Egg size and asymmetric sibling rivalry in red-winged blackbirds

How big to make an egg is a life history decision that in birds is made coincident with a series of other similar decisions (how many eggs to have, whether to fortify them with maternally derived hormones or immune system boosters, whether to hatch the eggs synchronously or asynchronously). Though within-population variation in egg size in birds has been well studied, its adaptive significance, if any, is unclear. Here we examine within-population variation in egg size in relation to asymmetric sibling rivalry in a 17-year study of red-winged blackbirds (Agelaius phoeniceus), an altricial songbird. Egg mass showed a twofold range of variation, with roughly 80% of the variation occurring across clutches. By commencing incubation before the clutch is complete, mothers create advantaged core and disadvantaged marginal elements within their brood. Previous work on this system has shown that sibling competition is asymmetric, and that core offspring enjoy priority access to food, and as a consequence show higher growth and lower mortality than marginal offspring. Here we examine the effect of initial egg size on nestling growth and survival in relation to these competitive asymmetries. Egg mass was strongly linked to hatchling mass, and remained significantly related to the mass of both core and marginal nestlings; the effect of egg size was stronger for core offspring early in the nestling period, but the disparity between core and marginal nestlings narrowed as they approached fledging age, and slower growing marginals fell victim to brood reduction. The effect of egg mass on survival differed dramatically between core and marginal nestlings. Egg mass was significantly related to the survival of marginal but not core nestlings: below average egg mass was associated primarily with very early mortality. Asymmetric sibling competition is clearly a strong determinant of the consequences of egg size variation.     

Body-Mass, Composition, and Survival of Nestling and Fledgling Starlings (

Earlier studies of the starling (Sturnus vulgaris) population at Belmont, Lower Hutt, New Zealand, showed that nest productivity was low compared with other populations in New Zealand and elsewhere. Therefore, we investigated possible trade-offs between offspring number and quality (as measured by body mass and composition). We also compared these measures of offspring condition with pre- and post-fledging survival. Nestling mass did not significantly differ with clutch size or brood size at any age. In starlings about to leave the nest, lean (i.e., fat-free) dry mass and water mass increased with body mass, but lipid mass increased approximately twice as much. When the effects of the other variables were controlled in a partial correlation analysis, lean dry mass, water mass, lipid mass, and mass of stomach contents were positively correlated with mass at nest-leaving; brood size was not correlated with mass at nest-leaving. Nest success was independent of clutch size and brood size, but lighter broods were more likely to fail totally than were heavier broods early in the nestling period. Nestling survival early, but not late, in the nestling period was positively correlated with nestling mass. The likelihood that a nestling raised in 1973-1979 would be recruited as a breeder was independent of its mass at brood- day 12. Thus, unlike some other passerines, larger, heavier starling nestlings did not seem to survive better than average ones. Low productivity was not accompanied by a decrease in body condition of those nestlings that survived the nestling period. Therefore, starlings at Belmont reduced offspring number rather than offspring quality when they encountered unfavourable conditions.     

Fitness-related consequences of egg mass in nestling house wrens

The fitness-related consequences of egg mass, independent of confounding influences associated with parental quality, remain poorly understood for wild birds in general and for passerines in particular. We performed cross-fostering experiments to test the hypothesis that egg mass, independent of parental quality, is the primary determinant of fitness-related traits in nestling house wrens (Troglodytes aedon), an insectivorous passerine. Nestling mass was significantly correlated with the mass of the eggs from which nestlings hatched early but not late in the nestling period in early-season broods. In contrast, in late-season broods, nestling mass was correlated with egg mass until nestlings achieved asymptotic mass. Neither nestling growth nor survival to nest leaving was related to egg mass in either early- or late-season broods; however, nestlings in late-season broods grew more slowly than did nestlings in early-season broods. We propose that nestling mass and egg mass remained correlated throughout the nestling period in late-season broods because decreased arthropod food resources late in the breeding season constrain parents'' ability to provision nestlings. We conclude that female house wrens in this population trade-off clutch size for greater egg mass to maximise reproductive success in late-season broods.     

