Parental and first generation effects of exogenous 17beta-estradiol on reproductive performance of female zebra finches (Taeniopygia guttata)

Variation in female reproductive tract morphology across the reproductive cycle in the zebra finch

Background

In seasonally breeding birds, the reproductive tract undergoes a dramatic circannual cycle of recrudescence and regression, with oviduct size increasing 5–220 fold from the non-breeding to the breeding state. Opportunistically breeding birds can produce multiple clutches sequentially across an extended period in response primarily to environmental rather than seasonal cues. In the zebra finch, it has been shown that there is a significant reduction in gonadal morphology in non-breeding females. However, the scale of recrudescence and regression of reproductive tissue within a single breeding cycle is unknown and yet important to understand the cost of breeding, and the physiological readiness to breed in such flexible breeders.

Methods

We examined the reproductive tissue of breeding female zebra finches at six stages in the nesting cycle from pre-breeding to fledging offspring. We quantified the wet mass of the oviduct, the volume of the largest pre-ovulatory follicle, and the total number of pre-ovulatory follicles present on the ovary.

Results

Measures of the female reproductive tract were highest during nesting and laying stages and declined significantly in the later stages of the breeding cycle. Importantly, we found that the mass of reproductive tissue changes as much across a single reproductive event as that previously characterized between birds categorized as breeding and non-breeding. However, the regression of the ovary is less dramatic than that seen in seasonal breeders. This could reflect low-level maintenance of reproductive tissues in opportunistic breeders, but needs to be confirmed in wild non-breeding birds.

Corticosterone implants produce stress-hyporesponsive birds

In birds, the use of corticosterone (Cort) implants is a frequent tool aimed at simulating systemic elevations of this hormone and studying effects on biological traits (e.g. physiology, morphology, behavior). This manipulation may alter adrenocortical function, potentially changing both baseline (Cort_BAS) and stress-induced (Cort_STRESS) plasma Cort levels. However, implant effects on the latter trait are rarely measured, disregarding downstream consequences of potentially altered stress responses. Here, we analyzed the effects of Cort implants on both Cort_BAS and Cort_STRESS in nestling and adult European white storks, Ciconia ciconia In addition, we performed a review of 50 studies using Cort implants in birds during the last two decades to contextualize stork results, assess researchers' patterns of use and infer current study biases. High and low doses of Cort implants resulted in a decrease of both Cort_BAS (31-71% below controls) and Cort_STRESS (63-79% below controls) in storks. Our literature review revealed that Cort_BAS generally increases (72% of experiments) whereas Cort_STRESS decreases (78% of experiments) following implant treatment in birds. Our results challenge and expand the prevailing assumption that Cort implants increase circulating Cort_BAS levels because: (i) Cort_BAS levels show a quadratic association with implant dose across bird species, and decreased levels may occur at both high and low implant doses, and (ii) Cort implants also decrease Cort_STRESS levels, thus producing stress-hyporesponsive phenotypes. It is time to work towards a better understanding of the effects of Cort implants on adrenocortical function, before addressing downstream links to variation in other biological traits.

Artificial selection reveals the energetic expense of producing larger eggs

BACKGROUND

The amount of resources provided by the mother before birth has important and long-lasting effects on offspring fitness. Despite this, there is a large amount of variation in maternal investment seen in natural populations. Life-history theory predicts that this variation is maintained through a trade-off between the benefits of high maternal investment for the offspring and the costs of high investment for the mother. However, the proximate mechanisms underlying these costs of reproduction are not well understood. Here we used artificial selection for high and low maternal egg investment in a precocial bird, the Japanese quail (Coturnix japonica) to quantify costs of maternal reproductive investment.

RESULTS

We show that females from the high maternal investment lines had significantly larger reproductive organs, which explained their overall larger body mass, and resulted in a higher resting metabolic rate (RMR). Contrary to our expectations, this increase in metabolic activity did not lead to a higher level of oxidative damage.

CONCLUSIONS

This study is the first to provide experimental evidence for metabolic costs of increased per offspring investment.

