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.     

Calculating egg volume when shape differs: when are equations appropriate?

ABSTRACT Egg volume is often calculated from length and breadth, assuming little variation in egg shape within species. However, egg shape may vary among females, over time, or within clutches. If this variation is not predictable, volume should be estimated using another method. For example, in some species, egg shape changes consistently with laying order and volume can be estimated using laying‐order specific equations. We measured the length, breadth, volume, and mass of 249 Magellanic Penguin (Spheniscus magellanicus) eggs and regressed volume on length and breadth for first and second eggs separately. Differences between measured and calculated volume averaged <1% for both eggs. Using linear measurements, we calculated egg volume for 7085 clutches from 1983 to 2006. First and second eggs had similar calculated volumes (P > 0.05) in 19 of 24 yr; when they differed, the second egg was larger. In contrast, using single equations typically employed to compute egg volume, we erroneously concluded that first eggs were usually (20 of 24 yr) significantly larger than second eggs. Our regression equations should be applicable to other Spheniscus penguins because ratios of elongation coefficients (length divided by breadth) of first and second eggs are similar among these species. In other species of birds where egg shape changes with laying order, the volume of a sample of eggs should be measured to develop regression equations specific to the species and laying order. Slight variation in egg volume can have important evolutionary and ecological implications that would be erroneously interpreted without field tests to measure volume.     

Resting eggs in rotifers

The biology of resting eggs of monogonont rotifers is reviewed, covering literature published since the last major review by Gilbert (1974). The topics examined include resting egg production, morphology and species specificity, hatching, and evolutionary significance. Four major determinants of resting egg production are identified: mictic female production, male activity and fertility, female susceptibility to fertilization, and fertilized female fecundity. Recent work in these four areas is discussed as well as resting egg production in natural populations. Resting egg morphology, particularly shell structure and internal organization, is compared among species. Recent reports on the control of resting egg hatching in the laboratory are examined and the importance of temperature, light, diet, and salinity is reviewed. Two hatching patterns are contrasted, the first where eggs hatch at regular intervals over extended periods and the second where hatching is synchronized to some environmental cue. A latent period after resting egg formation, during which no hatching occurs, is defined for several species. The adaptive features of resting eggs are outlined including their contribution to genetic variability through recombination, their provision for environmental escape by dormancy, and their colonizing function resulting from their ease of dispersal. The type of cue utilized to initiate mictic female production as well as the pattern of resting egg hatching is related to environmental predictability.     

Eggs of the Australian saltmarsh mosquito,Aedes camptorhynchus,survive for long periods and hatch in instalments: implications for biosecurity in New Zealand

The Australian saltmarsh mosquito, Aedes camptorhynchus (Diptera: Culicidae), is a significant biting pest and disease vector and is the subject of an eradication programme in New Zealand (NZ), where it has been resident for more than 10 years. To better understand the ecology of this common and widespread pest, we studied egg longevity and hatching patterns in the laboratory. By regularly testing for the presence of viable embryos, we found that eggs may last more than 15 months when stored dry (13% viable at this time). Eggs display instalment hatching, with no more than 56% of a batch hatching upon first inundation. Further hatching may occur for at least six inundations and some unhatched eggs may remain viable even after this. Variation in hatching rates can be observed using different water types, with weaker hatching media stimulating lower hatching rates spread over more inundations. By applying average hatching rates to a non‐linear model of natural egg attrition, we showed that egg batches exposed to three inundations should be exhausted (zero live eggs present) in approximately 11 months at the conditions tested here. These findings have implications for the current eradication programme for Ae. camptorhynchus in NZ and for our understanding of the ecology of a widespread and common disease vector in Australia.     