Differential effects of egg albumen content on barn swallow nestlings in relation to hatch order

In diverse animal taxa, egg mass variation mediates maternal effects with long-term consequences for offspring ontogeny and fitness. Patterns of egg mass variation with laying order differ considerably among birds, but no study has experimentally investigated the function of variation in albumen or yolk egg content in the wild. In barn swallows (Hirundo rustica), absolute and relative albumen mass increased with egg laying order. Experimental albumen removal delayed hatching, had larger negative effects on growth of late-hatched nestlings, and reduced nestling survival. Laying order positively predicted hatch order. Because nestling competitive ability depends on size, and albumen egg content influences hatchling size, present results suggest that by increasing albumen content of late eggs mothers reduce hatching asynchrony and enhance growth particularly of late-hatched nestlings. Thus, variation in albumen mass with laying order may function to mitigate the negative phenotypic consequences of hatching late in species that adopt a 'brood-survival' strategy.     

Egg size variation in birds with asynchronous hatching: is bigger really better?

In many animals large size at birth enhances offspring survival, but comparative evidence remains equivocal for birds. Failure to consider asynchronous hatching (ASH) may have confounded previous analyses. We assessed effects of egg size and ASH on growth and survival of common grackle (Quiscalus quiscula) nestlings to test the hypothesis that females adjust the size of last-laid eggs to modify effects of ASH. Although positive, the effect of egg size on nestling growth and survival was overwhelmed by the effect of ASH, with late-hatched nestlings being most likely to starve. Egg size did significantly affect growth late in the nestling period, but only because starvation had greatly reduced hatching asynchrony among surviving nestlings. Similarly, in experimentally synchronized nests, egg size and hatching asynchrony both affected offspring growth early in the nestling phase. Our results suggest that there is unlikely to be an adaptive advantage to females from varying the size of last-laid eggs in species with substantial ASH and that studies to assess the effect of a given maternal effect (e.g., varying egg size) should be done in the context of other maternal effects that may be operating simultaneously (e.g., ASH).     

Immune function and survival of great tit nestlings in relation to growth conditions

Life history theory predicts a trade-off between number and quality of offspring. Reduced quality with increasing brood size may arise from a decrease in body condition or in immunocompetence that would be important in fighting off virulent parasites by immunologically naive offspring. We tested the effect of rearing conditions on immune function of nestling great tits (Parus major) by reducing or increasing broods by two hatchlings. In the middle of the nestling period (on day 8), nestlings from enlarged broods developed lower T cell responses [as measured from the cutaneous swelling reaction to injection with phytohaemagglutinin (PHA)] and tended to have lower total leukocyte and lymphocyte concentrations in their peripheral blood than nestlings from reduced broods. Brood size manipulation affected the PHA response of nestlings most strongly in small clutches, suggesting that nestling immune function was dependent on their parents’ condition, as estimated by original clutch size. Intra-brood differences in nestling mortality were unrelated to immune parameters, but nestlings in broods without mortality had a stronger PHA response, higher concentration of lymphocytes and higher body mass on day 15 than nestlings in broods with mortality. These results support the prediction that the immune function of altricial birds is affected by rearing conditions, and that growth and immune parameters are related to inter-brood differences in nestling survival. Received: 1 February 1999 / Accepted: 19. July 1999     

The effect of egg mass on the growth and survival of blackbirds: a field experiment

