, we manipulated the level of sex-specific sibling competition in a great tit population (Parus major) by altering simultaneously the brood size and the brood sex ratio on two levels: the nest (competition for food among nestlings) and the woodlot where the parents breed (competition for food among adults). We investigated whether altered competition during the nestling phase affected nestling growth traits and survival in the nest and whether the effects differed between males, the larger sex, and females. 4. We found a strong negative and sex-specific effect of experimental brood size on all the nestling traits. In enlarged broods, sexual size dimorphism was smaller which may have resulted from biased mortality towards the less competitive individuals i.e. females of low condition. No effect of brood sex ratio on nestling growth traits was found. 5. Negative brood size effects on nestling traits were stronger in natural high-density areas but we could not confirm this experimentally. 6. Our results did not support the 'costly sex hypothesis' because males did not suffer from higher mortality under harsh conditions. The 'competitive advantage hypothesis' was also not fully supported because females did not suffer more in male-biased broods. 7. We conclude that male nestlings are not likely to be more expensive to raise, yet they have a size-related competitive advantage in large broods, leading to higher mortality of their on average lighter female nest mates.

Sex determination is a fundamental process governed by diverse mechanisms. Sex ratio selection is commonly implicated in the evolution of sex-determining systems, although formal models are rare. Here, we argue that, although sex ratio selection can induce shifts in sex determination, genomic conflicts between parents and offspring can explain why single-factor systems (e.g. XY/XX or ZW/ZZ) are common even in species that experience selection for biased sex ratios. Importantly, evolutionary shifts in sex determination do not always result in the biased production of sons and daughters sensu sex ratio theory. Thus, equal sex ratios might be an emergent character of sex-determining systems even when biased sex ratios are favored by selection.

The Seychelles warbler (Acrocephalus sechellensis) was an endangered endemic of the Seychelles islands where, until 1988, the entire population of ca. 320 birds was restricted to the one island of Cousin Island (29 ha). Additional breeding populations were successfully established on the islands of Aride (68 ha, 1988) and Cousine (26 ha, 1990) and now with the existence of ca. 2000 warblers on three islands the conservation status of the warbler has improved from endangered to vulnerable. Emigration from the island is extremely rare, so birds that disappeared were known to have died. Almost every bird on Cousin Island has been individually colour ringed and monitored throughout all breeding attempts during a 17-year period (1985-2002; total ca. 2400 birds). These birds were also blood sampled for molecular parentage and sex analyses. Therefore the lifetime reproductive success of many birds is known. Although warblers can breed independently in their first year, some individuals remain in their natal territory as subordinates, and often help by providing nourishment to non-descendant offspring. The frequency of 'helping' is affected by habitat saturation and variation in territory quality (insect prey availability). The long-term benefits of helping are higher for daughters than for sons, and it is therefore no wonder that most helpers are daughters from previous broods. Furthermore, on low-quality territories breeding pairs raising sons gain higher fitness benefits than by raising daughters, and vice versa on high-quality territories. Female breeders adaptively modify the sex of their single-egg clutches according to territory quality: male eggs on low quality and female eggs on high quality. The Seychelles warbler is a beautiful example of behavioural and life-history adaptations to restricted circumstances.

Females of some bird species have a high degree of control over the sex ratio of their offspring at laying. Although several mechanisms have been put forward to explain how females might control the sex of their eggs, virtually nothing is known. As females are the heterogametic sex in birds, adjustment of the clutch sex ratio could arise either by pre- or post-ovulation control mechanisms. The Seychelles warbler (Acrocephalus sechellensis) exhibits extreme adaptive egg sex ratio bias. Typically, warblers produce only single-egg clutches, but by translocating pairs to vacant habitat of very high quality, most females were induced to produce two-egg clutches. Overall, females skewed clutch sex ratios strongly towards daughters (86.6%). This bias was evident in the first egg, but critically, also in the second eggs laid a day apart, even when all absent, unhatched, or unsexed second eggs were assumed to be male. Although a bias in the first egg may arise through either pre- or post-ovulation mechanisms, the skew observed in second eggs could only arise through pre-ovulation control. Post-ovulation adjustment may also contribute to skewed hatchling sex ratios, but as sex-biased release of gametes is likely to be a more efficient process of control, pre-ovulation mechanisms may be the sole means of adjustment in this species. High fitness differentials between sons and daughters, as apparent in the Seychelles warblers, may be necessary for primary sex ratio adjustment to evolve.