Yolk steroids in great tit Parus major eggs: variation and covariation between hormones and with environmental and parental factors

Abstract

Avian mothers can potentially alter the phenotypes of their offspring by varying the concentration of steroid hormones in their eggs. We explored variation in androstenedione (A4), testosterone (T), 5α-dihydrotestosterone (DHT), 17β-estradiol (E2), and corticosterone (CORT) in the yolks of 12 free-living great tit Parus major clutches. We analyzed variation and covariation in greater detail than previous studies, using models for variation with laying sequence that take into account variable clutch size and comparing correlations between pairs of hormones at the within- and between-clutch levels. We also investigated relationships between hormone levels and various environmental, life history, and parental traits. For three of the five steroids, we found no significant correlates, but based on individual statistical tests (a) DHT varied between clutches with male age (1 year old vs older); (b) DHT and CORT were negatively correlated within clutches with the average temperature on the day (DHT and CORT) or 3 days (DHT only) preceding laying; and (c) DHT in the last egg of the clutch relative to the clutch mean was positively correlated with the interval between clutch completion and the onset of incubation (incubation delay). Relationships with ambient temperature and incubation delay have not previously been reported for any yolk hormone in birds. Intriguingly, the three relationships for DHT are consistent with more DHT being transferred to eggs in situations that could be more energetically challenging for the female. More research is needed to determine the generality of the patterns we found and to understand their functional significance.

Significance statement

The yolks of birds’ eggs contain steroid hormones produced by the mother which can affect the development and behavior of the resultant chicks. We analyzed five steroid hormones in the yolks of wild great tits and show for the first time that yolk hormone levels are related to ambient temperature in the day(s) just before laying and, in the last-laid egg, with the day it is laid relative to the onset of incubation, and that the concentrations of pairs of yolk hormones can vary with each other in a different way between and within clutches. These results contribute insights into the ways in which yolk hormones may adaptively modify the chicks or may reflect physiological processes occurring in the mother.

Electronic supplementary material

The online version of this article (doi:10.1007/s00265-016-2107-1) contains supplementary material, which is available to authorized users.

An assessment of in ovo toxicity of the flame retardant 1,2-dibromo-4-(1,2-dibromoethyl) cyclohexane (TBECH) in the zebra finch

Embryonic toxicity of a brominated flame retardant, TBECH (1,2-dibromo-4-(1, 2-dibromoethyl) cyclohexane) was investigated using the zebra finch (Taeniopygia guttata) as our model. Using in ovo dosing procedures, we injected the technical TBECH compound, consisting of 50:50 α-/β-isomers, directly into the yolk of freshly laid eggs at concentrations of 2.3-94 ng/g egg. No significant effects were observed in terms of growth or survival for either pre-hatch embryos or post-hatch chicks. Analysis of tissue samples at various developmental stages suggests that α-/β-TBECH was metabolized rapidly, and thus is unlikely to cause any direct, long-term effects on the development of zebra finch embryos or offspring.

Effect of estradiol-17β on follicle-stimulating hormone secretion and egg-laying performance of Japanese quail

The aim of this study was to measure the effect of estradiol-17β (E2) injection on follicle-stimulating hormone (FSH) secretion and egg-laying performance of Japanese quail. Female Japanese quail were housed in cages and fed ad libitum. After a 7-day adaptation period, the birds were randomly assigned to three groups, that is, one control group and two test groups. The birds were weighed, before every injection. The control group was subcutaneously injected with 0.2 ml sesame oil-ethanol mixture, whereas test groups were injected, twice in a week, with 0.2 ml sesame oil-ethanol mixture containing 0.1 or 0.2 mg E2 along the study. One day after the first injection, egg number, egg weight, eggshell strength and food conception were daily recorded. On the last day of the experiment, the birds were injected and 3 h later seven birds from each group were randomly selected for bleeding. Blood samples (2 ml/bird) were collected from the jugular vein for the measurements of serum concentrations of E2, FSH, calcium (Ca) and phosphorus (P). E2 injection did not cause any significant changes in serum FSH concentrations, daily egg laid/bird, food conception/bird, serum concentrations of the Ca and the P. Egg weight was significantly increased in the 0.1 mg E2-injected group as compared with the control and 0.2 mg E2-injected groups. Eggshell strength in the 0.2 mg E2-injected group was significantly high as compared with the control, whereas the difference between the 0.1 mg E2- and 0.2 mg E2-injected groups was not statistically important. These results show that serum FSH concentration was not increased even when slightly suppressed by subcutaneous injection of 0.1 or 0.2 mg E2. Different doses of E2 have different functions. The increase in BWs in the 0.1 mg E2-injected group was a result of the dose effect, which probably increased growth hormone secretion from the pituitary or IGF-1 synthesis from the liver or both. The dose, 0.2 mg E2, was ineffective in increasing the BW, but it significantly increased eggshell strength probably via the increase in Ca and P utilizations.

