Causes and consequences of egg mass variation between and within blue tit clutches

Causes of egg-size variation between and within clutches were studied in clutches of the blue tit ( Parus caeruleus L.). We measured the mass of each egg in the laying sequence in unmanipulated clutches, in clutches of parents experimentally supplied with extra food before egg-laying, and in clutches of parents supplemented with extra food after the start of egg-laying. Hatchlings were weighed at an age of two days and their mass was found to be positively related to egg mass. No general trend of decreasing or increasing egg mass was found within the laying sequence. Females provided with extra food before egg-laying laid clutches with significantly less variation in egg mass than did control females. The reason for this was that the first-laid egg of unmanipulated females was lighter than the rest of the eggs in the clutch. This pattern disappeared in clutches of females receiving extra food. Thus, the reduction in egg mass variation among clutches of foodsupplemented females depended on an ability of these females, in contrast to control females, to lay a first egg of the same mass as the rest of the clutch. Eggs laid after the initiation of incubation were significantly heavier than equivalent eggs in those clutches where incubation started after clutch completion. The difference was small, however, and the adaptive significance of the finding is questionable. We argue that intra-clutch variation in egg mass is connected with greater fitness consequences than in inter-clutch variation. Furthermore, our results indicate that energetical constraints on the laying female are more important as a cause of the observed intra-clutch variation in egg mass than are adaptive responses to the environment.     

Seasonal changes in egg-mass within and among clutches of birds: general explanations and a field study of the Blackbird Turdus merula

In many populations of birds there is a seasonal change in the mean mass of eggs in the clutch. This might be caused by seasonal changes in the costs of egg-production, or changes in the benefits of laying eggs of different size. In the Blackbird Turdus merula the mean mass of an egg correlated specifically with the air temperature during the period when it was predicted to be undergoing rapid follicular growth. There was no residual effect of date or day-length on egg-mass when statistically controlling for temperature during the period of rapid yolk synthesis, yet temperature still had a significant effect when controlling for date or day- length. Thus the seasonal increase in egg-mass appears to be due to changes in the cost of egg-production, not changes in the benefits of laying larger eggs. However, I could find no effect of food supply during laying on the mean mass of eggs in the clutch, either using an indirect measure of food availability, rainfall, or in a food-supplementation experiment. This could be because females responded to extra food by laying earlier, and probably larger, clutches, rather than by increasing egg-mass. The effect of temperature on egg-mass increased through the laying sequence and there was a small seasonal increase in the mass of the last-laid egg compared with the mean of the other eggs in the clutch. I propose that the mean mass of the last-laid egg relative to the clutch mean, which can characterize a species or population, could evolve in response to the environmentally-caused variance in the mass of the last-laid egg: when the variance is high, the mean may have to be high to avoid producing unviable eggs.     

Manipulation of laying effort reveals habitat-specific variation in egg production constraints in Great Tits (Parus major)

It has been suggested that a bird's clutch size is not limited by the amount of resources available at the time of laying but that differences in the availability of food for nestlings is the ultimate underlying factor determining spatio-temporal variations in clutch size. However, habitat-related variations in egg production ability has yet to be investigated explicitly. We studied the breeding of Great Tits Parus major in deciduous and coniferous forests in the same area. The sizes of both the clutches and the eggs were, on average, larger in the former habitat than in the latter. A number of females were induced to lay more eggs than usual by removing four eggs from designated experimental clutches early in the laying period. These manipulated females laid approximately one egg more than control females, with the number of additional eggs laid not differing between the habitats. However, in both study years the relative size of the extra eggs – relative to the mean size of earlier laid eggs of the same clutch – was smaller in the coniferous habitat than in the deciduous habitat, while there was no habitat-related difference in the relative size of the last-laid eggs of control clutches. This result indicates that some form of proximate limitation during egg-laying period can contribute to the relatively small clutches and eggs in the coniferous habitat. Our results emphasize the need to take egg production costs into account when attempting to account for spatial variation in the reproductive behaviour of birds.     