A number of studies on birds have shown a positive correlation between egg mass and the growth (or survival) of chicks. However, a correlation based on non-experimental data does not demonstrate that egg mass affects growth, because it could be confounded by parental or territory quality. One way to see if pre-hatching attributes affect growth or survival is to swap hatchlings between nests, so that parental or territory quality do not confound correlations. I conducted such a fostering experiment on the blackbird, Turdus merula . Apart from very light eggs, that did not hatch, egg mass did not affect hatching success. Heavier eggs produced both heavier and larger nestlings. Nestlings raised by their own parents showed a positive correlation between hatchling mass and mass and size both early (day-4) and late (day-8) in the nestling period. The mass and size of fostered nestlings correlated with the mean mass of their natural parents' hatchlings, with higher coefficients early rather than late in the nestling period. By contrast, early in the nestling period there were no significant correlations of nestling mass or size and mean hatchling mass of the foster parents, but there were significant correlations late in the nestling period. Thus pre-hatching attributes of the egg do affect nestling size but environmental effects, including parental or territory quality, have an affect late in the nestling period. Direct manipulations of components of egg mass are required before one can conclude that egg mass affects growth, rather than some correlated pre-hatching attribute. There was no clear effect of egg mass on the probability that a hatching bird would survive until two weeks after fledging (shortly before nutritional independence), despite the fact that nestling mass does correlate with fledgling survival. I suggest that egg mass affects the 'size' component of mass and that juvenile survival depends on the 'condition' component of mass.     

Discrepancy between factors affecting nestling growth and survival and maternal success in Common Grackles

ABSTRACT Multiple factors potentially affect nestling survival and maternal reproductive success. However, little is known about the relative importance of different factors when operating simultaneously or whether the same factors are important for nestlings and their mothers. We determined the effect of hatching asynchrony, individual egg size, mean egg size, nestling sex, and clutch initiation date on the survival of individual nestlings and on maternal reproductive success in Common Grackles (Quiscalus quiscula) from 2004 to 2006 in central Illinois. Factors most important to maternal success differed from those important for individual nestling growth and survival. Hatching asynchrony had the greatest within‐nest influence on the fate of nestlings; the earlier a nestling hatched relative to siblings, the greater its mass and likelihood of fledging. Clutch size had the greatest influence on maternal reproductive success, with females with larger clutches fledging more young. Thus, both nestling survival and maternal success were largely determined by a single, albeit different, factor. A possible explanation for the apparent unimportance of most factors we measured in determining maternal success is that we did not consider variation among females. Individual variation in maternal attributes such as condition, size, age, experience, or mate quality may result in females tailoring clutch attributes (i.e., egg size, sex, and degree of hatching asynchrony) in ways that allow them to maximize their reproductive success. The discordance between factors that benefited mothers versus their offspring illustrates the importance of considering the maternal consequences of any factor that appears to affect offspring survival. Factors that increase the mass and survival of some offspring may not result in increased maternal reproductive success.     

Energy expenditure, nestling age, and brood size: an experimental study of parental behavior in the great tit Parus major

A brood manipulation experiment on great tits Parus major was performedto study the effects of nestling age and brood size on parentalcare and offspring survival. Daily energy expenditure (DEE)of females feeding nestlings of 6 and 12 days of age was measuredusing the doubly-labeled water technique. Females adjusted theirbrooding behavior to the age of the young. The data are consistentwith the idea that brooding behavior was determined primarilyby the thermoregulatory requirements of the brood. Female DEEdid not differ with nestling age; when differences in body masswere controlled for, it was lower during the brooding periodthan later. In enlarged broods, both parents showed significantlyhigher rates of food provisioning to the brood. Female DEE wasaffected by brood size manipulation, and it did not level offwith brood size. There was no significant effect of nestlingage on the relation between DEE and manipulation. Birds wereable to raise a larger brood than the natural brood size, althoughlarger broods suffered from increased nestling mortality ratesduring the peak demand period of the nestlings. Offspring conditionat fledging was negatively affected by brood size manipulation,but recruitment rate per brood was positively related to broodsize, suggesting that the optimal brood size exceeds the naturalbrood size in this population.     