Avian egg collections: museum collection bias driven by shape and size

Avian eggs exhibit substantial intra- and interspecific variation in shape, size and colour. Considerable efforts have been made to better understand the evolutionary drivers behind such variation, often using museum egg collections. Usually it is assumed that museum collections accurately represent the variation seen in natural populations, but this may not be the case if there is collection bias. Collection bias may lead to the over-representation of certain egg traits in collections, due to the aesthetic (or other) preferences of collectors. The aim of this study is to begin to look for the occurrence of potential collection bias in museum egg collections by comparing three shape indices (pointedness/asymmetry, elongation and polar asymmetry) and egg volume between subsets of eggs in museum collections with those of recently sampled eggs in the field for three different bird species: common guillemot Uria aalge, razorbill Alca torda and northern fulmar Fulmarus glacialis. We found no evidence of collection bias in our sampled razorbill and northern fulmar museum collection eggs, but some evidence for a bias in sampled museum collection eggs of common guillemots. Since the guillemot's egg differs from most bird eggs in being pyriform, we suggest that collection bias by historic egg collectors may be more prevalent in species with extreme egg traits. Researchers using museum egg collections to examine questions relating to egg shape should be aware of collection bias risks and consider how to minimise the effect of these possible biases on accumulated datasets.     

Declining egg viability explains higher hatching failure in a suburban population of the threatened Florida scrub‐jay Aphelocoma coerulescens

Hatching failure occurs in approximately 10% of all avian eggs, but varies both within and among species. This reduction in viable offspring can have significant fitness consequences for breeding parents; therefore, it is important to understand which factors influence variation in hatching failure among populations. Previous research suggests that hatching failure is higher in a suburban than in a wildland population in the Florida scrub‐jay. From 2003 to 2007, we performed two experiments to examine whether increased hatching failure in the suburbs resulted from 1) increased length of off‐bouts during incubation (predation risk hypothesis, 2003–2004) or 2) increased exposure to ambient temperature during laying (egg viability hypothesis, 2005–2007). Hatching failure was higher for females that took fewer off‐bouts, but the length of those off‐bouts did not influence hatching failure. Thus, nest predation risk does not appear to explain higher hatching failure in the suburbs. Alternatively, hatching failure increased with increasing exposure of eggs to ambient conditions during the laying period. First‐laid eggs in the suburbs had the greatest pre‐incubation exposure to ambient temperature and the greatest rate of hatching failure, consistent with the egg viability hypothesis. Urbanization influences hatching failure through a series of complex interactions. Access to predictable food sources advances mean laying date in suburban scrub‐jays, leading to larger clutch sizes. Because scrub‐jays begin incubation with the ultimate egg, first‐laid eggs in the suburbs may be exposed to ambient temperatures for longer periods, thus reducing their viability.     

Egg shape mimicry in parasitic cuckoos

Parasitic cuckoos lay their eggs in nests of host species. Rejection of cuckoo eggs by hosts has led to the evolution of egg mimicry by cuckoos, whereby their eggs mimic the colour and pattern of their host eggs to avoid egg recognition and rejection. There is also evidence of mimicry in egg size in some cuckoo–host systems, but currently it is unknown whether cuckoos can also mimic the egg shape of their hosts. In this study, we test whether there is evidence of mimicry in egg form (shape and size) in three species of Australian cuckoos: the fan‐tailed cuckoo Cacomantis flabelliformis, which exploits dome nesting hosts, the brush cuckoo Cacomantis variolosus, which exploits both dome and cup nesting hosts, and the pallid cuckoo Cuculus pallidus, which exploits cup nesting hosts. We found evidence of size mimicry and, for the first time, evidence of egg shape mimicry in two Australian cuckoo species (pallid cuckoo and brush cuckoo). Moreover, cuckoo–host egg similarity was higher for hosts with open nests than for hosts with closed nests. This finding fits well with theory, as it has been suggested that hosts with closed nests have more difficulty recognizing parasitic eggs than open nests, have lower rejection rates and thus exert lower selection for mimicry in cuckoos. This is the first evidence of mimicry in egg shape in a cuckoo–host system, suggesting that mimicry at different levels (size, shape, colour pattern) is evolving in concert. We also confirm the existence of egg size mimicry in cuckoo–host systems.     

