# Egg Characteristics of Female Common Terns Are Repeatable, and Vary With Maternal Age and Laying Order

Coraline Bichet, Maria Moiron, Nathalie Kürten, Oscar Vedder, Sandra Bouwhuis

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Coraline Bichet, Maria Moiron, Nathalie Kürten, Oscar Vedder, Sandra Bouwhuis. Egg Characteristics of Female Common Terns Are Repeatable, and Vary With Maternal Age and Laying Order. Ecology and Evolution, 2025, 15 (11), pp.e72455. 10.1002/ece3.72455. hal-05373656

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RESEARCH ARTICLE

OPEN ACCESS

# Egg Characteristics of Female Common Terns Are Repeatable, and Vary With Maternal Age and Laying Order

Coraline Bichet<fnref n="1" />,² ID | Maria Moiron²,³ | Nathalie Kürten² | Oscar Vedder² ID | Sandra Bouwhuis² ID

**Correspondence:** Coraline Bichet (coraline.bichet@univ-lr.fr)

**Received:** 16 June 2025 | **Revised:** 10 October 2025 | **Accepted:** 21 October 2025

**Funding:** This work was supported by Alexander von Humboldt-Stiftung.

**Keywords:** egg camouflage | egg coloration | egg elongation | egg pointedness | egg quality | egg shape | egg volume | polar asymmetry | spottiness

## ABSTRACT

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.

## 1 | Introduction

Avian eggs exhibit striking inter-specific variability in the colour and maculation of their shells as well as their shape and size (Kilner 2006; Cassey et al. 2012; Hanley et al. 2015; Stoddard et al. 2017; Birkhead et al. 2018). These egg characteristics, however, do not only vary among species, but within species as well (Birkhead, Thompson, and Biggins 2017; Ducay et al. 2021, 202; Hauber et al. 2019; Holveck et al. 2017). Technical and analytical advances, combined with the availability of long-term individual-based studies, now offer the opportunity to deepen our understanding of factors explaining this variation.

One important aspect is the extent to which the colour, maculation, size and shape of eggs (hereafter referred to as “egg characteristics”) can vary within females. If intra-individual variation in egg characteristics is large, this would imply that these characteristics are mainly determined by varying individual factors (e.g., age, condition, epigenetics) and/or environmental factors (e.g., abiotic conditions, food availability). If, on the other hand, intra-individual variation would be low, egg characteristics could be mainly driven by stable individual characteristics that are genetically heritable or otherwise determined early in life (McCormack and Berg 2010), and/or stable environmental factors (e.g., micro-habitat effects). Elucidating

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

© 2025 The Author(s). *Ecology and Evolution* published by British Ecological Society and John Wiley & Sons Ltd.

<footnote n="1">¹Centre d'Etudes Biologiques de Chizé, UMR-CNRS 7372, La Rochelle Université, Villiers-en-Bois, France | ²Institute of Avian Research, Wilhelmshaven,</footnote>

<footnote>Germany | ³Department of Evolutionary Biology, Bielefeld University, Bielefeld, Germany</footnote>
describes the dominant wavelength of the colour, and varies
from 0 (red) to 0.36 (magenta). Saturation (S) is the amount
of the dominant colour (H) in the colour, and varies from 0
(for grey) to 1 (for a primary colour). Value (V) represents the
amount of light in the colour (i.e., the brightness), and also
varies from 0 (dark) to 1 (light). The HSV colour space has
been widely used across taxa to describe colouration in wild
populations (Stevens et al. 2007, 2009), and has been shown
to correlate with egg pigment concentrations in bird eggs
(Poláček et al. 2017).

2.3.3 | Egg Size and Shape

Egg shape can be characterized by many parameters, and which
the most appropriate ones are is still under debate (Biggins
et al. 2022). We decided to follow the recommendations pro-
vided by Birkhead et al. (2019), and used three parameters, de-
rived from Preston's method (Biggins et al. 2022; Preston 1968;
Preston 1953), to describe egg shape: pointedness, elonga-
tion, and polar asymmetry. These parameters were calculated
from the egg pictures using the R script provided by Biggins
et al. (2018) and the R package "EBImage" (Pau et al. 2010). Egg
volume (in mm³) was calculated according to the R script and
the equation provided by Biggins et al. (2018).

For the eggs that were photographed twice, we calculated the
means of the egg characteristics obtained from the pictures. In
addition, we tested the “measurement repeatability” of the two
measures obtained per egg, and found it to be very high in all
cases, ranging between 0.909 and 0.995 (Table S1).