Parent age differentially influences offspring size over the course of development in Laysan albatross

Offspring growth and survival are predicted to be higher for older parents, due to a variety of mechanisms, such as increased breeding experience or greater investment favored by low residual reproductive value. Yet the extent to which parent age affects offspring viability is likely to vary between different aspects of growth and survival, perhaps being most pronounced at the most stressful stages of reproduction. We studied the link between parent age and nestling growth and survival in the Laysan albatross, a long-lived seabird with a mean first breeding age of 8 years. Offspring of older parents were more likely to survive to fledging. Among those that did fledge, nestlings of older parents grew more rapidly. However, parent age did not influence the eventual asymptotic size that nestlings reached before fledging: fast-growing nestlings of older parents reached 90% of asymptotic size roughly 1 week sooner, but slow-growing nestlings of younger parents eventually caught up in size before fledging. Older parents bred c . 2 days earlier than younger parents, but hatch date did not explain observed variation in offspring success. The extent to which parent age accounted for variation in size of individual nestlings was not constant but peaked near the midpoint of development. This could reflect a time period when demands on parents reveal age-based differences in parental quality. Overall, growth and survival of offspring increased with parent age in this species, even though the late age of first breeding potentially provides a 7-year period for birds to hone their foraging skills or for selection to eliminate low-quality individuals.     

Sex-related effects of maternal egg investment on offspring in relation to carotenoid availability in the great tit

1. Maternal carotenoids in the egg yolk have been hypothesized to promote maturation of the immune system and protect against free radical damages. Depending on availability, mothers may thus influence offspring quality by depositing variable amounts of carotenoids into the eggs. Sex allocation theory predicts that in good quality environments, females should invest into offspring of the sex that will provide larger fitness return, generally males. 2. In a field experiment we tested whether female great tits bias their investment towards males when carotenoid availability is increased, and whether male offspring of carotenoid-supplemented mothers show higher body condition. We partially cross-fostered hatchlings to disentangle maternal effects from post-hatching effects, and manipulated hen flea Ceratophyllus gallinae infestation to investigate the relationship between carotenoid availability and resistance to ectoparasites. 3. As predicted, we found that carotenoid-supplemented mothers produced males that were heavier than their sisters at hatching, while the reverse was true for control mothers. This suggests that carotenoid availability during egg production affects male and female hatchlings differentially, possibly via a differential allocation to male and female eggs. 4. A main effect of maternal supplementation became visible 14 days after hatching when nestlings hatched from eggs laid by carotenoid-supplemented mothers had gained significantly more mass than control nestlings. Independently of the carotenoid treatment, fleas impaired mass gain of nestlings during the first 9 days in large broods only and reduced tarsus length of male nestlings at an age of 14 days, suggesting a cost to mount a defence against parasites. 5. Overall, our results suggest that pre-laying availability of carotenoids affects nestling condition in a sex-specific way with potentially longer-lasting effects on offspring fitness.     

Egg size and offspring performance in the collared flycatcher (<Emphasis Type="Italic">Ficedula albicollis</Emphasis>): a within-clutch approach

Adaptive within-clutch allocation of resources by laying females is an important focus of evolutionary studies. However, the critical assumption of these studies, namely that within-clutch egg-size deviations affect offspring performance, has been properly tested only rarely. In this study, we investigated effects of within-clutch deviations in egg size on nestling survival, weight, fledgling condition, structural size and offspring recruitment to the breeding population in the collared flycatcher (Ficedula albicollis). Besides egg-size effects, we also followed effects of hatching asynchrony, laying sequence, offspring sex and paternity. There was no influence of egg size on nestling survival, tarsus length, condition or recruitment. Initially significant effect on nestling mass disappeared as nestlings approached fledging. Thus, there seems to be limited potential for a laying female to exploit within-clutch egg-size variation adaptively in the collared flycatcher, which agrees with the majority of earlier studies on other bird species. Instead, we suggest that within-clutch egg-size variation originates from the effects of proximate constraints on laying females. If true, adaptive explanations for within-clutch patterns in egg size should be invoked with caution.     