A precise water displacement method for estimating egg volume

ABSTRACT.   Relationships between egg volume and an array of life-history traits have been identified for many bird species. Despite the importance of egg volume and the need for precise and accurate measurements, egg volume is usually estimated using a mathematical model that incorporates length and width measurements along with a shape variable. We developed an instrument that provides precise estimates of egg volume and can be easily used in the field. Using Clapper Rail ( Rallus longirostris ) eggs, we compared egg volumes measured using our instrument with estimates based on linear measurements. We found our instrument to be both precise and accurate. Compared with a method based on linear measurements of eggs, use of our instrument reduced variation in egg volume estimates by 1.6 cm³, approximately 8% of the volume of a Clapper Rail's egg. Further advantages of our technique include ease of use, increased accuracy of field-based volume estimates, and increased resolution of variation in egg volume estimates. In addition, our technique does not require postdata collection processing time and did not influence hatching success. Also, for Clapper Rails and similar species, our technique can be combined with other techniques (e.g., egg flotation) so that both egg volume and embryonic stage can be estimated at the same time.     

Proximate causes and fitness consequences of hatching failure in lesser kestrels Falco naumanni

Hatching failure is a pervasive phenomenon in birds, but factors affecting hatchability remain poorly understood. We studied proximate causes and fitness consequences of hatching failure in a long‐monitored population of the colonial lesser kestrel Falco naumanni. We investigated whether hatchability was related to clutch characteristics, parental traits, and social or environmental features. Hatching failure represents a cost for the parents in terms of immediate fitness, since it reduced both their number of young fledged and recruits in the breeding population, even when controlling for clutch size. Hatching failure showed a non‐linear relationship with clutch size, clutches of four eggs showing higher levels of hatching success than larger or smaller clutches. Hatchability could therefore play a role in the evolution of optimal clutch size in this species, at least constraining the maximum number of eggs the parents can afford to incubate. Contrary to most studies, the mean volume of the clutch and the individual egg volume were negatively related to hatching failure, indicating that large eggs have thermoregulatory and/or nutritional advantages. Mean daily maximum temperature during incubation affected hatching success negatively, but only for females in poor condition. This result seems to indicate that females are reluctant to jeopardize their own condition, but instead sacrifice incubation effort by paying the costs of a lower hatching success in circumstances of high temperatures. There was no evidence that hatching failure was related to the intrinsic properties of individuals or genetic similarity between the parents as indicated by low repeatabilities of: (1) males that bred with different females, (2) females that bred with different males, and (3) pairs breeding together in different years. Neither colony size nor subpopulation size affected hatchability. All these findings show how hatching failure is simultaneously influenced by several factors acting in a complex way, which could in part explain the apparently conflicting conclusions of empirical or even experimental studies carried out to date.     

Geographic variation in offspring size of a widespread lizard (Takydromus septentrionalis): importance of maternal investment

Geographic variation in offspring size is widespread, but the proximate causes of this variation have not yet been explicitly determined. We compared egg size and egg contents among five populations of a lizard (Takydromus septentrionalis, Günther, 1864) along a latitudinal gradient, and incubated eggs at two temperatures to determine the influence of maternal investment and incubation temperature on offspring size. The mean values for female size and egg size were both greater in the two northern populations (Chuzhou and Anji) than in the three southern populations (Lishui, Dongtou, and Ningde). The larger eggs were entirely attributable to the body size of females in the Anji population, but their increased size also stemmed from further enlargement of egg size relative to female body size in Chuzhou, the northernmost population sampled in this study. Eggs of the Chuzhou population contained more yolk and less water than those of southern populations. Despite the lower lipid content in the yolk, eggs from the Chuzhou population had higher energy contents than those from the two southern populations, owing to the larger egg size and increased volume of yolk. Hatchling size was not affected by incubation temperature, but differed significantly among populations, with hatchlings being larger in the Chuzhou population than in the other populations. Our data provide an inference that oviparous reptiles from cold climates may produce larger offspring, not only by increasing egg size but also by investing more energy into their eggs. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101 , 59–67.     