## 2.4 | Statistical Analyses

First, we tested the Pearson's correlations among the eight egg
characteristics: spottiness, Hue, Saturation, Value, volume,
pointedness, elongation, and polar asymmetry (all modelled with
a Gaussian error distribution). Second, we conducted a principal
component analysis (PCA) to assess how the eight egg charac-
teristics loaded onto fewer axes of variation (Tables S2 and S3).
The PCA was run using the R (version 3.6.1, R Core Team 2014)
function "prcomp" and visualized using the R packages "facto-
extra" (Kassambara and Mundt 2020) and "FactoMineR" (Lê
et al. 2008).

Third, to test whether the eight egg characteristics varied with
female age and differed between eggs of different laying order
and from differently sized clutches, we ran a series of eight
linear mixed models (LMMs) using the function "lmer," im-
plemented in the R package "lme4" (Bates et al. 2015). The
females' actual age was partitioned into an "average age" and
"delta age" component (following van de Pol and Wright 2009)
and both variables were added as explanatory variables.
Average age was calculated as the average of all ages at which
we measured a female's egg parameters, whereas delta age was
calculated as the difference between the female's actual age at
measurement and her average age (i.e., delta age = age – average
age). When modelled jointly, average age estimates the among-
individual age effect, and delta age the within-individual age
effect (van de Pol and Wright 2009). Should these effects differ

significantly from one another, this would mean that the effect
of age among females cannot be explained by changes with age
within females, indicating that there is selective (dis)appear-
ance of females with certain egg parameters from the older
age classes (van de Pol and Wright 2009). Since the effects of
laying order may not be independent from, or not identifiable
when not also accounting for, clutch size, we created a six-level
categorical variable ("CS.LO") representing a combination of
clutch size (CS: 1, 2 or 3 eggs) and laying order (LO: egg laid
1st, 2nd or 3rd) (also see Vedder, Zhang, and Bouwhuis 2017;
Vedder et al. 2019) and added it to each model. Female identity,
clutch identity (nested in female identity), and year were added
as random effects to account for the fact that eggs laid by the
same female, in the same nest by the same female or in the
same year are not independent from each other. Additionally,
we checked whether there was a linear change in egg charac-
teristics across the four years by fitting year as a continuous
fixed effect in our models. We did not find any significant lin-
ear trends for any of the eight egg characteristics and hence did
not include year as a fixed effect in the final models.

We calculated the repeatability of the eight egg characteristics (i) at the female level including egg data for all clutches produced across the years and (ii) at the clutch level including those eggs produced in the same clutch in a given year. Female and female-within-clutch repeatability estimates could be considered as "long-term" and "short-term" repeatability estimates, respectively (sensu Araya-Ajoy et al. 2015). Female repeatability is calculated as the variance explained by female identity divided by the total variance not attributable to fixed effects (Nakagawa and Schielzeth 2013); while female-within-clutch repeatability is calculated as the variance explained by female identity, plus the variance explained by clutch identity (nested in female identity), divided by the total variance not attributable to fixed effects. As such, female-within-clutch repeatability must be equal to, or higher than female repeatability.

We used the “sim” function from the R-package “arm” to sim-
ulate values from the posterior distributions of the model pa-
rameters (Gelman and Su 2024). Based on 5000 simulations,
we extracted 95% credible intervals (CI) around the mean of
each fixed and random effect. Model assumptions were tested
by checking the residuals' independence, normality and homo-
geneity, and autocorrelation. Assessment of statistical support
was obtained from the posterior distribution of each parameter.
We also provide *p* values to test the significance of fixed effects
(α=0.05).

Finally, using an ANOVA, we also tested the significance
(α=0.05) of the "CS.LO" effect on each egg parameter and con-
ducted a posteriori tests (Tukey HSD) to assess the differences
between two "CS.LO" categories. For this, we focused on differ-
ences between the laying order of eggs within a clutch (i.e., eggs
2-1 compared to 2-2 and eggs 3-1 compared to 3-2 and 3-3).