Maternal effects mediated by egg quality in the Yellow-legged Gull Larus michahellis in relation to laying order and embryo sex

BACKGROUND

Maternal effects mediated by egg size and quality may profoundly affect offspring development and performance, and mothers may adjust egg traits according to environmental or social influences. In avian species, context-dependency of maternal effects may result in variation in egg composition, as well as in differential patterns of covariation among selected egg components, according to, for example, position in the laying sequence or offspring sex. We investigated variation in major classes of egg yolk components (carotenoids, vitamins and steroid hormones) in relation to egg size, position in the laying sequence and embryo sex in clutches of the Yellow-legged Gull (Larus michahellis). We also investigated their covariation, to highlight mutual adjustments, maternal constraints or trade-offs in egg allocation.

RESULTS

Laying sequence-specific patterns of allocation emerged: concentration of carotenoids and vitamin E decreased, while concentrations of androgens increased. Vitamin A, estradiol and corticosterone did not show any change. There was no evidence of sex-specific allocation or covariation of yolk components. Concentrations of carotenoids and vitamins were positively correlated. Egg mass decreased along the laying sequence, and this decrease was negatively correlated with the mean concentrations of carotenoids in clutches, suggesting that nutritionally constrained females lay low quality clutches in terms of carotenoid content. Finally, clutches with smaller decline in antioxidants between first- and last-laid eggs had a larger increase in yolk corticosterone, suggesting that a smaller antioxidant depletion along the laying sequence may entail a cost for laying females in terms of increased stress levels.

CONCLUSIONS

Since some of the analyzed yolk components (e.g. testosterone and lutein) are known to exert sex-specific phenotypic effects on the progeny in this species, the lack of sex-specific egg allocation by mothers may either result from trade-offs between contrasting effects of different egg components on male and female offspring, or indicate that sex-specific traits are controlled primarily by mechanisms of sexual differentiation, including endogenous hormone production or metabolism of exogenous antioxidants, during embryonic development.

Phytoestrogens and avian reproduction: Exploring the evolution and function of phytoestrogens and possible role of plant compounds in the breeding ecology of wild birds

Phytoestrogens are secondary plant compounds, which can act to mimic estrogen and cause the disruption of estrogenic responses in organisms. Although there is a substantial body of research studying phytoestrogens, including their mechanisms of estrogenic effects, evolution, and detection in biological systems, little is known about their ecological significance. There is evidence, however, that an ecological relationship involving phytoestrogens exists between plants and animals-plants may produce phytoestrogens to reduce fecundity of organisms that eat them. Birds and other vertebrates may also exploit phytoestrogens to regulate their own reproduction-there are well known examples of phytoestrogens inhibiting reproduction in higher vertebrates, including birds. Also, common plant stressors (e.g., high temperature) increase the production of secondary plant compounds, and, as evidence suggests, also induce phytoestrogen biosynthesis. These observations are consistent with the single study ever done on phytoestrogens and reproduction in wild birds [Leopold, A.S., Erwin, M., Oh, J., Browning, B., 1976. Phytoestrogens adverse effects on reproduction in California quail. Science 191, 98-100.], which found that drought stress correlated with increased levels of phytoestrogens in plants, and that increased phytoestrogen levels correlated with decreased young. This review discusses the hypothesis that plants may have an effect on the reproduction of avian species by producing phytoestrogens as a plant defense against herbivory, and that birds may "use" changing levels of phytoestrogens in the vegetation to ensure that food resources will support potential young produced. Evidence from our laboratory and others appear to support this hypothesis.