Growth of yellow-headed blackbird Xanthocephalus xanthocephalus nestlings in relation to maternal body condition, egg mass, and yolk carotenoids concentrations

Condition‐dependent resource allocation to eggs can affect offspring growth and survival, with potentially different effects on male and female offspring, particularly in sexually dimorphic species. We investigated the influence of maternal body condition (i.e., mass‐tarsus residuals) and two measures of female resource allocation (i.e., egg mass, yolk carotenoid concentrations) on nestling mass and growth rates in the polygynous and highly size dimorphic yellow‐headed blackbird Xanthocephalus xanthocephalus. Egg characteristics and carotenoid concentrations were obtained from the third‐laid egg of each clutch and were correlated with the mass and growth rates of the first two asynchronously hatched nestlings. Maternal body condition was associated with the growth of first‐hatched, but not second‐hatched nestlings. Specifically, females in better body condition produced larger and faster growing first‐hatched nestlings than females in poorer body condition. As predicted for a polygynous, size‐dimorphic species, females that fledged first‐hatched sons were in better body condition than females that fledged first‐hatched daughters. Associations between egg mass, yolk carotenoid content, and nestling growth were also specific to hatching‐order. Egg mass was positively correlated with the mass and growth rates of second‐hatched nestlings, and yolk concentrations of β‐carotene were positively correlated with second‐hatched nestling mass. Surprisingly, the relationship between yolk lutein and hatchling growth differed between the sexes. Females with high concentrations of yolk lutein produced larger and faster growing first‐hatched sons, but smaller first‐hatched daughters than females with lower lutein concentrations. Mass and growth rates did not differ between first‐ and second‐hatched nestlings of the same sex, despite asynchronous hatching in the species. Results from this study suggest that maternal body condition and the allocation of resources to eggs have carotenoid‐, sex‐, and/or hatch‐order‐specific effects on yellow‐headed blackbird nestlings.     

Effects of intraclutch variation in egg size and hatching asynchrony on nestling development and survival in semi‐precocial Herring Gulls

Intraclutch egg size variation may non‐adaptively result from nutritional/energetic constraints acting on laying females or may reflect adaptive differential investment in offspring in relation to laying/hatching order. This variation may contribute to size hierarchies among siblings already established due to hatching asynchrony, and resultant competitive asymmetries often lead to starvation of the weakest nestling within a brood. The costs in terms of chick mortality can be high. However, the extent to which this mortality is egg size‐mediated remains unclear, especially in relation to hatching asynchrony which may operate concomitantly. I assessed effects of egg size and hatching asynchrony on nestling development and survival of Herring Gulls (Larus argentatus), where the smaller size and later hatching of c‐eggs may represent a brood‐reduction strategy. To analyze variation in egg size, I recorded the laying order and laying date of 870 eggs in 290 three‐egg clutches over a 3‐yr period (2010–2012). I measured hatchlings and monitored growth and survival of 130 chicks from enclosed nests in 2011 and 2012. The negative effect of laying date (β = ?0.18 ± SE 0.06, P = 0.002) on c‐egg size possibly reflected the fact that late breeders were either low quality or inexperienced females. The mass, size, and condition of hatchling Herring Gulls were positively related to egg size (all P < 0.0001). C‐chicks suffered from increased mortality risk during the first 12 d, identified as the brood‐reduction period in my study population. Although intraclutch variation in egg size was not directly related to patterns of chick mortality, I found that smaller relative egg size interactively increased differences in relative body condition of nestlings, primarily brought about by the degree of hatching asynchrony during this brood‐reduction period. Thus, the value of relatively small c‐eggs in Herring Gulls may lie in reinforcing brood reduction through effects on nestling body condition. A reproductive strategy Herring Gulls might have adopted to maintain a three‐egg clutch, but that also enables them to adjust the number of chicks they rear relative to the prevailing environmental conditions and to their own condition during the nestling stage.     

