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Avian eggs exhibit striking variability in size, shape, colour, and maculation, not only among but also within species. Technical and analytical advances in image analysis offer the opportunity to understand the factors underpinning this variability, especially when individual-based longitudinal data are available. Making use of such data, collected over four years, we investigated sources of variation in eight egg characteristics capturing the colour, spottiness, shape, and size of 1589 eggs from 687 clutches produced by 330 female common terns (Sterna hirundo) of known age. We found a high repeatability of the eight egg traits, both within clutches (range 0.48-0.77), and among clutches of the same female laid in different years (range 0.48-0.73). We also observed a within-female increase in egg size and spottiness with age, and evidence for selective disappearance of females producing spottier eggs, suggesting that egg maculation could reveal female quality. We also found that the size and shape of eggs were affected by their laying order within the clutch, suggesting that these traits may mediate intra-brood competition. We suggest further studies to identify the specific agents of selection that shape variation in egg size and morphology, to fully understand the eco-evolutionary significance of this extended female phenotype and its potential consequences for reproductive success and offspring survival.

There are many factors and environmental conditions affecting the quality and capacity of eggs produced by quail birds. We assessed the effect of storage on the mass dynamics of quail eggs after they were produced by the birds. The study was performed  on quail eggs to ensure their quality using a formula to calculate the theoretical value of the quail egg volume (cm3): V = 0.485 × D × d × d/1000, where D is the longitudinal diameter (mm); d is the transverse diameter (mm). Statistical processing of practical results was performed  using the data analysis package (SPSS). It was found that when eggs are stored for 10 days after laying at a temperature of  10-15 °С, the egg weight decreased on average by 2.6%, and the density by 2.1%. Eggs are considered one of the most important products due to containing a rich source of protein. Storing eggs in quails negatively affects the quality of eggs.

Avian eggs are products of consumer demand, with modern methodologies for their morphometric analysis used for improving quality, productivity and marketability. Such studies open up numerous prospects for the introduction of artificial intelligence (AI) and deep learning (DL). We first consider the state of the art of DL in the poultry industry, e.g., image recognition and applications for the detection of egg cracks, egg content and freshness. We comment on how algorithms need to be properly trained and ask what information can be gleaned from egg shape. Considering the geometry of egg profiles, we revisit the Preston–Biggins egg model, the Hügelschäffer’s model, universal egg models, principles of egg universalism and “The Main Axiom”, proposing a series of postulates to evaluate the legitimacy and practical application of various mathematical models. We stress that different models have pros and cons, and using them in combination may yield more useful results than individual use. We consider the classic egg shape index alongside other alternatives, drawing conclusions about the importance of indices in the context of applying DL going forward. Examining egg weight, volume, surface area and air cell calculations, we consider how DL might be applied, e.g., for egg storage. The value of DL in egg studies is in pre-incubation egg sorting, the optimization of storage periods and incubation regimes, and the index representation of dimensional characteristics. Each index can thus be combined to provide a synergy that is on the threshold of many scientific discoveries, technological achievements and industrial successes facilitated through AI and DL.

Variation in developmental conditions is known to affect fitness in later life, but the mechanisms underlying this relationship remain elusive. We previously found in jackdaws Corvus monedula that larger eggs resulted in larger nestlings up to fledging. Through a cross‐foster experiment of complete clutches we tested whether this association can be attributed to egg size per se, or to more proficient parents producing larger eggs and larger nestlings, with the latter effect being more or less independent of egg size. Due to other manipulations post‐hatching, we primarily investigated effects on nestling mass on day 5, which we show to predict survival until fledging. We introduce a new statistical approach to compare the competing hypotheses and discuss the multiple advantages of this approach over current practice of which we report the results for comparison. We conclude that 92% of the slope of the association between egg size and nestling mass can be attributed to a direct effect of egg size. The remaining 8% of the slope can be attributed to aspects of parental chick rearing ability as reflected in egg size, but this component did not deviate significantly from zero. Intriguingly, the effect of egg size on day 5 nestling mass was steeper (1.7 g cm−3) than the effect of egg size on day 1 hatchling mass (0.7 g cm−3). Early growth is exponential, and the difference in effect size may therefore be explained by hatchlings from large eggs being further in their development at hatching. The direct effect of egg size on nestling mass raises the question what causes egg size variation in jackdaws.