Independent effects of song quality and experience with photostimulation on expression of the immediate, early gene ZENK (EGR-1) in the auditory telencephalon of female European starlings

Age influences behavioral decisions such as reproductive timing and effort. In photoperiodic species, such age effects may be mediated, in part, by the individual's age-accrued experience with photostimulation. In female European starlings (Sturnus vulgaris) that do not differ in age, experimental manipulation of photostimulation experience (photoexperience) affects hypothalamic, pituitary, and gonadal activity associated with reproductive development. Does photoexperience also affect activity in forebrain regions involved in processing a social cue, the song of males, which can influence mate choice and reproductive timing in females? Female starlings prefer long songs over short songs in a mate-choice context, and, like that in other songbird species, their auditory telencephalon plays a major role in processing these signals. We manipulated the photoexperience of female starlings, photostimulated them, briefly exposed them to either long or short songs, and quantified the expression of the immediate-early gene ZENK (EGR-1) in the caudomedial nidopallium as a measure of activity in the auditory telencephalon. Using an information theoretic approach, we found higher ZENK immunoreactivity in females with prior photostimulation experience than in females experiencing photostimulation for the first time. We also found that long songs elicited greater ZENK immunoreactivity than short songs did. We did not find an effect of the interaction between photoexperience and song length, suggesting that photoexperience does not affect forebrain ZENK-responsiveness to song quality. Thus, photoexperience affects activity in an area of the forebrain that processes social signals, an effect that we hypothesize mediates, in part, the effects of age on reproductive decisions in photoperiodic songbirds.

Rapid decline in the concentrations of three yolk steroids during development: is it embryonic regulation?

Maternally derived yolk steroids have been found to elicit both short-term and long-term effects on offspring phenotype. Paradoxically, their effects can be strikingly specific given the often substantial concentrations present at oviposition, and they do not appear to uniformly affect all steroid-sensitive processes. To better understand the dynamics of yolk steroids across embryonic development, we quantified levels of progesterone, testosterone, and estradiol at 5-day intervals throughout development in eggs of the red-eared slider turtle (Trachemys scripta) incubated at both male- and female-producing temperatures. We also assessed the effect of season on yolk steroid levels. For all steroids assayed, the concentrations in yolk declined significantly by day 15 of embryonic development despite large differences in initial concentrations among steroids. We found that estradiol was the only steroid whose initial concentration varied significantly with season, while only the decline in testosterone was affected by incubation temperature. These findings illustrate the complex nature of yolk steroid dynamics and suggest that maternal steroids may be rapidly degraded or subject to embryonic processing, emphasizing the need for studies aimed at understanding the mechanisms through which yolk steroids may elicit their effects.

Hematological changes associated with egg production: estrogen dependence and repeatability

The `cost of reproduction' (i.e. the trade-off between current reproduction and future fecundity and/or survival) is a central concept in life history theory, yet we still know very little about the physiological mechanisms underlying such costs. Recently it has been recognized that reproduction itself or the regulatory (hormonal) mechanisms underlying reproduction might result in `costs' (cf. resource-allocation based mechanisms). As one example,it has been suggested that the decrease in hematocrit observed during egg production in birds might be due to antagonistic pleiotropic effects of estrogens. This could generate costs of reproduction by reducing oxygen-carrying capacity during subsequent aerobically demanding stages such as chick-provisioning. Here we show that the reduction in hematocrit during egg-laying is dependent on receptor-mediated actions of endogenous estrogens:blocking estrogen receptors using the anti-estrogen tamoxifen reduces the decrease in hematocrit during egg production in female zebra finches(Taeniopygia guttata) such that hematocrit at the 1-egg stage is not significantly different than pre-breeding, baseline values. We also show that both pre-breeding hematocrit and the decrease in hematocrit associated with egg production are repeatable, and that females with the highest pre-breeding hematocrit values tend to show the largest decreases in hematocrit during egg production. We suggest that hematological changes during egg production are a good candidate mechanism for a regulatory-network based trade-off involving antagonistic pleiotropic effects of estrogens, which otherwise have essential reproductive functions.