安徽滁州雌性丽斑麻蜥繁殖特征

在安徽滁州地区丽斑麻蜥(Eremias argus)年产两窝卵。窝卵数及窝卵重与雌体体长呈正相关,相对窝卵重与雌体体长无关,卵重与窝卵数无关。窝卵数、窝卵重及卵重在窝序间无明显的差异。卵长径与卵短径呈正相关,卵长径与窝卵数呈负相关,而卵短径与窝卵数无关。雌体主要通过增加窝卵数增加繁殖输出。     

Time-dependent reproductive decisions in the blue tit

Many breeding attempts in birds do not result in any fledged young due to predation on eggs or young. Consequently, the influence of time constraints on reproductive decisions are integrated parts of the reproductive behaviour of birds breeding within short, seasonal climate zones. In this study, I mimicked nest predation by removing blue tit (Parus caeruleus) clutches shortly after completion. Around 75% of the removed clutches were followed by a repeat clutch. Females producing their first clutch early in the season and females with an early onset of incubation in the laying sequence (an indication of high parental or territory quality) were most likely to initiate a repeat clutch. A trade‐off between the benefits of a repeat clutch and survival likely stopped late females in bad condition from investing more in the current reproductive season. Females producing a repeat clutch laid fewer eggs, had an earlier onset of incubation in the laying sequence and produced larger eggs than they did when producing their original first clutch. Eggs produced after the onset of incubation were especially large in the repeat clutches. Since food availability was presumably higher when the female produced her repeat clutch compared with her first clutch, females made a strategical decision when reducing clutch size, whereas onset of incubation and egg size may have been energetically constrained when producing the first clutch. Females that produced a relatively large clutch, had a relatively early onset of incubation, and laid relatively large eggs in their first clutch also did so when producing a repeat clutch, indicating that some of the variation in breeding parameters are due to differences in parental or territory quality. Differences between years in the temperature‐dependent development rate of caterpillars seem to affect the time constraints on breeding. A year with a predicted early seasonal decline in caterpillars resulted in short intervals between removal and relaying, small clutches and an early onset of incubation.     

The effects of low dietary calcium during egg-laying on eggshell formation and skeletal calcium reserves in the Zebra Finch Taeniopygia guttata

Many small passerines forage intensively for calcium-rich foods during laying. Increased incidences of shell defects in eggs of small passerines have been reported, particularly in western Europe, and these have been explained in terms of declining calcium availability in soils, resulting from prolonged anthropogenic acid deposition. Studies in the field have provided laying birds, nesting in areas of low calcium availability, with calcium supplements. An alternative approach was adopted in this study by allowing captive Zebra Finches Taeniopygia guttata to lay first clutches on ad libitum calcium, switching them to a low calcium diet for 72 hours for the formation of all but the first egg of the second clutch and reinstating ad libitum calcium for the final clutch. Control females had access to ad libitum calcium for all three clutches. Clutch sizes did not vary significantly between birds on low calcium and controls. The former took over three days longer to lay clutch 3 than did controls but the difference was not statistically significant. Birds on low calcium laid eggs that declined in shell ash mass with laying sequence, indicating that birds may have been calcium-limited. Although not statistically significant, eggshell thickness also declined with laying sequence in clutches laid by females on low calcium. The remaining egg measurements (shell mass, shell surface area and volume] of clutches laid by birds on low calcium did not differ significantly from those of controls. Furthermore, females on low calcium did not resort to skeletal reserves to provide sufficient calcium for egg formation. Dietary calcium appears to be of paramount importance in providing sufficient calcium for clutch formation.     

Does the cost of incubation set limits to clutch size in common eiders Somateria mollissima?