We review some recent theoretical and empirical developments in the study of sex allocation in birds. The advent of reliable molecular sexing techniques has led to a sharp increase in the number of studies that report biased offspring sex ratios in birds. However, compelling evidence for adaptive sex allocation in birds is still very scant. We argue that there are two reasons for this: (i) standard sex allocation models, very helpful in understanding sex allocation of invertebrates, do not sufficiently take the complexities of bird life histories and physiology into account. Recent theoretical work might bring us a step closer to more realistic models; (ii) experimental field and laboratory studies on sex allocation in birds are scarce. Recent experimental work both in the laboratory and in the field shows that this is a promising approach.

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. [ABSTRACT FROM PUBLISHER]

1. The parents of sexually size-dimorphic offspring are often assumed to invest more resources producing individuals of the larger sex. A range of different methods have been employed to estimate relative expenditure on the sexes, including quantifying sex-specific offspring growth, food intake, energy expenditure and energy intake, in addition to measures of parental food provisioning and energy expenditure. These methods all have the potential to provide useful estimates of relative investment, but each has particular problems of interpretation, and few studies have compared the estimates derived concurrently from more than two of these measures. 2. In this study we compared these surrogate measures of parental investment in the brown songlark Cinclorhamphus cruralis, which exhibits one of the most extreme cases of sexual size dimorphism among birds. 3. At 10 days of age we found that male chicks, on average, were 49% heavier, received 42% more prey items, expended 44% more energy and ingested 50% more metabolizable energy than their sisters. 4. Furthermore, we created, experimentally, both all-male and all-female broods of 10-day-old chicks and found that mothers delivered 43% more prey items and expended 27% more energy when provisioning all-male broods, providing the first direct evidence for a change in parental energy expenditure in relation to brood sex ratio. 5. These data reveal remarkable agreement between these estimates of investment and suggest that all may provide quantitatively useful information on sex allocation. However, the lower variance associated with estimates of relative mass and energy intake suggest that these methods may be of greater utility, although this may primarily reflect the shorter period over which our provisioning data were collected. [ABSTRACT FROM AUTHOR]

In sexually dichromatic birds, females may adaptively adjust the sex ratio of their offspring prior to hatching in relation to male ornamentation, for example, by producing more sons when paired to a highly attractive partner. However, to our knowledge no studies have investigated offspring sex ratio modification in species in which both sexes are ornamented, and it is unknown whether such a process would be adaptive. Here we examine variation in offspring sex ratio in the mutually ornamented Black Swan Cygnus atratus. Brood sex ratio was not related to the degree of ornament elaboration in either parent, or to extra-pair paternity. We suggest that parental attractiveness may not be inherited in a sex-linked manner, or may be largely non-heritable. Thus, females may not benefit from biasing the sex ratio of their offspring in relation to parental attractiveness. [ABSTRACT FROM AUTHOR]

Tests the prediction that brood sex ratio does not vary systematically with either environmental or parental characteristics in the sexually monomorphic species fairy martin Petrochelidon ariel, and examines the assumption that parental provisioning is unrelated to brood sex ratio. Study population; Molecular sexing and parentage analysis.

Demonstrates that biased hatching sex ratios are caused by biased production and not by differential embryo mortality in Young Seychelles warblers. The use of random amplified polymorphic DNA (RAPD) markers; The warblers also being known as Acrocephalus sechellensis.