Experimental evidence that corticosterone affects offspring sex ratios in quail

Recent studies have shown that some species of birds have a remarkable degree of control over the sex ratio of offspring they produce. However, the mechanism by which they achieve this feat is unknown. Hormones circulating in the breeding female are particularly sensitive to environmental perturbations, and so could provide a mechanism for her to bias the sex ratio of her offspring in favour of the sex that would derive greatest benefit from the prevailing environmental conditions. Here, we present details of an experiment in which we manipulated levels of testosterone, 17beta-oestradiol and corticosterone in breeding female Japanese quail (Coturnix coturnix japonica) using Silastic implants and looked for effects on the sex ratio of offspring produced. Offspring sex ratio in this species was significantly correlated with faecal concentrations of the principal avian stress hormone, corticosterone, and artificially elevated levels of corticosterone resulted in significantly female-biased sex ratios at laying. Varying testosterone and 17beta-oestradiol had no effect on sex ratio alone, and faecal levels of these hormones did not vary in response to corticosterone. Our results suggest that corticosterone may be part of the sex-biasing process in birds.

Laying-sequence-specific variation in yolk oestrogen levels, and relationship to plasma oestrogen in female zebra finches (Taeniopygia guttata)

We investigated the relationship between plasma and yolk oestrogens in laying female zebra finches (Taeniopygia guttata) by manipulating plasma oestradiol (E2) levels, via injection of oestradiol-17beta, in a sequence-specific manner to maintain chronically high plasma levels for later-developing eggs (contrasting with the endogenous pattern of decreasing plasma E2 concentrations during laying). We report systematic variation in yolk oestrogen concentrations, in relation to laying sequence, similar to that widely reported for androgenic steroids. In sham-manipulated females, yolk E2 concentrations decreased with laying sequence. However, in E2-treated females plasma E2 levels were higher during the period of rapid yolk development of later-laid eggs, compared with control females. As a consequence, we reversed the laying-sequence-specific pattern of yolk E2: in E2-treated females, yolk E2 concentrations increased with laying-sequence. In general therefore, yolk E2 levels were a direct reflection of plasma E2 levels. However, in control females there was some inter-individual variability in the endogenous pattern of plasma E2 levels through the laying cycle which could generate variation in sequence-specific patterns of yolk hormone levels even if these primarily reflect circulating steroid levels.

Prior experience with photostimulation enhances photo-induced reproductive development in female European starlings: a possible basis for the age-related increase in avian reproductive performance

Reproductive performance in female birds improves with age, and this is generally attributed to experiences obtained during breeding. In temperate-zone species, experience with photostimulation during the first breeding year may prime the hypothalamo-pituitary-gonadal axis to respond to photic cues more rapidly or robustly in subsequent years. To test this idea, we captured 32 photorefractory juvenile (hence naive to photostimulation) female European starlings (Sturnus vulgaris) and held half of them (naive group) on a photoperiod of 8L:16D for 32 wk and the other half (experienced group) on 8L:16D for 12 wk, 16L:8D for 12 wk, and then 8L:16D for 8 wk. When we subsequently transferred all birds to 16L:8D, the increase in body mass, which may presage egg laying in the wild, was more robust in experienced than in naive females. Experienced females also showed a more robust elevation in plasma concentrations of the yolk-precursor protein vitellogenin, although naive females showed an initial rapid but transient rise in vitellogenin that we attribute to their extended exposure to short-day photoperiods prior to photostimulation. Finally, the photo-induced increase in diameter of the largest ovarian follicle, in plasma concentrations of luteinizing hormone, and in the number of septo-preoptic fibers relative to the number of cell bodies immunoreactive to GnRH was greater in experienced than in naive females. Thus, prior experience with photostimulation enhances some initial phases of photo-induced reproductive development and may explain, in part, why reproductive performance improves with age in temperate-zone birds.