3 | Results

The mean (±SE), minimum and maximum, as well as the dis-
tribution of all eight egg characteristics assessed (spottiness,
Hue, Saturation, Value, volume, pointedness, elongation, polar
<table><caption>**TABLE 1** | Descriptive statistics for, and distributions of, eight egg characteristics measured for 1589 eggs laid in 687 clutches by 330 female common terns between 2017 and 2020.</caption><thead><tr><th>Egg parameter</th><th>Mean ± SD</th><th>Minimum</th><th>Maximum</th><th>Distribution</th></tr></thead><tbody><tr><td>Spottiness</td><td>0.236 ± 9.323 × 10<sup>-2</sup></td><td>0.006</td><td>0.776</td><td></td></tr><tr><td>Hue</td><td>0.136 ± 1.693 × 10<sup>-2</sup></td><td>0.089</td><td>0.206</td><td></td></tr><tr><td>Saturation</td><td>0.359 ± 7.584 × 10<sup>-2</sup></td><td>0.133</td><td>0.549</td><td></td></tr><tr><td>Value</td><td>0.788 ± 6.668 × 10<sup>-2</sup></td><td>0.538</td><td>0.981</td><td></td></tr><tr><td>Volume (mm<sup>3</sup>)</td><td>18.872 ± 1.466</td><td>9.642</td><td>23.548</td><td></td></tr></tbody></table>

(Continues)
<table><caption>TABLE 1 | (Continued)</caption><thead><tr><th>Egg parameter</th><th>Mean ± SD</th><th>Minimum</th><th>Maximum</th><th>Distribution</th></tr></thead><tbody><tr><td>Pointedness</td><td>0.566 ± 1.478 × 10<sup>-2</sup></td><td>0.510</td><td>0.633</td><td></td></tr><tr><td>Elongation</td><td>1.383 ± 6.241 × 10<sup>-2</sup></td><td>1.210</td><td>1.640</td><td></td></tr><tr><td>Polar asymmetry</td><td>2.258 ± 4.544 × 10<sup>-1</sup></td><td>1.148</td><td>4.015</td><td></td></tr></tbody></table>

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asymmetry) are reported and visualized in Table 1. The four egg characteristics related to colour and maculation (spottiness, Hue, Saturation, Value) were correlated with each other, as were those describing size and shape (volume, pointedness, elongation, and polar asymmetry) (Figure S2). The PCA showed that the first two axes could explain 34.6% and 21.5% of the variation, respectively (Figure 1 and Table S2), with colour/maculation indices having high loading values for the first axis (negatively for Hue and Value, positively for Saturation and spottiness), and shape/size indices having high loading values for the second axis (positively for all indices: volume, pointedness, elongation, and polar asymmetry). This means that bluer/greener eggs were also brighter, but less spotty and saturated; and that large eggs were also pointy, long, and asymmetric.

The characteristics of all eggs produced by the same female across the years were moderately to highly repeatable, with female repeatability coefficients varying from 0.48 for polar asymmetry to 0.73 for Hue (Figure 2 and Table S4), meaning that individual females lay similar eggs in their different clutches produced in different years. Eggs from the same clutch were equally or only slightly more similar than eggs from different clutches of the same female among years, with female-within-clutch repeatability coefficients varying from 0.48 for polar asymmetry to 0.77 for Hue (Figure 2 and Table S4).

Regarding the within-female effect of age, we found eggs to become spottier as females grew older ($\beta$=7.55 × 10⁻³, 95% CI=2.91 × 10⁻³–1.22 × 10⁻³, Figure 3a and Table 2). In addition, there was a difference between the effects of average age and delta age for egg spottiness ($\beta$=8.79 × 10⁻³, 95% CI=3.51 × 10⁻³–1.41 × 10⁻²), indicating that females producing spottier eggs were less likely to be found breeding at older ages (suggesting selective disappearance of females producing spottier eggs, Figure 3a and Table 2). Egg volume also tended to increase with age within females ($\beta$=1.41 × 10⁻¹, 95% CI=1.56 × 10⁻²–2.76 × 10⁻¹, Figure 3b and Table 2), but for this trait there was no indication of selective disappearance of females with a certain egg size ($\beta$=9.66 × 10⁻², 95% CI=-4 × 10 × 10⁻²–3.36 × 10⁻¹, Figure 3b and Table 2).

We did not find any differences in shell spottiness and colour between eggs of a different laying order in clutches of a different size (all *p* values of CS.LO > 0.05 in ANOVA tests). For egg shape/size parameters, however, we did find such an effect (all *p* values of “CS.LO” < 0.001 in ANOVA tests): later laid eggs from two- and three-egg clutches were smaller (Figure 4a) and less pointy (Figure 4b) with eggs in three-egg clutches also becoming less asymmetric (Figure 4d) over the laying sequence. Finally, the second eggs from three-egg clutches were less elongated than the first and the third eggs (Figure 4c).
<figure><img src="image_12.png" /><figcaption>**FIGURE 1** | Results from a principal component analysis of eight egg characteristics (spottiness, Hue, Saturation, Value, volume, pointedness, elongation, polar asymmetry) of 1589 eggs (grey circles) from 687 clutches laid by 330 female common terns between 2017 and 2020. The arrows represent the projection of the eight characteristics onto the first two PCA axes (PC1 and PC2), which explained 34.6% and 21.5% of the variation, respectively (also see Tables S2 and S3).</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**FIGURE 2** | Estimates of the repeatability of eight egg characteristics for female common terns across years (grey circles) and within clutches (black circles), with their 95% credible intervals (bars), assessed using 1589 eggs from 687 clutches laid by 330 individual female common terns in the years 2017–2020.</figcaption></figure>