Nestling telomere shortening,but not telomere length,reflects developmental stress and predicts survival in wild birds

Developmental stressors often have long-term fitness consequences, but linking offspring traits to fitness prospects has remained a challenge. Telomere length predicts mortality in adult birds, and may provide a link between developmental conditions and fitness prospects. Here, we examine the effects of manipulated brood size on growth, telomere dynamics and post-fledging survival in free-living jackdaws. Nestlings in enlarged broods achieved lower mass and lost 21% more telomere repeats relative to nestlings in reduced broods, showing that developmental stress accelerates telomere shortening. Adult telomere length was positively correlated with their telomere length as nestling (r = 0.83). Thus, an advantage of long telomeres in nestlings is carried through to adulthood. Nestling telomere shortening predicted post-fledging survival and recruitment independent of manipulation and fledgling mass. This effect was strong, with a threefold difference in recruitment probability over the telomere shortening range. By contrast, absolute telomere length was neither affected by brood size manipulation nor related to survival. We conclude that telomere loss, but not absolute telomere length, links developmental conditions to subsequent survival and suggest that telomere shortening may provide a key to unravelling the physiological causes of developmental effects on fitness.     

Maternal effects reduce oxidative stress in female nestlings under high parasite load

Mothers can adjust the phenotype of their offspring to the local environment through a modification of their egg investment and/or nestling provisioning. However, offspring health may be severely impaired if the conditions experienced by nestlings do not match with those anticipated by the mother. If maternal effects differentially affect the sexes or if one sex is more strongly affected by an environmental stressor, fitness benefits may also differ between male and female offspring. Here, we study maternal effects in male and female great tit Parus major nestlings by means of an ectoparasite treatment before egg‐laying combined with a partial cross‐foster experiment between broods of infested and uninfested nests. Nestlings that were raised in their own nest experienced the same conditions before and after cross‐fostering (either in parasite infested or uninfested nests), while cross‐fostered ones experienced different conditions (either changing from infested to uninfested or the other way around). We measured effects on nestling plasma levels of oxidative stress [reactive oxygen metabolites (ROMs) and total antioxidant capacity (OXY)], body condition (body size and mass) and post‐fledging survival. Daughters, but not sons, from matching conditions showed the lowest ROM and high OXY levels when exposed to parasites, while there was no effect of parasite exposure in any of both sexes in case of a mismatch. In contrast, body condition and post‐fledging survival were not (or only slightly) affected by any of the experimental treatments. Results of this study show that maternal effects can affect oxidative stress levels of nestlings in a sex‐specific way and that the outcome depends on the exposure to environmental stressors, such as parasites.     

Cross-fostering reveals an effect of spleen size and nest temperatures on immune responses in nestling European starlings

Immunocompetence may be a good measure of offspring quality, however, factors affecting variation in immune responses are not clear. Research suggests that immune function can vary due to differences in genetics, development conditions and individual quality. Here, I examined factors affecting variation in immune response among nestling European starlings through a split-nest cross-fostering brood manipulation that included two important covariates: spleen size and nest temperatures. Immunocompetence was assessed via a cell-mediated immune response to phytohaemagglutinin (PHA). This paper provides the first direct evidence that individuals with large spleens also mount strong immune responses. Exposure to PHA did not cause splenomegaly, as there was no difference in spleen size between control birds and those injected with PHA. Offspring immune function was affected by common origin and by rearing environment, though rearing environment appeared to exert its influence only through nest temperatures. A comparison of the immune performance of siblings reared in their home nest versus those reared in other nests revealed a strong effect of maternal quality. As the difference in natal clutch size increased, the magnitude of the difference in immune performance between home-reared nestlings versus out-reared nestlings increased. Overall, nestling immune function appears to be determined by the combination of genetic, maternal and environmental effects.     