Effects of 17-beta-estradiol treatment of female zebra finches on offspring sex ratio and survival

Treatment of female zebra finches (Taeniopygia guttata) with 17-beta-estradiol leads to a female-biased sex ratio in their offspring at the age of independence [Horm. Behav. 35 (1999) 135]. It is unclear whether this is due to a bias of the primary sex ratio or to sex-specific survival. We replicated this experiment and found again a significantly higher total number of daughters than sons at independence in the estradiol-treated group. This was due to higher embryonic survival of daughters compared with sons in the estradiol-treated group and the reverse in the control group. There was no effect of the hormone treatment on the primary sex ratio. Treatment with 17-beta-estradiol led to a significantly shorter hatching time and to heavier offspring at day 7 after hatching. This weight was correlated with maternal plasma estradiol levels on the day of the first egg, which were significantly higher in the estradiol-treated group than in the control group. The results do not support the idea that maternal estradiol levels influence the primary sex ratio. They indicate that maternal estradiol differentially affects survival of sons and daughters via an influence on the embryonic environment, possibly enhancing offspring growth.

Individual variation in plasma estradiol-17beta and androgen levels during egg formation in the European starling Sturnus vulgaris: implications for regulation of yolk steroids

While it is clear that maternal transfer of steroids to egg yolk can have significant effects on offspring phenotype, an unresolved question is whether females can facultatively adjust yolk hormone levels independently of their own plasma levels or whether yolk steroid levels are simply a direct consequence of temporal variation in the female's hormonal status. In part, this is because we lack detailed information about the day-to-day pattern of changes in plasma hormone levels during the laying cycle for non-domesticated birds. Here, we describe changes in plasma estradiol-17beta (E2) and androgens, throughout laying in relation to specific stages of ovarian follicular development in the European starling (Sturnus vulgaris). Plasma E2 levels increased rapidly from the onset of rapid yolk development (RYD) to reach maximum levels in birds with a complete follicle hierarchy (> or = 4 yolky follicles). However, levels decreased linearly throughout the later stages of follicle development returning to pre-breeding values before the final yolky follicle was ovulated. In females with > or = 4 yolky follicles there was 10-fold variation in plasma E2 levels among individual females, but this was not related to plasma levels of the main yolk precursor vitellogenin or to the total mass of yolky follicles developing at the time of blood sampling. In contrast to E2, plasma androgen levels showed only a very gradual linear decline throughout the laying cycle from pre-RYD to clutch completion. Furthermore, androgen levels showed less individual variability: 4-fold variation among females with > or = 4 yolky follicles, although this was also independent of our measures of reproductive function. Data on inter- and intra-individual variation in female hormone levels are important to set-up a priori predictions for, and interpretation of, studies of yolk hormone levels.

Primary sex ratios in birds: problems with molecular sex identification of undeveloped eggs

Sex allocation studies seek to ascertain whether mothers manipulate offspring sex ratio prior to ovulation. To do so, DNA for molecular sexing should be collected as soon after conception as possible, but instead neonates are usually sampled. Here, we aim to identify and quantify some of the problems associated with using molecular techniques to identify the sex of newly laid avian eggs. From both fertilized and unfertilized chicken (Gallus gallus) eggs, we sampled (1) the blastoderm/disc, (2) vitelline membrane and (3) a mixture of (1) and (2). Thus, we replicated scenarios under which contaminated samples are taken and/or unfertilized eggs are not identified as such and are sampled. We found that two commonly used molecular sexing tests, based on the CHD-1 genes, differed in sensitivity, but this did not always predict their ability to sex egg samples. The vitelline membrane was a considerable source of maternal and probably paternal contamination. Fertile eggs were regularly assigned the wrong sex when vitelline membrane contaminated the blastoderm sample. The membrane of unfertilized eggs was always female, i.e. maternal DNA had been amplified. DNA was amplified from 47 to 63% of unfertilized blastodiscs, even though it was highly unlikely that DNA from a single haploid cell could be amplified reliably using these polymerase chain reaction (PCR) techniques. Surprisingly, the blastodiscs were identified as both males and females. We suggest that in these cases only maternal DNA was amplified, and that 'false' males, Z not ZZ, were detected. This was due to the reduced ability of both sets of primers to anneal to the W chromosome compared to the Z chromosome at low DNA concentrations. Overall, our data suggested that estimates of primary sex ratios based on newly laid eggs will be appreciably inaccurate.

