Yolk antioxidants vary with male attractiveness and female condition in the house finch (Carpodacus mexicanus)

The manipulation of egg content is one of the few ways by which female birds can alter offspring quality before hatch. Lipid-soluble vitamins and carotenoids are potent antioxidants. Female birds deposit these antioxidants into eggs in variable amounts according to environmental and social conditions, and the quantities deposited into eggs can have effects on offspring health and immunological condition. Allocation theory posits that females will alter the distribution of resources according to mate quality, sometimes allocating resources according to the differential allocation hypothesis (DAH), investing more in offspring sired by better-quality males, and other times allocating resources according to a compensatory strategy, enhancing the quality of offspring sired by lower-quality males. It is unknown, however, whether antioxidants are deposited into eggs according to the DAH or a compensatory strategy. We examined deposition patterns of yolk antioxidants (including vitamin E and three carotenoids) in relation to laying order, mate attractiveness, female condition, and yolk androgen content in the house finch (Carpodacus mexicanus). Female house finches deposited significantly more total antioxidants into eggs sired by less attractive males. Additionally, yolk antioxidant content was significantly positively correlated with female condition, which suggests a cost associated with the deposition of antioxidants into eggs. Finally, concentrations of antioxidants in egg yolks were positively correlated with total yolk androgen content. We suggest that yolk antioxidants are deposited according to a compensatory deposition strategy, enabling females to improve the quality of young produced with less attractive males. Additionally, yolk antioxidants may act to counter some of the detrimental effects associated with high levels of yolk androgens in eggs and, thus, may exert a complementary effect to yolk androgens.     

Laying‐sequence variation in yolk carotenoids and egg characteristics in the red‐winged blackbird Agelaius phoeniceus

In many bird species with asynchronous hatching, smaller, later‐hatched nestlings are out‐competed for food by their larger, earlier‐hatched siblings and therefore suffer increased mortality via starvation. It is thought that female birds can either maintain or reduce the survival disadvantage of later‐hatched nestlings by differentially allocating maternal resources across the eggs of a clutch. Carotenoid pigments are an example of resources that female birds allocate differentially when producing a clutch, but laying sequence patterns for these pigments remain poorly studied in North American songbirds. We examined intraclutch variation in yolk carotenoids and egg metrics in 27 full clutches of red‐winged blackbird Agelaius phoeniceus eggs collected from eight wetlands in central Alberta, Canada. We predicted that carotenoids would decrease across the laying sequence, as in this species, later‐hatched, marginal nestlings suffer greater mortality than earlier‐hatched, core nestlings. We found nine carotenoid pigments in red‐winged blackbird egg yolks, including two that have never been described from avian yolks: α‐doradexanthin and adonirubin. As predicted, concentrations and amounts of most carotenoids decreased across the laying sequence, suggesting that female red‐winged blackbirds depleted their carotenoid resources as they laid more eggs. However, egg mass and yolk mass both increased across the laying sequence, suggesting that female red‐winged blackbirds may use other maternal resources to compensate for the size and survival disadvantage experienced by later‐hatched, marginal nestlings.     

Social environment affects female and egg testosterone levels in the house sparrow (Passer domesticus)

Maternal effects can have an adaptive value if they improve the performance of offspring. As such, the transfer of maternal testosterone (T) to the eggs has been suggested as a mechanism for adaptive maternal control of offspring phenotype in birds, although recent studies have shown negative effects of testosterone on hatching rate and chick survival. Here, we experimentally investigated whether socially stressful conditions experienced by female house sparrows during egg laying affected their circulating levels of androgens and the amount transferred to the eggs. Social stress was simulated by the intrusion of a foreign male placed near the nest box during the egg‐laying sequence. We found that (1) both female and yolk testosterone titres were positively related to breeding density; (2) yolk testosterone was negatively correlated with maternal testosterone; (3) yolk testosterone was positively correlated with the behavioural response of females towards the intruder and (4) the interaction between social intrusion and breeding density affected the amount of testosterone transferred to the eggs. Altogether, our results suggest that females may be able to modulate the amount of testosterone they allocate to their eggs according to the social environment they experience during egg laying.     

