Do group size and laying order influence maternal deposition of testosterone in smooth-billed ani eggs?

The avian egg contains maternal hormones that affect behavior, growth, morphology, and offspring survival. Evidence to date suggests that patterns of yolk androgen deposition could provide females with a means to manipulate sibling competition and, thereby, increase their fitness. We examined yolk testosterone (T) concentrations in eggs of the smooth-billed ani (Crotophaga ani) to understand patterns of androgen deposition in eggs of this plural-breeding joint-nesting cooperatively breeding species. We tested the hatching asynchrony adjustment hypothesis, which states that increases in yolk androgen levels over the laying sequence function to mitigate the disadvantage of being a later-hatched chick in species without adaptive brood reduction. We also investigated the effect of group size on yolk T deposition to test the hypothesis that females in multi-female groups could give a competitive edge to their own chicks by depositing higher T levels in their eggs. Predictions of the hatching asynchrony adjustment hypothesis were supported in both single- and multi-female groups as yolk testosterone levels increased from early- to late-laid eggs. This suggests that ani females can influence nestling competition and chick survival by within-clutch differential T allocation. Unexpectedly, we did not observe an effect of group size on yolk T deposition. Yolk testosterone concentrations may not be a mere reflection of a female's hormonal status as female plasma circulating levels of T did not vary in the same direction as yolk T levels. Results of this study therefore support the idea that females may adaptively manipulate chick behavior through hormonal deposition in eggs.     

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‐yolk androgen and carotenoid deposition as a function of maternal social environment in barn swallows Hirundo rustica

Evidence is mounting that female animals use egg‐yolk compounds (e.g. steroids, antioxidants) to adaptively engineer the quality of their offspring as a function of several maternal and environmental factors. Though adjustments to yolk allocation have been well‐characterized as a function of parental phenotypes, we know very little about how an individual's social environment influences yolk composition. Here, we consider how two types of yolk compounds, androgens and carotenoids, relate to the maternal social environment during the egg‐laying period, controlling statistically for known correlations between various aspects of parental quality and egg yolk compounds. Barn swallows Hirundo rustica erythrogaster breed in groups of highly variable size and spacing, allowing us to test whether or not the social environment is correlated with these maternal effects. We found no relationship between carotenoid levels in eggs as a function of colony size, colony density, or nearest‐neighbor distance. However, eggs from females in larger groups had lower concentrations and total amounts of yolk androgens than those from females in smaller, less dense social settings. Our results counter previous predictions and literature, showing that females breeding in large groups deposit more androgen in eggs, mechanistically, because they compete more with conspecifics and have higher circulating androgen levels themselves and, functionally, because it could be advantageous for their offspring to show high androgen‐mediated competitive abilities early in life. Instead, because group size in this species is governed largely by site fidelity and the availability of old nests for re‐use, and because reproductive output does not differ as a function of group size, it may be that competition is greater for limited nests in small groups, thus elevating androgen levels. Further, yolk androgens were previously shown to be affected by male quality, and the greater concentrations and amounts of yolk androgens in smaller sites may reflect differential allocation to darker males found at these sites.     

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.     

Interfemale variation in egg yolk androgen allocation in the European starling: do high-quality females invest more?

Androgenic hormones occur naturally in bird egg yolk and are known to enhance growth in canary and gull chicks. Gil et al. (1999, Science, 286, 126-128) have recently proposed that female allocation of androgens to eggs represents a form of costly maternal investment. This hypothesis predicts that females of high quality or high reproductive potential should invest more yolk androgens in their eggs than females of low quality or low reproductive potential. We tested these predictions by examining interfemale variation in allocation of the androgenic steroids testosterone and androstenedione to eggs in a wild population of European starlings, Sturnus vulgaris. We collected 30 full clutches of eggs and captured 22 of the females that laid these clutches for phenotypic measurements. In agreement with the hypothesis, we found that there was significant interfemale variation in yolk androgen concentrations. Furthermore, older females deposited more androstenedione and testosterone in eggs than 1-year-old females, and females laying early or large clutches deposited more testosterone in eggs than females laying late or small clutches. However, females in good body condition did not deposit more androgens in eggs than females in poor body condition. Large-yolked eggs had a higher total yolk androgen content than small-yolked eggs, but larger eggs did not have higher concentrations of yolk androgens. The results in general support the investment hypothesis of yolk androgen allocation. However, further data are needed regarding the costs of yolk androgen allocation and the effects of yolk androgens on offspring to substantiate the assumptions of the hypothesis. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

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.     