Potential mechanisms of avian sex manipulation

The aim of this review is to consider the potential mechanisms birds may use to manipulate the sex of their progeny, and the possible role played by maternal hormones. Over the past few years there has been a surge of reports documenting the ability of birds to overcome the rigid process of chromosomal sex determination. However, while many of these studies leave us in little doubt that mechanisms allowing birds to achieve this feat do exist, we are only left with tantalizing suggestions as to what the precise mechanism or mechanisms may be. The quest to elucidate them is made no easier by the fact that a variety of environmental conditions have been invoked in relation to sex manipulation, and there is no reason to assume that any particular mechanism is conserved among the vast diversity of species that can achieve it. In fact, a number of intriguing proposals have been put forward. We begin by briefly reviewing some of the most recent examples of this phenomenon before highlighting some of the more plausible mechanisms, drawing on recent work from a variety of taxa. In birds, females are the heterogametic sex and so non-Mendelian segregation of the sex chromosomes could conceivably be under maternal control. Another suggestion is that follicles that ultimately give rise to males and females grow at different rates. Alternatively, the female might selectively abort embryos or 'dump lay' eggs of a particular sex, deny certain ova a chance of ovulation, fertilization or zygote formation, or selectively provision eggs so that there is sex-specific embryonic mortality. The ideas outlined in this review provide good starting points for testing the hypotheses both experimentally (behaviourally and physiologically) and theoretically.

Pre-ovulation control of hatchling sex ratio in the Seychelles warbler

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.

Adaptive sex allocation in birds: the complexities of linking theory and practice

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 effects of natural and synthetic steroid estrogens in relation to their environmental occurrence

Laboratory-based acute toxicity data and physiological studies relating to natural and synthetic steroid estrogens in a range of animals and plants are reviewed. Steroid estrogens may induce adverse effects in animals that do or do not express the estrogen receptor, and in plants, and they may mimic other hormones or induce nonestrogenic effects. Although the findings of such studies should be treated with caution when extrapolated to possible environmental effects, the available data indicate that a wide range of effects may be manifested in a diversity of species. The environmental occurrence of the compounds and possible environmental exposure routes are also reviewed and discussed in relation to the laboratory-based acute toxicity data. While there are likely to be difficulties in relating some of the observed laboratory data to possible environmental effects, studies undertaken on fish are directly relevant because exposure pathways and concentrations were related to those occurring in the environment. Effects that may occur in the environment are discussed in relation to their significance to the individual and at the species level.

Avian egg size: variation within species and inflexibility within individuals

Egg size is a widely-studied trait and yet the causes and consequences of variation in this trait remain poorly understood. Egg size varies greatly within many avian species, with the largest egg in a population generally being at least 50% bigger, and sometimes twice as large, as the smallest. Generally, approximately 70% of the variation in egg mass is due to variation between rather than within clutches, although there are some cases of extreme intra-clutch egg-size variation. Despite the large amount of variation in egg size between females, this trait is highly consistent within individuals between breeding attempts; the repeatability of egg size is generally above 0.6 and tends to be higher than that of clutch size or laying date. Heritability estimates also tend to be much higher for egg size (> 0.5) than for clutch size or laying date (< 0.5). As expected, given the high repeatability and heritability of egg size, supplemental food had no statistically significant effect on this trait in 18 out of 28 (64%) studies. Where dietary supplements do increase egg size, the effect is never more than 13% of the control values and is generally much less. Similarly, ambient temperature during egg formation generally explains less than 15% of the variation in egg size. In short, egg size appears to be a characteristic of individual females, and yet the traits of a female that determine egg size are not clear. Although egg size often increases with female age (17 out of 37 studies), the change in egg size is generally less than 10%. Female mass and size rarely explain more than 20% of the variation in egg size within species. A female's egg size is not consistently related to other aspects of reproductive performance such as clutch size, laying date, or the pair's ability to rear young. Physiological characteristics of the female (e.g. endogenous protein stores, oviduct mass, rate of protein uptake by ovarian follicles) show more promise as potential determinants of egg size. With regards to the consequences of egg-size variation for offspring fitness, egg size is often correlated with offspring mass and size within the first week after hatching, but the evidence for more long-lasting effects on chick growth and survival is equivocal. In other oviparous vertebrates, the magnitude of egg-size variation within populations is often as great or greater than that observed within avian populations. Although there are much fewer estimates of the repeatability of egg size in other taxa, the available evidence suggests that egg size may be more flexible within individuals. Furthermore, in non-avian species (particularly fish and turtles), it is more common for female mass or size to explain a substantial proportion of the variation in egg size. Further research into the physiological basis of egg-size variation is needed to shed light on both the proximate and ultimate causes of intraspecific variation in this trait in birds.