Hatching asynchrony and brood reduction influence immune response in Common Kestrel Falco tinnunculus nestlings

The onset of incubation before the end of laying imposes asynchrony at hatching and, therefore, a size hierarchy in the brood. It has been argued that hatching asynchrony might be a strategy to improve reproductive output in terms of quality or quantity of offspring. However, little is known about the mediating effect of hatching asynchrony on offspring quality when brood reduction occurs. Here, we investigate the relationship between phenotypic quality and hatching asynchrony in Common Kestrel Falco tinnunculus nestlings in Spain. Hatching asynchrony did not increase breeding success or nestling quality. Furthermore, hatching asynchrony and brood reduction had different effects on nestlings’ phytohaematogglutinin (PHA)‐mediated immune response and nestling growth. In asynchronous and reduced broods (in which at least one nestling died), nestlings showed a stronger PHA‐mediated immune response and tended to have a smaller body size compared with nestlings raised in synchronous and reduced broods. When brood reduction occurred in broods hatched synchronously, there was no effect on nestling size, but nestlings had a relatively poor PHA‐mediated immune response compared with nestlings raised in asynchronous and reduced broods. We suggest that resources for growth can be directed to immune function only in asynchronously hatched broods, resulting in improved nestling quality, as suggested by their immune response. We also found that males produced a greater PHA‐mediated immune response than females only in brood‐reduced nests without any effect on nestling size or condition, suggesting that females may trade off immune activities and body condition, size or weight. Overall, our results suggest that hatching pattern and brood reduction may mediate resource allocation to different fitness traits. They also highlight that the resolution of immune‐related trade‐offs when brood reduction occurs may differ between male and female nestlings.     

Body condition variation in kestrel (<Emphasis Type="Italic">Falco tinnunculus</Emphasis>) nestlings in relation to breeding conditions

The body condition index (i.e., body mass corrected for age or size differences) is commonly used to investigate offspring condition in nestling birds. The body condition index reflects different parameters related to the general nutritional state of nestlings and may predict survival prospects. Since conditions experienced during the growth period can affect the fitness of nestlings in adulthood, we investigated proximate and ultimate factors underlying body condition index variation in kestrel (Falco tinnunculus) nestlings in a 9-year field study and we carried out two cross-fostering experiments to disentangle the origin (genetics plus maternal effects) and rearing (environment effect) components of body condition index variation. In total, we sampled 2,065 nestlings from 464 broods and used 121 nestlings from 24 broods in the cross-fostering experiments. We found that nestlings from larger broods had higher body condition index than nestlings from smaller ones, but this pattern did not emerge in two of the 9 years of study; nestlings born later in the breeding season had lower body condition index in some years but not in others; the decrease of body condition index over the breeding season emerged in all but three-chick broods; males and females did not differ neither in body condition index nor in the covariation between body mass and wing length, while this result was limited to one of the nine field study years; the annual mean value of body condition index did not covary with the total rainfall; both the origin and rearing components explained body condition index variation, but their relative contributions varied from a year to another. Overall, these results suggest that the brood size is not a good predictor of body condition index; the rule “nesting early in the season is better” is less general than previously thought; the body condition index may contain origin variance, whose expression may be modulated by environmental conditions.     

The separate effects of egg size and parental quality on the development of ornamental plumage coloration

The fitness‐related consequences of egg size, independent of the influences of parental quality, are poorly understood in altricial birds. Not only can egg size and parental quality influence growth and survival, but each could influence the development of condition‐dependent plumage coloration in offspring. The Eastern Bluebird Sialia sialis is an altricial, multi‐brooded, cavity‐nesting passerine in which juveniles display dichromatic UV‐blue plumage. Previous research suggests that plumage coloration acts as a signal of individual quality among juvenile and adult Eastern Bluebirds. Here, we separate the effects of egg size and parental quality (defined by egg size laid) on nestling growth and plumage ornamentation by exchanging clutches of large eggs with clutches of small eggs. Nestlings were significantly larger immediately post‐hatching when hatched from a large egg, but to maintain a larger size, nestlings needed to have hatched from a large egg and to have been reared by high‐quality parents. Nestlings were brighter when reared by high‐quality parents and this relationship was strongest later in the breeding season. Nestlings exhibited greater UV chroma if hatched early in the season, but UV chroma was not significantly affected by egg size or parental quality. These findings demonstrate varying influences of both egg size and parental quality on offspring growth and plumage ornamentation but suggest that quality of post‐hatching investment is more influential than pre‐hatching investment.