We examined the effect of natural clutch size on the cost of incubation in a population of common eiders Somateria mollissima nesting in Tromsø, northern Norway. The body condition of females at day 5 in the incubation period was not related to clutch size (3–6 eggs), but females incubating large clutches lost more mass and had a lower body condition at day 20 in the incubation period than females incubating small clutches. Females incubating large clutches had a slightly shorter incubation period and a lower egg predation rate. The results do not support the hypothesis that the female's ability to produce eggs is the only ultimate control of clutch size in eider. Instead the results suggest that there may be an interaction between the allocation of body reserves to eggs and incubation, and that females producing large clutches allocate more of their body reserves to incubation than females producing small clutches, in order to shorten the incubation period and to minimise the risk of predation on eggs.     

Intraclutch egg-size variation and offspring survival in the Kentish Plover Charadrius alexandrinus

We studied the consequences of intraclutch egg-size variation in Kentish Plovers Charadrius alexandrinus in southern Spain to test the hypothesis that females allocate resources preferentially to eggs with the greatest survival potential. Second eggs were larger and had a greater hatching success than the other eggs in a clutch. However, we did not find unequivocal support for the differential allocation hypothesis, because egg failures according to laying order were not due to the causes predicted by the hypothesis, and the change in egg-size with laying order was not consistent between successive clutches of renesting females. This suggests that nutrient availability during egg formation could act as a proximate factor affecting egg-size independently of laying order. Larger eggs produced heavier chicks and, within broods, heavier chicks were most likely to be recruited into the breeding population. By laying clutches with a larger mean egg volume in replacement clutches, females took longer to lay again after failure of the first clutch. Thus, opposing selective forces may act to increase and decrease egg volume. Although a trade-off between egg-size and clutch-size has not been found in precocial birds, our results suggest that there may be a trade-off between egg-size and fecundity, since an increase in the size of eggs within clutches may limit the time remaining within the season to lay second or replacement clutches.     

Within‐ and between‐season repeatability of eggshell colouration in the great reed warbler Acrocephalus arundinaceus

While avian eggshell colouration has attracted biologists for decades, little is known about its variation within individuals. The main goal of this study was to explore within‐ and between‐season repeatability of eggshell appearance in the great reed warbler Acrocephalus arundinaceus. To achieve this, we measured eggshell reflectance of first and replacement clutches of individual females within one breeding season, and the reflectance of their first clutches in two following breeding seasons. As environmental conditions may affect egg colouration, repeatability was estimated from linear mixed‐effects models where we initially included temperature and rainfall during egg laying, as well as year, clutch order and egg‐laying date as fixed effects, and clutch identity nested in females as random effects. Eggshell appearance within clutches showed moderate repeatability, while both within‐ and between‐season repeatability in clutch colouration for individual females was low. Our findings indicate that low intra‐clutch variation may have a function within the context of brood parasitism, i.e. in facilitating host recognition of alien eggs. With variable eggs between successive clutches, however, host females may need to relearn the appearance of their eggs with every clutch they lay. This could represent a significant constraint for host egg‐discrimination abilities. Yet, whether environmental or intrinsic physiological factors are responsible for the variation in eggshell colouration between successive clutches of the same females still remain to be discovered.     

Measuring temporal variation in reproductive output reveals optimal resource allocation to reproduction in the northern grass lizard, Takydromus septentrionalis

We measured the reproductive output of Takydromus septentrionalis collected over 5 years between 1997 and 2005 to test the hypothesis that reproductive females should allocate an optimal fraction of accessible resources in a particular clutch and to individual eggs. Females laid 1–7 clutches per breeding season, with large females producing more, as well as larger clutches, than did small females. Clutch size, clutch mass, annual fecundity, and annual reproductive output were all positively related to female size (snout–vent length). Females switched from producing more, but smaller eggs in the first clutch to fewer, but larger eggs in the subsequent clutches. The mass-specific clutch mass was greater in the first clutch than in the subsequent clutches, but it did not differ among the subsequent clutches. Post-oviposition body mass, clutch size, and egg size showed differing degrees of annual variation, but clutch mass of either the first or the second clutch remained unchanged across the sampling years. The regression line describing the size–number trade-off was higher in the subsequent clutch than in the first clutch, but neither the line for first clutch, nor the line for the second clutch varied among years. Reproduction retarded growth more markedly in small females than in large ones. Our data show that: (1) trade-offs between size and number of eggs and between reproduction and growth (and thus, future reproduction) are evident in T. septentrionalis ; (2) females allocate an optimal fraction of accessible resources in current reproduction and to individual eggs; and (3) seasonal shifts in reproductive output and egg size are determined ultimately by natural selection.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 91 , 315–324.     