Sex-ratio theory states that if the fitness costs to the parents of producing one offspring’s sex relative to the other are higher, parents should discount these costs by producing fewer individuals of the more costly sex. In the co-operatively breeding Seychelles warbler (Acrocephalus sechellensis) mothers adaptively modify the sex of their single egg toward daughters, the helping sex, when living on territories with rich resources where helpers increase parental reproductive success, but toward sons, the dispersing sex, when living on territories where resources are scarce and/or no helping benefits accrue. By modifying offspring sex ratio, parents maximize their inclusive fitness benefits. Pairs in high-quality territories gained significantly more inclusive fitness benefits (through helping and reproducing offspring) from the production of daughters than from sons, and vice versa in low-quality territories (through reproducing offspring). Experimental manipulation of the offspring’s sex shows that the consequences of sex allocation are adaptive for parents on high-quality territories. On high-quality territories with female production, breeding pairs raising step-daughters gained significantly higher inclusive benefits (through indirect and direct fitness gains) than by raising step-sons. [ABSTRACT FROM AUTHOR]

Social interactions (e.g. aggression, mating and parenting) often depend on the social environment, and recent studies suggest that the adult sex ratio (ASR, the proportion of males in the adult population) modulates these social interactions. While observational studies and phylogenetic analyses provide strong support for the proposition that the relative frequencies of adult males and females impact social decisions, the experimental evidence for the impacts of ASR on social interactions is moot. In this study, we manipulated ASR in dung beetles, Onthophagus taurus , to test the responses of males and females in contest behaviour, courtship behaviour and breeding success. We found that ASR influenced the intensity of both contest and courtship, with both contests and courtships being more frequent in male-biased ASRs than in female-biased and unbiased treatments. The weight of brood balls did not differ significantly between ASR treatments, suggesting that ASR does not affect the breeding success of dung beetles by changing the parental investment of females and males. Moreover, the mean number of brood balls per male was fewer in male-biased treatments than in female-biased and unbiased treatments, while there was no significant difference in the mean number of brood balls per female between treatments, supporting the hypothesis that a male-skewed ASR can impair the breeding success of organisms by influencing the reproductive capacity of males but not females. Our results extend the understanding of the influence of ASR on the reproductive strategy decisions of males and females in breeding success. Future analyses are needed to understand the physiological and genetic implications of mate choice and contest behaviour in the context of a variable social environment, as represented by ASR. • An increased male-biased ASR frequently intensifies contest and courtship behaviour. • ASR shows no impact on parental care, evidenced by consistent brood ball weights. • ASR shifts male, not female, reproductive capacity, impacting breeding success. • There is no ASR effect on the parental care–fecundity trade-off in our experiment. [ABSTRACT FROM AUTHOR]

In cooperatively breeding species, sexually mature individuals may delay natal dispersal and become subordinates, helping a dominant pair raise offspring. To understand how cooperative breeding evolved, it is important to determine the mechanisms leading to delayed dispersal. Adult sex ratio (ASR) variation may affect dispersal by limiting breeding vacancies available to the more abundant sex, and cooperative breeders often have a more biased ASR than noncooperative breeders. However, no studies of cooperative breeders have related ASR at both the local and population level with dispersal. Using the long-term Seychelles warbler, Acrocephalus sechellensis , data set, we tested the influence of population-wide and local ASR, and density, on natal dispersal of yearlings. Our ASR - density hypothesis predicts that the probability of natal dispersal is lower when the ASR is biased towards the sex of the focal individual, but only when the population density is high. Dispersal was associated with population density and population-wide ASR in males, but not in females; males were more likely to delay dispersal when ASR was male biased and density was high. Our findings illustrate a complex association between demographic factors and cooperative breeding and suggest that individuals alter their dispersal behaviour in response to the demographic composition of the population. • Natal dispersal is associated with ASR in a facultative cooperative breeder. • Male but not female yearlings disperse in response to population ASR and density. • Population-wide rather than local demographic composition predicted male dispersal. • Our study links demographic factors with behaviour that drives cooperative breeding. [ABSTRACT FROM AUTHOR]