Penguin Chicks Benefit from Elevated Yolk Androgen Levels under Sibling Competition

Crested penguins (genus Eudyptes) have a peculiar hatching pattern, with the first-laid egg (A-egg) hatching after the second-laid egg (B-egg) and chicks from A-eggs typically having a much lower survival probability. Maternal yolk androgens have been suggested to contribute to the competitive superiority of the B-chick in southern rockhopper penguins Eudyptes chrysocome, given their important role in mediating sibling competition in other species. We therefore increased the yolk androgen levels in freshly-laid eggs and examined the consequences for sibling competition - via effects on embryonic developmental times, chick growth and early survival. We placed one androgen-treated egg and one control egg into each foster nest, matching them for mass, laying date and laying order. The androgen treatment did not significantly affect embryonic developmental times or chick measurements at hatching. However, elevated yolk androgen levels benefitted chick growth in interaction with the number of siblings in a brood. Chicks from androgen-treated eggs had faster growth in the presence of a sibling than chicks from control eggs. Under these circumstances they also had a higher survival probability. Thus maternal androgens appear to reinforce the observed hatching pattern, facilitating brood reduction. This contrasts to most previous studies in other species where yolk androgens have been shown to compensate for the negative consequences of delayed hatching within the brood hierarchy.     

Maternal androgens in black-headed gull (Larus ridibundus) eggs: consequences for chick development

We tested the hypothesis that mother birds counterbalance the negative effects of hatching asynchrony for later-hatched chicks by increasing the yolk androgen concentrations in consecutive eggs of their clutch. In doing so, they may adaptively tune each offspring's competitive ability and, thus, growth and survival. However, evidence in support of this hypothesis is contradictory. The yolk concentrations of maternal androgens in the eggs of black-headed gulls increase significantly with the laying order of the eggs in a clutch. We experimentally tested the functional consequences of this increase on chick development under natural conditions by injecting eggs with either an oil or androgen solution. We created experimental clutches in which androgen levels either stayed constant or increased with laying order while controlling for differences in egg quality by using only first-laid eggs. We then compared development, growth and survival between these broods. Androgen treatment enhanced embryonic development because androgen-treated eggs hatched half a day earlier than controls, while their size at hatching was similar to oil-treated controls. Androgen treatment did not increase chick survival, but it enhanced growth. Androgen-treated, third-hatched chicks had a higher body mass and longer legs than third-hatched chicks that hatched from oil-treated eggs. At the same time, growth of first chicks (which were all oil treated) was reduced by the presence of two androgen-treated siblings, suggesting that yolk androgens enhance the competitive ability of later-hatched chicks. Our results support the hypothesis that transfer of different amounts of androgens to the eggs of a clutch is a mechanism by which mothers maximize their reproductive output.     

Experimental evidence for genetic heritability of maternal hormone transfer to offspring

In many animal species, embryos are exposed to maternal hormones that affect their development. Maternal hormone transfer varies with environmental conditions of the mother and is often interpreted as being shaped by natural selection to adjust the offspring to prevailing environmental conditions. Such hormone transfer requires genetic variability, which has not yet been experimentally demonstrated. Our study reports direct evidence for additive genetic variance of maternal androgens through a bidirectional selection on yolk testosterone (T) levels in Japanese quail. Lines selected for high egg T (HET) and low egg T (LET) concentration differed in yolk levels of this androgen, resulting in high realized heritability (h2 = 0.42)Correlated responses to selection on other gonadal hormones indicated that selection specifically targeted biologically active androgens. Eggs of HET quail contained higher androstenedione and lower estradiol concentrations than did those of LET quail, with no line differences in yolk progesterone concentration. Plasma T concentrations in adult females were not affected by selection, seriously challenging the hypothesis that transfer of maternal hormones to offspring is constrained by hormone levels in a mother's circulation. Our results suggest that transfer of maternal T represents an indirect genetic effect that has important consequences for the evolution of traits in offspring.     

Prenatal androgen exposure modulates cellular and humoral immune function of black-headed gull chicks

Avian eggs contain considerable amounts of maternal yolk androgens, which have been shown to beneficially influence the physiology and behaviour of the chick. As androgens may suppress immune functions, they may also entail costs for the chick. This is particularly relevant for colonial species, such as the black-headed gull (Larus ridibundus), in which the aggregation of large numbers of birds during the breeding season enhances the risk of infectious diseases for the hatching chick.To test the effect of maternal yolk androgens on the chick's immune function, we experimentally manipulated, in a field study, yolk androgen levels within the physiological range by in ovo injection of either androgens (testosterone and androstenedione) or sesame oil (control) into freshly laid eggs. We determined cell-mediated immunity (CMI) and humoral immunity of the chicks at the beginning of the nestling period to evaluate early modulatory effects of yolk androgens on immune function.Embryonic exposure to elevated levels of androgens negatively affected both CMI and humoral immunity in nestling gull chicks. Consequently, maternal yolk androgens not only entail benefits of enhanced competitiveness and growth as previously shown, but also costs in terms of immunosuppression. The outcome of embryonic yolk androgen exposure thus likely depends on the post-hatching circumstances for the developing offspring such as parasite exposure and degree of sibling competition.     