Do purely capital layers exist among flying birds? Evidence of exogenous contribution to arctic-nesting common eider eggs

The strategy of relying extensively on stored resources for reproduction has been termed capital breeding and is in contrast to income breeding, where needs of reproduction are satisfied by exogenous (dietary) resources. Most species likely fall somewhere between these two extremes, and the position of an organism along this gradient can influence several key life-history traits. Common eiders (Somateria mollissima) are the only flying birds that are still typically considered pure capital breeders, suggesting that they depend exclusively on endogenous reserves to form their eggs and incubate. We investigated the annual and seasonal variation in contributions of endogenous and exogenous resources to egg formation in eiders breeding at the East Bay colony in the Canadian Arctic. We collected prey items along with females and their eggs during various stages of breeding and used two complementary analytical approaches: body reserve dynamics and stable isotope [δ(13)C, δ(15)N] mixing models. Indices of protein reserves remained stable from pre-laying to post-laying stages, while lipid reserves declined significantly during laying. Similarly, stable isotope analyses indicated that (1) exogenous nutrients derived from marine invertebrates strongly contributed to the formation of lipid-free egg constituents, and (2) yolk lipids were constituted mostly from endogenous lipids. We also found evidence of seasonal variation in the use of body reserves, with early breeders using proportionally more exogenous proteins to form each egg than late breeders. Based on these results, we reject the hypothesis that eiders are pure capital layers. In these flying birds, the fitness costs of a strict capital breeding strategy, such as temporary loss of flight capability and limitation of clutch and egg size, may outweigh benefits such as a reduction in egg predation rate.     

Relationship between clutch size,egg volume and hatching success in a Yellow-legged Gull Larus michahellis colony in south-eastern Tunisia

This study aimed to test the hypothesis that clutch size covaries with egg volume and hatching success in the Yellow-legged Gull Larus michahellis. We determined clutch size and egg volume in a sample of 131 nests, and we used the data to check whether egg volume varied among nests according to clutch size, while taking into account the effects of egg laying order. We also estimated hatching success rate and investigated the relationship between hatching success and clutch size. Egg volume varied among clutches according to clutch size, with eggs being larger in three-egg clutches than in two-egg clutches. Moreover, three-egg clutches showed higher daily survival rates, and hence hatching success, than two-egg clutches. Overall, our results suggest that in the Yellow-legged Gull clutch size covaries with egg volume and hatching success, which could possibly reflect an age effect through different mechanisms. Indeed, older females could be hypothesised to exhibit greater breeding performance than younger females because of their higher experience in tapping energy resources for egg formation and defending nests from dangers. Moreover, due to their age, older females are likely to have lower residual reproductive potential and should invest more heavily in current breeding attempts.     