Individual quality and food availability determine yolk and egg mass and egg composition in tree swallows Tachycineta bicolor

Variation in egg size and composition can have important consequences for the quality of offspring. We investigated the factors influencing the yolk mass and egg mass of tree swallows breeding in Ithaca, NY. Using a nondestructive technique to estimate yolk mass via standardized digital-candler photographs, we compared yolk size and egg size of tree swallows Tachycineta bicolor in response to variation in food availability and individual quality. Insect availability one to three days prior to laying, but not four to six days, predicted yolk mass, while insect availability two to three days prior to laying predicted total egg mass. This suggests that, while eggs are formed over longer periods, food availability closer to time of laying has the greatest influence on egg size. These results were supported by collected eggs, as yolk rings revealed that tree swallow eggs are formed over 5–6 days. There was an influence of female quality as well, with early laying birds, independent of food availability, laying larger eggs. Eggs laid later in the laying sequence had larger yolks and greater egg mass. Overall, variation in egg quality appears to be due to a combination of environmental conditions, reflected in food resources, individual quality, and allocation tradeoffs during the laying period.     

Eggshell Spottiness Reflects Maternally Transferred Antibodies in Blue Tits

Blue-green and brown-spotted eggshells in birds have been proposed as sexual signals of female physiological condition and egg quality, reflecting maternal investment in the egg. Testing this hypothesis requires linking eggshell coloration to egg content, which is lacking for brown protoporphyrin-based pigmentation. As protoporphyrins can induce oxidative stress, and a large amount in eggshells should indicate either high female and egg quality if it reflects the female''s high oxidative tolerance, or conversely poor quality if it reflects female physiological stress. Different studies supported either predictions but are difficult to compare given the methodological differences in eggshell-spottiness measurements. Using the blue tit Cyanistes caeruleus as a model species, we aimed at disentangling both predictions in testing if brown-spotted eggshell could reflect the quality of maternal investment in antibodies and carotenoids in the egg, and at improving between-study comparisons in correlating several common measurements of eggshell coloration (spectral and digital measures, spotted surface, pigmentation indices). We found that these color variables were weakly correlated highlighting the need for comparable quantitative measurements between studies and for multivariate regressions incorporating several eggshell-color characteristics. When evaluating the potential signaling function of brown-spotted eggshells, we thus searched for the brown eggshell-color variables that best predicted the maternal transfer of antibodies and carotenoids to egg yolks. We also tested the effects of several parental traits and breeding parameters potentially affecting this transfer. While eggshell coloration did not relate to yolk carotenoids, the eggs with larger and less evenly-distributed spots had higher antibody concentrations, suggesting that both the quantity and distribution of brown pigments reflected the transfer of maternal immune compounds in egg yolks. As yolk antibody concentrations were also positively related to key proxies of maternal quality (egg volume, number, yellow feather brightness, tarsus length), eggshells with larger spots concentrated at their broad pole may indicate higher-quality eggs.     

Female variation in allocation of steroid hormones,antioxidants and fatty acids: a multilevel analysis in a wild passerine bird

The environment where an embryo develops can be influenced by components of maternal origin, which can shape offspring phenotypes and therefore maternal fitness. In birds that produce more than one egg per clutch, females differ in the concentration of components they allocate into the yolk along the laying sequence. However, identification of processes that shape female yolk allocation and thus offspring phenotype still remains a major challenge within evolutionary ecology. A way to increase our understanding is by acknowledging that allocation patterns can differ depending on the level of analysis, such as the population versus the among‐female (within‐population) level. We employed mixed models to analyze at both levels the variation in allocation along the laying sequence of four steroid hormones, three antioxidants, and four groups of fatty acids present in the egg yolks of wild great tits Parus major. We also quantified repeatabilities for each component to study female consistency. At a population level, the concentrations/proportions of five yolk components varied along the laying sequence, implying that the developmental environment is different for offspring developing in first versus last eggs. Females varied substantially in the mean allocation of components and in their plasticity along the laying sequence. For most components, these two parameters were negatively correlated. Females were also remarkably repeatable in their allocation. Overall, our data emphasize the need to account for female variation in yolk allocation along the laying sequence at multiple levels, as variation at a population level is underpinned by different individual patterns. Our findings also highlight the importance of considering both levels of analysis in future studies investigating the causes and fitness consequences of yolk compounds. Finally, our results on female repeatability confirm that analyzing one egg per nest is a suitable way to address the consequences of yolk resource deposition for the offspring.     