Reproductive effort involves trade‐offs among offspring and with homeostasis. In birds, a crucial parental investment concerns the allocation of resources to each egg. Variation in egg investment has led to the development of hypotheses regarding whether females favor the eldest nestlings (“brood reduction”) or distribute resources more evenly (“brood survival”). Alternative explanations include the maturation of reproductive organs and trade‐offs with environmental constraints, such as thermoregulation. The earliest tests of these hypotheses relied on the deviation of the last egg's volume from the clutch mean (D). More recent studies have often overlooked non‐linearity and temporal autocorrelation in egg investments. Here, we analyze 145 broods comprising 1084 eggs from a great tit (Parus major) population in eastern Spain using generalized additive mixed‐effects models (GAMMs). Laying order influenced egg volume, with most variation occurring as an increase from the first to middle eggs in the clutch, followed by a slight decrease. We favor the interpretation that females adopt a brood survival strategy because (1) volume changes were of small magnitude (approx. 5%); (2) no significant negative effects of the number of eggs developing simultaneously in the ovary, total clutch size, or minimum temperatures were found; and (3) the laying order effect prevailed throughout the breeding season. However, within this brood survival strategy, females slightly favored middle eggs. In addition, negative correlations among deviations in volume from the main strategy suggest that energetic trade‐offs were present. Finally, most variation occurred between clutches, reinforcing previous findings of limited plasticity in egg volume in birds. Our results further caution against relying on mean egg volume (D) as a measure of investment strategies and highlight the importance of considering non‐linearity and temporal autocorrelation in analyses of reproductive investment. The use of GAMMs provides a robust approach to overcoming previous analytical limitations in the study of within‐clutch variation in egg investment. Reproductive investment in eggs can be made egalitarian or unevenly distributed within clutches. We investigated the pattern of investment in great tits (Parus major) from a Spanish population using generalized additive mixed‐effects models. Our results indicate that females consistently invest less in the first eggs.

There is great variation in the size and shape of teleost eggs from species to species. The size of the teleost egg depends on the amount of yolk accumulated in the egg, which is an important factor directly affecting the survival of hatchlings. Egg shape also contributes significantly to spawning ecology and survival during the prehatching stage. In this study, we used bitterlings, which show a wide variety of egg volumes and shapes, to elucidate whether these factors are determined by germ cells or somatic cells. Reciprocal transplantations of germ cells between two bitterling species revealed that the egg volume was identical to that of the germ cell donor species in both combinations. The egg shape was also very similar to that of the species providing the germ cells. These results suggest that the volume and shape of teleost eggs are greatly influenced by germ cell autonomy.

In the egg industry, it is necessary to estimate the egg volume accurately when estimating egg quality or freshness in a non-destructive method. Egg volume and weight could obtain egg density and could be used to determine egg freshness. Therefore, the egg geometric must be obtained first to establish a volume equation with a geometric shape. This research proposes an innovative idea to derive the mathematical model and volume equation of egg shape, calculate its volume, and verify the accuracy of the mathematical equation proposed using the volume displacement method. Using the proposed equation, the minimum error between the calculated egg volume) and actual egg volume is 0.01%. The maximum volume error does not exceed 2%. The egg shape equation can accurately draw the outer contour curve of the egg by the half-length of the maximum long axis and maximum breadth of the short axis, and the distance from the center point of the egg to the maximum breadth (xm).