Influence of egg size differences within egg clutches on larval parameters in nine libellulid species (Odonata)

Abstract.  1. In libellulids, egg size differs between species and populations. There are also size differences within egg clutches of individual females.
2. Past experiments suggest that there are two different types of egg clutches in libellulids. Egg size decreases significantly during oviposition in species that perform non-contact guarding during oviposition. In contrast, in species ovipositing in tandem, egg size is randomly distributed.
3. This study deals with the possible consequences of egg size variation within the different egg clutch types. The study examined whether there is a correlation between egg development time, offspring sex or larval size and egg size.
4. The current experiments were conducted in Namibia and Germany. Five non-contact guarding and four tandem guarding libellulid species were used.
5. In some species larger eggs needed more time to develop, in some species no correlation between egg size and egg development time could be found, whereas in other species larger eggs developed faster.
6. The sex ratio was biased towards females in Leucorrhinia dubia and in Sympetrum striolatum and egg size was not associated with gender.
7. In both egg clutch types larger eggs resulted in larger larvae. In this study, evidence was found that the effects of egg size diminished with progressing larval development under good conditions. However, it is possible that the effects may have a greater influence under harsh circumstances.     

Influence of egg size on embryos and larvae of Fundulus heteroclitus (L.)

Egg size for Fundulus heteroclitus (L.) populations is concordant with the distribution of the two F . heteroclitus subspecies, i.e. F. h. heteroclitus eggs are considerably larger than F. h. macrolepidotus eggs. The influence of egg size on survival of embryos during incubation and survival and growth of newly-hatched larvae was estimated for four populations representing both subspecies along the Atlantic coast of the United States and in Delaware Bay. Survival of embryos was determined for incubation periods of 14, 21 and 28 days. Greatest differences in survival were detected following the longest incubation period where less than 50 per cent of the smaller F. h. macrolepidotus eggs survived while little or no mortality was detected among the larger F. h. heteroclitus eggs . Influence of egg size on larval survival was also greatest among those larvae hatched after 28 days where F. h. macrolepidotus larvae survived without food, for an average of 6 days, while F. h. heteroclitus larvae survived 11–12 days. F. h. heteroclitus larvae were significantly larger at hatching than F. h. macrolepidotus larvae. Larval growth rates were the same (0.4 mm day⁻¹) in both subspecies. As a result, size differences at hatching were still maintained after 42 days of growth. The differences in egg size along with other morphological and reproductive characteristics of F. heteroclitus populations probably represent genetically based adaptations to environmental conditions, of which the length of the spawning season is one of the major components stimulating the coevolution of these traits.     

The body size of the New Zealand orb-weaving spider Waitkera waitakerensis (Uloboridae) is directly related to temperature and affects fecundity

Abstract. Waitkera waitakerensis occupies lowland forests of New Zealand's North Island, where temperatures decrease in a southwestward direction. The mean annual temperatures of 18 collecting sites, as extracted from GIS data, are directly related to the first femur length of adult females. Neither site elevation nor phylogeny affected spider size or other variables examined. The direct relationship between spider body size and environmental temperature followed a pattern observed in other terrestrial arthropods with a univoltine life cycle and can probably be explained by the longer growing season of warmer regions. Egg diameter was uniform across the species. Site temperature and female first femur length were each directly related to the number of eggs deposited in egg sacs. The date of egg sac collection was inversely related to egg number, suggesting that clutch size declines during the reproductive season. Females deposit eggs beneath a triangular platform and then cover them with a lower silk sheet. The area of this upper platform and the volume of the egg sac were each directly related to egg number, but not to female first femur length. The depth of the lower covering was not related to egg number or to spider first femur length. This suggests that spiders use information about the volume of eggs in their abdomens to construct an egg sac whose volume will accommodate the volume of eggs to be laid and that females do so principally by adjusting the size of the sac's upper triangular platform.     