Indeterminate laying and flexible clutch size in a capital breeder, the common eider

Clutch size control in capital breeders such as large waterfowl has been much debated. Some studies have concluded that clutch size in ducks is determined before the start of laying and does not change in response to egg additions or removals. The response, however, may depend on the timing of tests, and experiments may have been too late for females to alter the number of eggs. We here study clutch size responses to predation of first and second eggs in the common eider, using protein fingerprinting of egg albumen to verify that the same female continues laying in the nest after predation. Sixty of 79 females with early egg predation (one or both of the two first eggs) deserted the nest. Among the 19 females that stayed and continued laying, the mean number of eggs produced was 4.4, significantly higher than the 3.7 in non-predated nests. The staying females had similar egg size and clutch initiation date as females that deserted, and their body mass and clutch initiation date was similar to that of females whose clutches were not predated. Even capital-breeding common eiders may therefore be indeterminate layers, as many females in which early eggs are removed lay more eggs than others. A previous study has shown that they can reduce their laying if eggs are added. Our results add to increasing evidence that ducks have more flexible egg production than previously thought.     

Arctic waders and the capital‐income continuum: Further tests using isotopic contrasts of egg components

Birds migrating annually to high‐latitude breeding grounds may benefit from the transport of endogenous nutrient reserves that ultimately contribute to reproduction. Shorebirds represent a diverse group of Arctic breeders that typically arrive on the breeding grounds with body reserves enriched in ¹³C and ¹⁵N due to wintering and staging in marine or estuarine habitats. Such isotopic differences between endogenous macronutrient reserves and local foodwebs allow the use of stable isotopes to test for the source of nutrient allocations to eggs. We examined δ¹³C and δ¹⁵N values in lipid‐free yolk and albumen and δ¹³C values in yolk lipid of first clutches of ten species of sandpiper and plover breeding near Churchill, Manitoba, Canada in 2003. Most birds had egg isotope values indicating a C3 terrestrial biome, which fits primarily an income (exogenous) breeding strategy. Two exceptions were single sandpiper and plover with strong marine isotope values. Among species, strong positive relationships for each isotope between egg tissue components suggest that egg proteins and lipids tended to be derived from the same isotopic source. Correlations of egg δ¹³C values for lipids and proteins approached theoretical relationships expected for exogenous breeding strategies, based on captive studies. Significant positive correlations between clutch initiation date and δ¹³C values of egg lipids and albumen suggest some endogenous nutrient contributions to later laid eggs but the circumstances under which this may occur are unstudied. Where possible, we recommend that researchers use blood and fat biopsies from laying females as a means of anchoring endogenous and exogenous endpoints for modeling of each reproductive event. We encourage the isotopic analysis of egg albumen, yolk and yolk lipids among individuals and species and tests of correlations among these components as a means of inferring origins of nutrients to eggs.     

Fatty acid signatures of female Brünnich's guillemots Uria lomvia suggests reliance on local prey for replacement egg production

The laying of smaller replacement eggs has been described as a time-saving adaptation because chicks generally grow faster once hatched than inside the egg. However, chicks hatched from smaller eggs have, potentially, lower survival. Consequently, the smaller replacement egg represents a benefit to the female in terms of preserving its own condition at a cost to their offspring. We test these ideas by measuring adult mass changes and plasma lipid concentration changes in male and female Brünnich's guillemots Uria lomvia breeding on Coats Island, Nunavut. Though males lost more mass than females, these differences were not significant. Between laying the first and replacement egg, plasma fatty acid concentrations declined in females and increased in males, suggesting that females mobilise less lipid to preserve their condition after laying the replacement egg. In females, plasma lipid concentrations of the dominant fatty acids found in the eggs (16:0 and 18:1) declined between the laying of first and replacement eggs while plasma concentrations of 20:1 increased in both males and females. We compared the fatty acid signatures of first and replacement egg to look for evidence of differences between the lipid sources for their production. Principal component and discriminant function analyses showed that the fatty acid signatures of replacement eggs were closer to the signatures of the local prey than those of first eggs. We suggest that females rely on local sources of energy to a greater degree for the production of the replacement egg than the first egg, but that endogenous reserves of certain nutrients are important for the production of both eggs.     