Elevated testosterone levels affect female breeding success and yolk androgen deposition in a passerine bird

Although it is well documented that testosterone (T) is an important mediator in the regulation of behaviour in male vertebrates, its functional significance in females is less understood. Experimentally increased T in adult female birds has been found to have both advantageous and detrimental effects on behaviour and fitness. In addition, T may also mediate maternal effects when it is deposited into the egg yolk, and variations in androgen concentration between eggs contribute to differences in offspring phenotype and fitness. In this study we examined the effects of experimentally elevated female T on reproductive success and yolk androgen deposition in the spotless starling. The administration of exogenous T in female spotless starlings before egg laying caused negative effects on reproductive performance: when compared to control females T-females laid fewer eggs and raised fewer chicks. We also found an effect of elevated female T on yolk androgen deposition: T-females laid eggs with greater amounts of yolk T than control females, whereas yolk androstenedione levels were not affected. Although some of these effects likely involved a direct interference of female T with female reproductive function, some of them could be due to effects operating in eggs through maladaptive high T levels.     

Elevated corticosterone during egg production elicits increased maternal investment and promotes nestling growth in a wild songbird

Glucocorticoids circulating in breeding birds during egg production accumulate within eggs, and may provide a potent form of maternal effect on offspring phenotype. However, whether these steroids affect offspring development remains unclear. Here, we employed a non-invasive technique that experimentally elevated the maternal transfer of corticosterone to eggs in a wild population of house wrens. Feeding corticosterone-injected mealworms to free-living females prior to and during egg production increased the number of eggs that females produced and increased corticosterone concentrations in egg yolks. This treatment also resulted in an increase in the amount of yolk allocated to eggs. Offspring hatching from these eggs begged for food at a higher rate than control offspring and eventually attained increased prefledging body condition, a trait predictive of their probability of recruitment as breeding adults in the study population. Our results indicate that an increase in maternal glucocorticoids within the physiological range can enhance maternal investment and offspring development.     

Transgenerational immunity in a bird–ectoparasite system: do maternally transferred antibodies affect parasite fecundity or the offspring's susceptibility to fleas?

During egg formation, female birds deposit antibodies against parasites and pathogens they were exposed to before egg laying into the yolk. In captive bird species, it has been shown that these maternal immunoglobulins (maternal yolk IgGs) can protect newly hatched offspring against infection. However, direct evidence for such benefits in wild birds is hitherto lacking. We investigated (1) if nestling Great Tits Parus major originating from eggs with naturally high levels of maternal yolk IgG are less susceptible to a common, nest-based ectoparasite, (2) if maternal yolk IgGs influence nestling development and in particular, their own immune defence, and (3) if there is a negative correlation between levels of maternal yolk IgG in host eggs and the reproductive success of ectoparasitic fleas feeding on the nestlings. Counter to expectations, we found no indication that maternally transferred yolk IgGs have direct beneficial effects on nestling development, nestling immune response or nestling resistance or tolerance to fleas. Furthermore, we found no negative correlation between host yolk IgG levels and parasite fecundity. Thus, whereas previous work has unequivocally shown that prenatal maternal effects play a crucial role in shaping the parasite resistance of nestling birds, our study indicates that other egg components, such as hormones, carotenoids or other immuno-active substances, which bird females can adjust more quickly than yolk IgG, might mediate these effects.     

Elevated plasma corticosterone decreases yolk testosterone and progesterone in chickens: linking maternal stress and hormone-mediated maternal effects

Despite considerable research on hormone-mediated maternal effects in birds, the underlying physiology remains poorly understood. This study investigated a potential regulation mechanism for differential accumulation of gonadal hormones in bird eggs. Across vertebrates, glucocorticoids can suppress reproduction by downregulating gonadal hormones. Using the chicken as a model species, we therefore tested whether elevated levels of plasma corticosterone in female birds influence the production of gonadal steroids by the ovarian follicles and thus the amount of reproductive hormones in the egg yolk. Adult laying hens of two different strains (ISA brown and white Leghorn) were implanted subcutaneously with corticosterone pellets that elevated plasma corticosterone concentrations over a period of nine days. Steroid hormones were subsequently quantified in plasma and yolk. Corticosterone-implanted hens of both strains had lower plasma progesterone and testosterone levels and their yolks contained less progesterone and testosterone. The treatment also reduced egg and yolk mass. Plasma estrogen concentrations decreased in white Leghorns only whereas in both strains yolk estrogens were unaffected. Our results demonstrate for the first time that maternal plasma corticosterone levels influence reproductive hormone concentrations in the yolk. Maternal corticosterone could therefore mediate environmentally induced changes in yolk gonadal hormone concentrations. In addition, stressful situations experienced by the bird mother might affect the offspring via reduced amounts of reproductive hormones present in the egg as well as available nutrients for the embryo.     