Generational shape shifting: changes in egg shape and size between sexual and asexual generations of a cyclically parthenogenic gall former

Successive generations of multivoltine species experience selection specific to the spatiotemporal environments encountered that may lead to adaptive divergence in reproductive traits among generations. To compare reproductive effort within and between generations, appropriate volumetric models, selected on the basis of the analysis of egg shape, are required to estimate the sizes (volumes) of individual eggs. We assessed the shape and estimated the volume of individual eggs produced by the temporally and spatially segregated sexual and asexual generations of the gall former, Belonocnema treatae Mayr (Hymenoptera: Cynipini: Cynipidae). Egg shape, indexed as the difference between the polar and equatorial axes of the ellipsoidal eggs, was independent of egg size, but differed between generations. The relationship of egg shape and female body size within and between generations confirmed that egg shape is an intrinsic property of each generation. Generational differences in egg shape then informed the selection of volumetric models to estimate egg size. We modeled asexual generation eggs as both spheres and prolate spheroids, and sexual generation eggs as both cylinders and prolate spheroids. Choice of volumetric model changed estimates of egg size within the asexual generation by 23% and within the sexual generation by 50%. Comparisons between generations based on the above models produced estimated differences in egg volume that ranged from 16 to 114%. In both generations, a prolate spheroid was the most parsimonious model of egg volume. Based on this model, sexual generation eggs averaged 43% larger than asexual generation eggs. The increased size of sexual eggs was achieved via conservation of the egg’s equatorial axis and elongation of the polar axis. The shift in egg shape between sexual and asexual B. treatae is the first documented dimorphism in an egg characteristic expressed between generations of a cyclically parthenogenic organism.     

The point of a Guillemot's egg

The adaptive significance of avian egg shape in birds is poorly understood. The pyriform (pear‐like) shape of the Common Guillemot's Uria aalge egg has long been considered to be an adaptation to prevent eggs rolling off the bare cliff ledges on which this species breeds. Rolling was thought to be prevented either by the egg spinning like a top, which is not the case, or by rolling in an arc, which it does but with little influence on whether the egg will fall from a ledge. We therefore sought alternative explanations for the pyriform shape of the Common Guillemot's egg. This species breeds in extremely dense colonies, which makes their eggs vulnerable to mechanical damage from conspecifics, and to contamination by debris such as faeces and soil. We present evidence consistent with both these possible explanations. First, the pyriform shape of Common Guillemot eggs means that a higher proportion of the eggshell lies in contact with the substrate and this may minimize the effect of impacts. Resistance to impacts may be further enhanced because their eggshells are especially thick where they are in contact with the substrate. Secondly, Common Guillemot eggs are often heavily contaminated with faecal material and other debris during incubation. Most contamination is on the pointed end of the egg where it is in contact with the substrate; the pyriform shape thus keeps the blunt end of the egg, which has the highest porosity, relatively free of contamination, which in turn may facilitate both gas exchange during incubation and the hatching process, because the chick emerges from the blunt end of the egg.     

FITNESS CONSEQUENCES OF VARIATION IN EGG SIZE AND FOOD ABUNDANCE IN BROOK TROUT SALVELINUS FONTINALIS

Relationships between egg size and juvenile survival in brook trout, Salvelinus fontinalis, were determined experimentally at two levels of food abundance and then incorporated into a model that related maternal fitness to egg size and food supply. Egg volume was positively correlated with juvenile size at hatching and size at yolk sac resorption but had no significant effect on embryonic survival or development time. Juvenile survival was linearly related to egg size throughout the first 50 days of exogenous feeding at high and low food levels. The effects of egg size and food abundance on juvenile survival were not additive. Decreased food abundance significantly increased mortality among the smallest eggs but had a negligible effect on the largest eggs. Model simulations indicate that maternal fitness is a curvilinear function of egg size and that food supply influences both the height and the shape of the function. The fitness functions provide empirical support for the hypothesis that selection favors an increase in offspring size with reductions in resource abundance.     