Experimental manipulation of female reproduction reveals an intraspecific egg size-clutch size trade-off

A negative relationship, or trade-off, between egg size and clutch size is a central and long-standing component of life-history theory, yet there is little empirical evidence for such a trade-off, especially at the intraspecific level. Here, I show that female zebra finches (Taeniopygia guttata) treated chronically during egg formation with the anti-oestrogen tamoxifen lay smaller eggs (by 8%) but produce larger clutches (on average two eggs more) than controls. Decreased egg mass in tamoxifen-treated females was associated with a 50% decrease in plasma levels of the two yolk precursors, vitellogenin and very-low-density lipoprotein. Although tamoxifen-treated females laid more, smaller eggs (and had a higher total expenditure in their clutch), they did not differ from controls in the number of chicks fledged, the mass or size of these chicks at fledging, or the chicks' egg-production performance at three months of age. However, tamoxifen-treated females had lower relative hatching success: they laid more eggs but hatched the same number of chicks. Among individual tamoxifen-treated females, birds that laid the smallest eggs early in their laying sequence laid the largest number of additional eggs, that is, there was a negative correlation, or trade-off, between egg size and clutch size.

Experimental (tamoxifen-induced) manipulation of female reproduction in zebra finches (Taeniopygia guttata)

Experimental manipulation of reproductive phenotype is a potentially powerful approach for understanding the fitness relationships of traits such as egg size, egg composition, and egg number. In this study, I investigated the effect of the antiestrogen tamoxifen on multiple, estrogen-dependent reproductive traits in female zebra finches (Taeniopygia guttata). Short-term tamoxifen treatment of egg-laying females (two daily injections before laying) had no effect on the timing or the pattern of egg laying compared to sham controls. However, tamoxifen treatment caused (1) a marked, but transient, decrease in egg size; (2) increased within-clutch egg-size variation; (3) a reduction in plasma vitellogenin (VTG) levels; and (4) lower dry yolk and yolk protein content of tamoxifen-treated females. Plasma levels of the second yolk precursor, very low density lipoprotein (VLDL), were not affected by tamoxifen, and tamoxifen appeared to have no effect on oviduct function in egg-laying females. These results are consistent with tamoxifen blocking estrogen receptors in the liver, suppressing VTG production, and decreasing the plasma pool of yolk precursors below a level required to maintain yolk formation at the normal rate. Tamoxifen treatment can therefore be used successfully to manipulate several components of the female reproductive phenotype (egg composition, intraclutch egg-size variation) to further explore the fitness consequences of these traits.

Effects of exogenous 17(β)-estradiol on the reproductive physiology and reproductive performance of european starlings (Sturnus vulgaris)

Egg mass shows large intraspecific variation in birds and yet the mechanisms underlying this variation remain unknown. We hypothesized that estradiol would play a central role in determining egg mass, since this hormone stimulates the production of yolk precursors (vitellogenin and very-low density lipoprotein, VLDL) by the liver, and of albumen by the oviduct. We gave European starlings (Sturnus vulgaris) silastic implants containing estradiol prior to egg formation, which we predicted would increase egg mass. As expected, exogenous estradiol stimulated a marked (49 %) increase in plasma vitellogenin levels at the beginning of laying. At clutch completion, plasma VLDL levels and oviduct mass were also elevated in estradiol-treated females compared with controls. However, estradiol had no effect on fresh egg mass or clutch size. Estradiol treatment actually decreased the mass of yolk protein and lipid, perhaps by decreasing the rate of uptake of yolk precursors at the ovary. The failure of estradiol to increase egg mass indicates that this phenotype may be regulated at higher levels of organization (e.g. negative feedback, uptake of yolk precursors) than those studied in this experiment. Despite elevating yolk precursor levels, treatment with estradiol had no effect on the mass of the liver or endogenous stores of protein and lipid at clutch completion.