FACTORS AFFECTING THE EGG SIZE OF RED-BILLED GULLS LARUS NOVAEHOLLANDIAE SCOPULINUS

The factors influencing the egg size of the Red-billed Gull Larus novaehollandiae scopulinus were studied at Kaikoura, New Zealand, between 1964 and 1972. In two- and three-egg clutches there was a trend for the eggs to become smaller in the sequence of laying. Length, breadth and volume of eggs of one-, two- and three-egg clutches declined significantly as the season progressed. The size of eggs from single-egg clutches tended to be smaller than eggs from two-egg clutches laid at the same time. There were correlations between the proportions of one-egg and of three-egg clutches being laid at a given period and the mean egg volume of two-egg clutches. When the mean egg volume of two-egg clutches increased there was a corresponding increase in the proportion of two- and three-egg clutches laid. When the mean egg volume of two-egg clutches decreased there was an increase in the proportion of single-egg clutches laid. The egg size of the Red-billed Gull showed no direct correlation with the abundance or availability of food; the largest eggs were produced early in the season when food was in short supply. In spite of an increase in the food supply in the middle of the breeding season, birds laying at this time produced smaller eggs than birds which laid earlier in the season. However, early breeders which relayed at the peak in food abundance on average produced a larger replacement clutch than originals laid early in the season. It is suggested that the birds nesting early in the season are able to produce the largest eggs because they are the most efficient foragers for food, and those which nest later in the season produce smaller eggs, even at peak food abundance, because of their inefficiency or inexperience. Early breeders laying replacement clutches tended to lay larger eggs and larger clutches than birds which are producing their first clutches at the same time. Two-year-old females laid eggs which were significantly shorter than older aged birds while the breadth and volume of the egg increased with the age of the female up to the fifth year. There was a trend for females to lay larger eggs when mated with older rather than younger males. No statistical differences in egg size were detected between females changing or retaining the partner of the previous season. Female body weight and egg volume were positively correlated in females weighing less than 275 g but not for heavier females. It is suggested that the seasonal decline in egg size and clutch size results from a decrease in the availability of food and the ability of the individual to exploit the resource.     

Maturation increases early reproductive investment in Adélie Penguins Pygoscelis adeliae

Correlations between female investment in egg production and age, breeding experience and laying date have been reported in several seabird species. In general, clutch and egg sizes increase with female age and breeding experience but decrease with laying date. Positive correlations of clutch and egg size with age and breeding experience can be caused by an increase in reproductive investment with maturation or they may be an artefact of lower survival rates for individuals with poor-quality phenotypes. Negative correlations of clutch and egg size with laying date might signal an adaptive reduction in egg production or be due in part to variation among individuals. We examined the interactions of female age, breeding experience, laying date and clutch and egg size in Adélie Penguins Pygoscelis adeliae . Breeding experience strongly affected clutch size with 87.3% of all one-egg clutches laid by first-time breeders. In addition, increasing age had a positive influence on egg size and was associated with earlier laying dates. However, there was little evidence to suggest that either clutch or egg sizes are influenced by laying date. Laying dates and clutch and egg sizes did not affect a female's probability of returning to breed in the following year, indicating that increased investment is a product of maturation and not of the loss of poor-quality breeders from the population. Our results suggest that as female Adélie Penguins gain foraging and breeding experience they are able to initiate breeding earlier, to lay complete clutches of two eggs and to lay larger eggs.     

Mate provisioning, nutritional requirements for egg production, and primary reproductive effort of female Common Terns Sterna hirundo