SEVERE INBREEDING DEPRESSION AND RAPID FITNESS REBOUND IN THE BUTTERFLY BICYCLUS ANYNANA (SATYRIDAE)

We established inbred laboratory lines of the satyrid Bicyclus anynana with one, three and 10 pairs of butterflies, which were subsequently allowed to increase freely to a maximum size of 300 butterflies. Minimally inbred control lines were established with 300 randomly selected virgin butterflies of equal sex ratio. We measured fecundity, egg weight, egg hatching, adult emergence, adult size, and the proportion of crippled adults in generations F₂, F₃, F₅, and F₇ (the latter two for the one pair bottleneck lines only). The most striking result was an unexpectedly large decrease in egg hatching with increase in inbreeding (25% per 10% increase in inbreeding). Such a level of inbreeding depression has not been reported previously for any insect. The distribution of egg hatching rate for individual clutches within inbred lines was markedly skewed, with a large fraction of clutches producing no eggs at all. This is interpreted as a relatively lower ratio of detrimental to lethal (or sterile) mutation loads than is found in Drosophila, the only insects for which mutation loads have been well characterized. Possible explanations for this severe inbreeding depression include a relatively high rate of mutation to recessive alleles with substantial damaging effects and infrequent episodes of inbreeding in nature. In the experiments, average egg hatching rate recovered rapidly between F₂ and F₇ in three of the six one-pair lines. We discuss the implications of these results for survival of populations through extreme bottlenecks in nature and in captivity.     

Temperature‐mediated plasticity and genetic differentiation in egg size and hatching size among populations of Crepidula (Gastropoda: Calyptraeidae)

Offspring size is a key characteristic in life histories, reflecting maternal investment per offspring and, in marine invertebrates, being linked to mode of development. Few studies have focused explicitly on intraspecific variation and plasticity in developmental characteristics such as egg size and hatching size in marine invertebrates. We measured over 1000 eggs and hatchlings of the marine gastropods Crepidula atrasolea and Crepidula ustulatulina from two sites in Florida. A common‐garden experiment showed that egg size and hatching size were larger at 23 °C than at 28 °C in both species. In C. ustulatulina, the species with significant genetic population structure in cytochrome oxidase I (COI), there was a significant effect of population: Eggs and hatchlings from the Atlantic population were smaller than those from the Gulf. The two populations also differed significantly in hatchling shape. Population effects were not significant in C. atrasolea, the species with little genetic population structure in COI, and were apparent through their marginal interaction with temperature. In both species, 60–65% of the variation in egg size and hatching size was a result of variation among females and, in both species, the population from the Atlantic coast showed greater temperature‐mediated plasticity than the population from the Gulf. These results demonstrate that genetic differentiation among populations, plastic responses to variation in environmental temperature, and differences between females all contribute significantly to intraspecific variation in egg size and hatching size. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99 , 489–499.     

The importance of energetic versus pelvic constraints on reproductive allocation by the eastern fence lizard (Sceloporus undulatus)

In ectothermic species, females often produce larger eggs in colder environments. Models based on energetic constraints suggest that this pattern is an adaptation to compensate for the slower growth of offspring in the cold. Yet, females in cold environments also tend to be larger than females in warm environments. Consequently, thermal clines in egg size could be caused by pelvic constraints, which stem from the inability of large eggs to pass through a small pelvic aperture. Models based on energetic constraints and models based on pelvic constraints predict similar relationships between maternal size and egg size. However, pelvic constraints should produce these relationships both within and among populations, whereas energetic constraints would not necessarily do so. If pelvic constraints are important, we might also expect small females to compensate by producing eggs that are relatively rich in lipids (i.e. high energy density). The present study aimed to assess whether energetic or pelvic constraints generate geographical variation in egg size of the lizard Sceloporus undulatus . Pelvic width is very highly correlated with body length in S. undulatus , making maternal size a suitable measure of pelvic constraint. Although maternal size and egg mass (dry and wet) covaried among populations, these variables were generally not related within populations. Energetic density of eggs tended to increase with decreasing egg mass (dry and wet), but this relationship was strongest in populations where no relationship between maternal size and egg mass was observed. Our results do not support the pelvic constraint model and thus indicate energetic constraints play a greater role in generating geographical variation in egg size.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 91 , 513–521.     

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.