Transfer of Maternal Antibodies against Avian Influenza Virus in Mallards (Anas platyrhynchos)

Maternal antibodies protect chicks from infection with pathogens early in life and may impact pathogen dynamics due to the alteration of the proportion of susceptible individuals in a population. We investigated the transfer of maternal antibodies against avian influenza virus (AIV) in a key AIV host species, the mallard (Anas platyrhynchos). Combining observations in both the field and in mallards kept in captivity, we connected maternal AIV antibody concentrations in eggs to (i) female body condition, (ii) female AIV antibody concentration, (iii) egg laying order, (iv) egg size and (v) embryo sex. We applied maternity analysis to the eggs collected in the field to account for intraspecific nest parasitism, which is reportedly high in Anseriformes, detecting parasitic eggs in one out of eight clutches. AIV antibody prevalence in free-living and captive females was respectively 48% and 56%, with 43% and 24% of the eggs receiving these antibodies maternally. In both field and captive study, maternal AIV antibody concentrations in egg yolk correlated positively with circulating AIV antibody concentrations in females. In the captive study, yolk AIV antibody concentrations correlated positively with egg laying order. Female body mass and egg size from the field and captive study, and embryos sex from the field study were not associated with maternal AIV antibody concentrations in eggs. Our study indicates that maternal AIV antibody transfer may potentially play an important role in shaping AIV infection dynamics in mallards.     

Egg colour reflects the amount of yolk maternal antibodies and fledging success in a songbird

Blue-green colours in avian eggs have been proposed as post-mating signals of female phenotypic quality to their mates. Egg colour may not only indicate female value, but also the quality of the eggs themselves and of resulting offspring. To date, there has been no demonstration of an association between egg colour and egg or offspring immune quality. We here show that the intensity of blue-green colour of pied flycatcher Ficedula hypoleuca eggs reflects the amount of maternal antibodies in the yolk, a form of passive immunity crucial for offspring performance. Moreover, egg colour predicts fledging success. Also, incubating females in better condition lay more colourful eggs. The immunoglobulin level in incubating females is positively associated with that in the eggs. These results support the signalling hypothesis of eggshell coloration, underlining its role as an indicator of trans-generational transmission of immune defences in birds.     

Maternal resource variation across the laying sequence in Canada geese Branta canadensis maxima

Variation in maternal investments to offspring presumably reflects an optimization of resource allocation such that a female's fitness is maximized. In birds, both egg size and yolk constituents are examples of resources that can vary among offspring within a clutch. Egg size and maternally-derived steroid hormone concentrations present in yolk have been characterized for many species that lay small clutches or have altricial young, but little information is available for species that lay moderate to large clutches of precocial young. In this study, we recorded laying position, measured fresh egg mass and determined maternally-derived testosterone and estradiol concentrations present in yolks for whole clutches of free-living Canada geese Branta canadensis maxima to assess variation in maternal resources within clutches. We found that egg size varied non-linearly across the laying sequence such that first laid eggs were small, the largest eggs in the clutch occurred in the second and third positions, and size declined in eggs laid in subsequent positions. Concentration of testosterone in the yolk followed a pattern in which the first and second laid eggs have the highest concentrations within a clutch and declining concentrations in subsequently laid eggs. In contrast, maternally-derived yolk estradiol concentrations (measured in a subset of clutches) did not change across the laying sequence.     