Egg size is a useful metric for maternal investment, offspring quality, and contaminant studies. Yet these values and the egg shape coefficients required to estimate egg size are not available for many species, including White-faced-Ibis (Plegadis chihi). We provide egg morphometrics derived from 319 White-faced Ibis eggs sampled at Bear River Migratory Bird Refuge, Great Salt Lake, Utah, from 2010 to 2012. Measured egg length (mean ± SD) was 51.20 ± 1.99 mm, egg width was 36.08 ± 1.15 mm, and whole egg mass was 34.1 ± 3.3 g. Estimated whole egg volume was 34.63 ± 3.73 cm3 and estimated egg shape coefficients were 0.507 for Kv (whole egg and egg contents), 0.547 for Kw (whole egg), and 0.524 for Kw (egg contents only). In addition, we documented expected declines in egg mass over time due to incubation (–0.22 g/d) and desiccation during storage (–0.03 g/d), that should be accounted for prior to analyses that use egg mass of freshly laid eggs.

Age and environment are important determinants of reproductive parameters in long-lived organisms. These factors may interact to determine breeding responses to environmental change, yet few studies have examined the environmental dependence of aging patterns across the entire life span. We do so, using a 20-yr longitudinal data set of reproductive phenotypes in long-lived female Nazca boobies (Sula granti), a monogamous seabird breeding in the eastern tropical Pacific. Young and old females may suffer from inexperience and senescence, respectively, and/or practice reproductive restraint. Breeding performance (for breeding participation, breeding date, clutch size, egg volume, and offspring production) was expected to be lower in these age classes, particularly under environmental challenge, in comparison with middle-aged breeders. Sea surface temperature anomalies (SSTA) represented interannual variation in the El Ni˜no–Southern Oscillation (ENSO) and were one proxy for environmental quality (a population count of clutch initiations was a second). Although only females lay eggs, both sexes care for eggs and nestlings, and the male partner’s age, alone or in interaction with female age, was evaluated as a predictor of breeding performance. Middle-aged females performed better than young and old birds for all reproductive traits. Pairing with a young male delayed breeding (particularly for old females) and reduced clutch size, and pairing with an old male reduced offspring production. Challenging environments increased age effects on breeding probability and breeding date across young to middle ages and for offspring production across middle to old ages. However, important exceptions to the predicted patterns for clutch size and fledging success across young to middle ages suggested that trade-offs between fitness components may complicate patterns of trait expression across the life span. Relationships between breeding participation, environment, and individual quality and/or experience in young females may also contribute to unexpected patterns for clutch size and fledging success, traits expressed only in breeders. Finally, independent of age, breeding responses of female Nazca boobies to the ENSO did not follow expectations derived from oceanic forcing of primary productivity. During El Niño-like conditions, egg-laying traits (clutch size, breeding date) improved, but offspring production declined, whereas La Niña-like conditions were “poor” environments throughout the breeding cycle.

Aim: Both theory and empirical data suggest that life histories of insular species slow down (the \"island syndrome\"). Insular individuals are hypothesized to lay smaller clutches of larger eggs compared with individuals belonging to closely related mainland species. Most lizards have variable clutch sizes and can lay any number between one egg and a species-specific maximum which can be well over 50 eggs. Many lizards, such as geckos and anoles, however, lay invariant small clutches of one or two eggs, and may thus be unable to manifest some aspects of the island syndrome. We tested whether insular species with either variable or invariant clutch sizes respond to insularity differently. Location: Worldwide. Methods: We assembled data on egg, clutch, hatchling and female sizes and brood frequencies of 2,511 lizard species. We tested whether the predictions of the island syndrome were met for lizards laying invariant versus variable clutches, controlling for female size and phylogenetic non-independence. We further examined whether geckos and anoles, the major clades with invariant clutches, differ from other lizards in the way they respond to insularity. Results: On islands, species with variable clutch sizes lay smaller clutches of larger eggs, from which larger hatchlings emerge, compared with mainland species. Lizards with invariant clutch sizes, however, decrease clutch size and increase clutch frequency but not hatchling or egg size, compared with mainland species. Main conclusions: Lizards with invariant clutch sizes may be unable to increase egg and hatchling sizes because of limitations set by the female body cavity and pelvic opening, or by arboreality. They nonetheless lay smaller clutches on islands and increase clutch frequency. We suggest that this may result from lower seasonality of tropical islands, leading to a greater spread of reproductive effort. Alternatively, the higher clutch frequency on tropical islands could result from fluctuations in population densities caused by tropical storms.