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.     

Yolk androgens do not appear to mediate sexual conflict over parental investment in the collared flycatcher Ficedula albicollis

Males and females are in conflict over parental care, as it would be favourable for one parent to shift labour to the other. Yolk hormones may offer a mechanism through which female birds could influence offspring traits in ways that increase the relative investment by the male. We studied the role of yolk androgens in mediating sexual conflict over parental care in the collared flycatcher (Ficedula albicollis). In a cross-fostering experiment, the male's proportion of total feeding visits increased with increasing androgen levels in the foster eggs. This could suggest that sexual conflict over parental care may be influenced by the female's differential allocation of yolk androgens or a maternal effect associated with yolk androgens. However, when we experimentally elevated yolk androgen levels, male feeding rates did not differ between control and androgen-manipulated nests. This suggests that other egg components correlated with yolk androgen levels, rather than yolk androgen levels per se, may influence male parental effort. In conclusion, yolk androgens per se do not appear to mediate sexual conflict over parental investment in the collared flycatcher.     

Genetic differences in yolk testosterone levels influence maternal hormone deposition in the second laying cycle in Japanese quails

Maternally-derived yolk androgens exhibit distinct among- and within-female variations but limited data refer to inter-seasonal changes of maternal hormones in the yolk. We investigated the deposition of yolk testosterone (T) across two laying cycles in Japanese quail. To test how genetically-determined differences influence between cycle variations in yolk androgens we compared females from low (LET) and high (HET) egg T lines at the end of the first and at the beginning of the second laying cycle after an induced moult. Line differences in yolk T levels exhibited high consistency exceeding two reproductive cycles. Yolk T concentrations increased in the second laying cycle in HET but not in LET females. Plasma T levels did not differ between cycles in both lines and no line differences were found either before or after the moult indicating the presence of mechanisms limiting the increase of T concentrations in the circulation. Differences in the yolk T levels were not accompanied by changes in the egg and yolk mass. The HET quail laid eggs with heavier eggshell than the LET quail. Our results demonstrate different abilities of mothers to deposit T in their eggs over two reproductive seasons with expected consequences on the development of their progeny.     

Food availability affects the maternal transfer of androgens and antibodies into eggs of a colonial seabird

Mothers can improve the quality of their offspring by increasing the level of certain components in their eggs. To examine whether or not mothers increase deposition of such components in eggs as a function of food availability, we food-supplemented black-legged kittiwake females (Rissa tridactyla) before and during egg laying and compared deposition of androgens and antibodies into eggs of first and experimentally induced replacement clutches. Food-supplemented females transferred lower amounts of androgens and antibodies into eggs of induced replacement clutches than did non-food-supplemented mothers, whereas first clutches presented no differences between treatments. Our results suggest that when females are in lower condition, they transfer more androgens and antibodies into eggs to facilitate chick development despite potential long-term costs for juveniles. Females in prime condition may avoid these potential long-term costs because they can provide their chicks with more and higher quality resources.     

Maternal transfer of androgens in eggs is affected by food supplementation but not by predation risk

Mothers may affect the future success of their offspring by varying allocation to eggs and embryos. Allocation may be adaptive based on the environmental conditions perceived during early breeding. We investigated the effects of food supplementation and predation risk on yolk hormone transfer in the pied flycatcher Ficedula hypoleuca. In a food supplementation experiment, females were food‐supplemented prior to and during egg‐laying and androgen concentrations were measured throughout the laying order. Predation risk was investigated in three different studies combining both correlative data, where flycatchers bred in close proximity to two different predator species that prey upon adult flycatchers (either Tengmalm's owl Aegolius funereus or pygmy owl Glaucidium passerinum), and an experimental manipulation, where flycatchers were exposed to cues of a nest predator (least weasel Mustela nivalis). Females receiving food supplementation laid eggs with lower concentrations of androstenedione (A4) than females not receiving food supplements. Yolk testosterone (T) concentration showed the same pattern but the difference was not statically significant. Testosterone (but not A4) concentration increased within clutches, from the first to the last egg, independently of the food supplementation. Females breeding under high predation risk did not differ from control females in their yolk androgen levels (A4, T or progesterone). However, concentrations of A4 tended to be lower in the proximity of pygmy owls, which could indirectly increase offspring survival after fledging. Food supplementation during egg‐laying seems to have a stronger impact on maternal transfer of androgens than predation risk. Food availability and predation risk could differentially affect the trade‐offs of androgen allocation for the offspring when raised in good vs. dangerous environments.     