Maternal androgens in the pied flycatcher: timing of breeding and within-female consistency

Maternal hormones can have substantial phenotypic effects in the progeny of many vertebrates. It has been proposed that mothers adaptively adjust hormone levels experienced by particular young to optimize their reproductive output. In birds, systematic variation in egg hormone levels has been related to different female reproductive strategies. Because in many bird species prospects of the offspring change seasonally and with brood number, strategic adjustment of yolk androgen levels would be expected. To test this idea, we induced pied flycatcher (Ficedula hypoleuca) females to nest twice during the same season by removing their first clutches shortly after clutch completion. We collected eggs of first and replacement clutches to measure yolk concentrations of androstenedione (A4) and testosterone (T) and captured the females that laid these clutches for phenotypic measurements. Although average egg androgen levels were remarkably consistent within females, hormone patterns differed considerably between first and replacement clutches. Eggs of replacement clutches were heavier with larger yolks compared to first clutches, but they contained on average lower levels of androgens. Within clutches, androgen concentration increased over the laying sequence in the first clutch, but decreased or remained more constant over the laying sequence in the replacement clutch. Mean yolk T, but not A4 levels, were negatively associated with laying date for both breeding attempts. Moreover, females in good body condition produced eggs containing lower levels of androgens than females in poor condition. Our results are consistent with the idea that differences in yolk androgen levels may be one mechanism underlying seasonal variation in reproductive success and it is possible that changes in egg androgen patterns may reflect a change in female reproductive strategy. High within-female consistency also highlights the possibility that there may be some underlying genetic variation in yolk androgen levels.     

Egg quality and parental ornamentation in the blue tit Parus caeruleus

Components of avian egg quality often show remarkable variation between females and yet the causes of variation in this fitness-related trait remain poorly understood. We investigated the egg size and yolk carotenoid investment of blue tit Parus caeruleus females in relation to multiple parental traits. We show that females produce eggs with more colourful yolks when mated to ultraviolet (UV) attractive males. Since yolk carotenoids may enhance offspring fitness, but may be available to females in limiting amounts, the correlation we found suggests adaptive egg quality adjustment in response to mate attractiveness, but an experimental approach is required to rule out alternative explanations. Yolk colour was also correlated with laying date and yolk mass. Though the effect of laying date could not be explained by ambient temperature during egg formation, it suggests a proximate constraint of general carotenoid availability on yolk composition. Egg size was not affected by male attractiveness; however, females with a brighter crown produced larger eggs, suggesting that crown coloration might indicate individual quality in females too.     

Consequences of prenatal androgen exposure for the reproductive performance of female pheasants (Phasianus colchicus)

Maternal hormones in vertebrate eggs can mediate important forms of maternal effects. However, the function of hormone transfer to the eggs is still debated, especially because long-term fitness consequences have been little studied. We investigated the effect of prenatal exposure to physiologically elevated yolk testosterone (T) levels on reproduction of female pheasants (Phasianus colchicus) in captivity. We found that females hatching from T-injected eggs (T-females) had a lower egg-laying rate than controls, and their eggs were more frequently infertile than those laid by control females. There were no effects of prenatal maternal treatment on egg size and yolk T concentration, but eggs carrying a female embryo laid by T-females had smaller yolks than eggs with a male embryo, while there was no sex difference in yolk size among the eggs laid by control females. Progeny sex ratio was unaffected by maternal treatment. These findings suggest that the transfer of high androgen levels to the eggs by the mother is constrained by complex trade-offs between direct effects on her daughters' reproduction and by trans-generational differential consequences on male and female descendants.     

Sexual dimorphism of egg size in the European Blackbird Turdus merula

Recently, a number of studies have shown that female birds are able to control the sex of their progeny at the stage of the gamete. There is also some evidence that females adjust their investment in offspring depending on the sex of the embryo during egg formation. Differential maternal investment to the eggs depending on their sex is usually interpreted as an adaptive strategy, by which females can increase competitive abilities of the smaller sex, or preferentially invest towards the sex with the potentially higher fitness returns. Here, we studied variation in egg size in relation to embryo sex and laying order in the European Blackbird Turdus merula. We found male and female eggs to differ in size, with larger eggs containing male embryos, as well as a significant interaction between embryo sex and laying order. This interaction resulted from the fact that egg size increased with the laying sequence among eggs bearing females but did not change with laying order among eggs bearing males. There was no relationship between offspring sex and the laying sequence within a clutch. We suggest that sexual dimorphism in egg size recorded in the European Blackbird may reflect favouritism of the sex which may give higher fitness returns.     