We assessed the nutritional importance of mate provisioning to females during egg production and its effects on clutch parameters (egg size, length of the laying period) in Common Terns Sterna hirundo: (1) we estimated the costs of egg production by modeling the daily protein, lipid, and energy requirements of laying females, and (2) compared these costs to both the amount, and the timing, of the male's contribution via mate provisioning. Net lipid, net protein, and gross energy requirements for a three‐egg clutch were estimated to be 5.4 g, 8.6 g, and 569 kJ respectively. Peak protein and lipid requirements occurred one (day ?1) and two (day ?2) days before laying, respectively. Peak energy requirement occurred on day ?1; a cost of 127% to 157% above maintenance. Variation in male provisioning effort (in terms of energy and nutrients delivered) paralleled variation in predicted female requirements for egg production at the level of individual pairs. Males delivered protein in excess of the female's requirements on all days investigated. Male lipid delivery accounted for 45% of female net requirement on the day when demand was greatest (day ?2), but exceeded requirements on all other days. However, the proportion of the female's total energy budget (egg production, maintenance, and activity costs) that was supplied by her mate rose from an average of 29% on day ?2 to 76% during the interval between second and third eggs. Paradoxically, females that were fed at higher rates during the interval between first and second eggs produced clutches with lower total volumes, smaller last‐laid eggs, and clutches with a greater egg‐size hierarchy than conspecifics receiving less food from their mates. Also, females fed at higher rates during the interval between second and third eggs took longer to produce their clutch. These negative relationships between mate provisioning and clutch parameters contrast with previous studies in this and other species.     

Patterns of variation in size of Boat-tailed Grackle Quiscalus major eggs

Boat-tailed Grackle Quiscalus major eggs averaged 8^.09 g wet weight. Mean egg weight represented 8 05 % of female body weight. Based on egg weight, Grackle eggs have a shorter incubation period than the general passerine pattern.
Egg weight varied significantly with sequence of laying and between clutches in both two-egg and three-egg clutches. Last laid eggs weighed less than first laid eggs. The mean weight of the first two eggs laid in three-egg clutches did not differ from the mean egg weight of two-egg clutches. The average clutch of two and three eggs weighed 16.33 g and 24.16 g, respectively.
Mean egg weight varied between study colonies and with season. Grackles at East Lake laid eggs that weighed less than grackles at either Alligator Lake or North Lake. Eggs laid during March averaged less than eggs laid later in the season. The locality variation reflects the different timing of nesting between sites rather than food abundance. As nutrient provisioning increases with an increase in egg weight, the seasonal change in egg weight probably reflects improved feeding conditions for the female. This suggests that selection favours beginning laying before reserves of the female are sufficient to lay the largest egg possible.
The size of the young at hatching was correlated with egg size. Newly hate had young averaged 79.6% of fresh egg weight. Egg weight did not determine the probability of hatching or starvation of the young. Females do not appear to adjust egg size facultatively depending on the sex of the young. Eggs producing males and females did not vary significantly in weight, whether comparisons were made within or between clutches.     

Two eggs,two different constraints: a potential explanation for the puzzling intraclutch egg size dimorphism in Eudyptes penguins

Phenotypic plasticity and phenotypic stability are major components of the adaptive evolution of organisms to environmental variation. The invariant two-egg clutch size of Eudyptes penguins has recently been proposed to be a unique example of a maladaptive phenotypic stability, while their egg mass is a plastic trait. We tested whether this phenotypic plasticity during reproduction might result from constraints imposed by migration (migratory carry-over effect) and breeding (due to the depletion of female body reserves). For the first time, we examined whether these constraints differ between eggs within clutches and between egg components (yolk and albumen). The interval between colony return and clutch initiation positively influenced the yolk mass, the albumen mass, and the subsequent total egg mass of first-laid eggs. This time interval had only a slight negative influence on the yolk mass of second-laid eggs and no influence on their albumen and subsequent total masses. For both eggs, female body mass at laying positively influenced albumen and total egg masses. Female investment into the entire clutch was not related to the time in the colony before laying but increased with female body mass. These novel results suggest that the unique intraclutch egg size dimorphism exhibited in Eudyptes penguins, with first-laid eggs being consistently smaller than second-laid eggs, might be due to a combination of constraints: a migratory carry-over effect on the first-laid egg and a body reserve depletion effect on the second-laid egg. Both these constraints might explain why the timing of reproduction, especially egg formation, is narrow in migratory capital breeders.