Maternal androgens in eggs of communally breeding guira cuckoos (Guira guira)

Variation of maternal androgens in avian eggs may be a mechanism of maternal influence on offspring development, growth, and/or behavior. We studied yolk androgen concentrations in eggs of guira cuckoos (Guira guira) to understand how females might enhance the success of offspring in a complex communal breeding system. We measured concentrations of androstenedione, 5alpha-dihydrotestosterone, and testosterone in yolks and identified eggs and clutches of individuals in joint nests by yolk protein electrophoresis. Androstenedione had the highest yolk concentration, at least 10 times higher than that of testosterone and 5alpha-dihydrotestosterone. The first eggs of individual females that laid two or three eggs in a joint nest had lower androstenedione concentrations than their second and third eggs, the latter having a lower probability of being ejected from the nest. This implies that guira cuckoo females may influence offspring survival and competitiveness in communal nests by means of differential allocation of androstenedione and laying tactics. There was significant variation in yolk androstenedione among females, but the order in which females entered laying in the communal clutch had no effect on the concentrations. Androstenedione yolk concentrations increased with communal clutch size, which may indicate that higher levels of competition in larger groups lead to higher yolk androgen concentrations. Finally, androstenedione concentrations were higher in clutches in the later wetter periods of the rainy season than during the earlier drier period. This may be explained by the high frequency of large clutches in the later periods, with more females contributing to a joint clutch.     

Maternal dietary carotenoids mitigate detrimental effects of maternal GnRH on offspring immune function in Japanese quail Coturnix japonica

Maternal resources deposited in eggs can affect the development of several offspring phenotypic traits and result in trade‐offs among them. For example, maternal androgens in eggs may be beneficial to offspring growth and competitive ability, but detrimental to immunocompetence and oxidative stress. In contrast, maternal antioxidants in eggs may be beneficial if they mitigate oxidative stress and immunosuppressive effects of androgens. We investigated possible interactive effects of maternal steroids and carotenoids on aspects of offspring physiology and phenotype, by simultaneously manipulating levels of androgens (via gonadotropin‐releasing hormone, GnRH‐challenges) and carotenoids (via diet supplementation) in captive female Japanese quail Coturnix japonica during egg laying. Carotenoid supplementation of hens, which elevates yolk concentrations of carotenoid and vitamins A and E, enhanced egg hatching success, offspring survival to age 15 d, and size of the bursa of Fabricius in offspring. In contrast, repeated maternal GnRH challenges, which elevated yolk testosterone concentrations, enhanced offspring neonatal size, but negatively affected bursa size. However, interaction among the treatments suggests that the positive effect of maternal carotenoid supplementation on plasma bactericidal capacity was mediated by maternal GnRH challenges. Chicks originating from carotenoid‐supplemented hens were less immunosuppressed than those originating from carotenoid‐supplemented + GnRH‐challenged hens, which were less immunosuppressed than chicks from GnRH‐challenged females not supplemented with carotenoids. Females availability of carotenoid enriched diets allows them to enhance the development of offspring immune system via carotenoids and vitamins deposited in egg yolks and offset detrimental effects of androgens deposited by GnRH‐challenged females.     

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.     

Selection for rapid embryo development correlates with embryo exposure to maternal androgens among passerine birds

Greater offspring predation favors evolution of faster development among species. We hypothesized that greater offspring predation exerts selection on mothers to increase levels of anabolic androgens in egg yolks to achieve faster development. Here, we tested whether (1) concentrations of yolk androgens in passerine species were associated with offspring predation and (2) embryo and nestling development rates were associated with yolk androgen concentrations. We examined three androgens that increase in potency along the synthesis pathway: androstenedione (A(4)) to testosterone (T) to 5 alpha -dihydrotestosterone (5 alpha -DHT). Concentrations of none of these steroids were related to clutch size; only A(4) was allometrically related to egg volume. Species that experience greater predation showed higher yolk concentrations of T and 5 alpha -DHT. Higher concentrations of T and particularly 5 alpha -DHT were strongly correlated with faster development during the embryo period and less so during the nestling period. Development rates were most strongly correlated with 5 alpha -DHT, suggesting that potency increases along the androgen synthesis pathway and that effects are mediated by the androgen receptor pathway. These results are consistent with the hypothesis that selection for faster development by time-dependent offspring mortality may be achieved epigenetically by varying embryo exposure to maternal anabolic steroids.     

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.