Maternal adjustment or constraint: differential effects of food availability on maternal deposition of macro‐nutrients,steroids and thyroid hormones in rock pigeon eggs

In oviparous species like birds, eggs provide the direct environment in which embryos are developing. Mothers may adjust different egg components in different ways in reaction to environmental cues either to adjust offspring development or because of constraints. In this study, we investigated the effects of food quality and quantity before and during egg laying on three different aspects of egg quality: macro‐nutrients (egg and yolk mass), androgens (testosterone and androstenedione), and thyroid hormones (3,5,3′‐triiodothyronine, T3 and l ‐thyroxine, T4), using the rock pigeon (Columba livia). As expected, egg and yolk mass were significantly reduced for the eggs laid under the poor‐food condition, indicating a maternal trade‐off between offspring and self in allocating important resources. We did not find any significant change in yolk testosterone or their within‐clutch pattern over the laying sequence. This is consistent with the fact that, in contrast with nutrients, these hormones are not costly to produce, but does not support the hypothesis that they play a role in adjusting brood size to food conditions. In contrast, we found that T3 levels were higher in the egg yolks under the poor‐food condition whereas the total T4 content was lower. This change could be related to the fact that iodine, the critical constituent of thyroid hormones, might be a limiting factor in the production of this hormone. Given the knowledge that food restriction usually lead to reduction of circulating T3 levels, our results suggested that avian mothers can independently regulate its concentrations in their eggs from their own circulation. The study demonstrates that environmentally induced maternal effects via the egg can be a result of a combination of constrained resources and unconstrained signals and that thyroid hormones might be an interesting case of both. Therefore, this hormone and the interplay of different maternal effects on the offspring phenotype deserve much more attention.     

The effect of maternal state on the steroid and macronutrient content of lesser black-backed gull eggs

It has been proposed that female birds can influence the phenotype of their offspring by provisioning eggs with variable amounts of nutrients and maternal hormones. Egg quality is strongly influenced by maternal body reserves and the amount of food available at the time of egg formation. This study investigated the effects of maternal state and food availability on the capacity of female lesser black-backed gulls Larus fuscus to provision their eggs with macronutrients and steroid hormones. Maternal state was reduced by increasing egg-production effort, whereas extra food was provided to reverse this effect. Compared with eggs of first clutches, eggs of experimentally induced replacement clutches exhibited a lower yolk/albumen ratio and contained more yolk testosterone. During one of the three years in which the study was performed, replacement eggs also contained more 17β-estradiol. Food provisioning during the relaying interval did not affect changes in yolk/albumen ratio or steroid concentrations, but fed females produced bigger eggs in their replacement clutch. This study demonstrates significant within-female consistency in egg size, macronutrient content, and yolk steroid concentration, and it shows that these egg characteristics are influenced by maternal state, food availability, and the timing of breeding.     

Maternal condition, yolk androgens and offspring performance: a supplemental feeding experiment in the lesser black-backed gull (Larus fuscus)

It has been proposed that the maternal androgens in avian egg yolk enhance offspring fitness by accelerating growth and improving competitive ability. Because egg quality is strongly influenced by maternal condition, we predicted that females in good condition would produce high-quality eggs with relatively high androgen content. We experimentally enhanced maternal condition by supplementary feeding lesser black-backed gulls (Larus fuscus) during egg formation and compared the concentrations of androstenedione (A4), 5alpha-dihydrotestosterone (DHT) and testosterone (T) in their eggs with those in eggs laid by control females. We also measured circulating levels of T in females immediately after laying. Egg androgens could affect offspring performance directly through chick development and/or indirectly through changes in the competitive ability of a chick relative to its siblings. To avoid confounding these two routes, and to separate effects operating through the egg itself with those operating through experimental changes in parental chick rearing capacity, we fostered eggs from both maternal treatment groups singly into the nests of unmanipulated parents. Contrary to expectation, mothers with experimentally enhanced body condition laid eggs with lower levels of androgens, while exhibiting higher circulating T concentrations post-laying. Despite these lower levels of egg androgen, offspring hatched from eggs laid by mothers in good condition did not show reduced growth or survival when reared in the absence of sibling competition. Our results demonstrate that yolk androgen concentrations vary with the body condition of the female at the time of egg formation and that females in good condition reduced the yolk androgen content of their eggs without altering offspring performance.