4 | Discussion

Birds are unique amniotes in the sense that they have evolved huge among- and within-species variation in the colour,

maculation, shape, and size of their eggs. In this study, performed on common terns across four breeding seasons, we found that all eight egg characteristics that we quantified were moderately to highly repeatable between and within clutches of the same female, and organized along two main axes of variation: one describing the colour and spottiness of egg shells, and one describing the size and shape of eggs. While some characteristics changed with age within females (egg volume and spottiness), others did not (colour variables and shape variables). Similarly, some egg characteristics varied with the laying order of eggs within a clutch (size and shape variables), while others did not (spottiness and colour variables). As such, we show that different egg characteristics are differently related to important life-history variables, hinting at differences in adaptive significance among them.

The result that all egg characteristics that we measured were moderately to highly repeatable across females, regardless of whether they were measurements of eggs within clutches, or from clutches produced in different years, shows that females consistently produce eggs of a distinct morphology and suggests that these characteristics are not purely stochastic or environmental by-products that form during the production of the egg and its shell. Such individual consistency has also been found in other studies (Beech et al. 2022; Dearborn et al. 2012; Hauber et al. 2019; Sulc et al. 2022), and may be mechanistically explained by a strong genetic basis of the process that creates these unique “signatures” (Gosler et al. 2000; Guo et al. 2020; Morales et al. 2010). From an adaptive point of view, this consistency could facilitate egg recognition and reduce the risk of brood parasitism (Fernandez-Duque et al. 2025; Li et al. 2020). However, personal observations in the study colony taught us that incubating common terns readily accept any fake egg that roughly resembles a real egg (Figure S3), and maintain incubation when their real eggs are temporarily artificially incubated (e.g., Vedder, Kuerten, Bouwhuis, et al. 2017; Kürten et al. 2021). Alternatively, egg camouflage may favor the production of very similar-looking eggs within a nest to avoid predator attraction (Stoddard et al. 2011; Westmoreland 2008; Westmoreland and Kiltie 2007), without narrowing down the large and consistent differences between females in egg characteristics that we observed. Indeed, experimental work on corvids suggests that their “hunting by searching image” can simultaneously select for crypsis and polymorphism, with polymorphism in prey appearance being maintained by frequency-dependent selection (Bond and Kamil 2002). Perhaps, the large variation in egg characteristics between female common terns serves a similar function; the absence of a single egg phenotype among clutches in the population may make it more difficult for visual predators to develop an efficient searching image. Further research should thus focus on identifying the agents of selection on egg characteristics and quantifying how they act.

Some of the variables that determine egg characteristics are expected to be costly for the female and therefore to be related to female “quality” (Morales et al. 2006; Moreno and Osorno 2003; Reynolds et al. 2009). As such, we expected to find egg characteristics to vary with age, either based on the general increase in reproductive performance with age within females in our study species (Nisbet et al. 2020; Vedder
<figure><img src="image_14.png" /><figcaption>**FIGURE 3** | Relationship between female age and (a) eggshell spottiness and (b) egg volume. Dots represent the values for 1589 eggs from 687 clutches laid by 330 female common terns, solid lines the model predictions with their standard errors (grey areas). The black lines represent the within-female effect, while the grey lines represent the among-female effect.</figcaption></figure>

et al. 2021; Zhang et al. 2015a, 2015b), or due to an association with lifespan, causing the egg characteristics of high-quality females to be overrepresented among older females (Vedder and Bouwhuis 2018). The majority of the egg characteristics we measured (6 of 8), however, did not show any age specificity. We did find that eggshell spottiness increased with age within individual females, while at the same time finding that females producing spottier eggs seemed less represented among the females of older age classes, suggesting their selective disappearance from the breeding population. Following the hypothesis that spots reflect a female's oxidative level, as they are mainly due to the pigment protoporphyrin that can act as a prooxidant (Martinez et al. 2003; Martínez-de la Puente et al. 2007), our result may suggest that oxidative stress increases with age within females and that females with high oxidative stress in early life, reflected in their spottier eggs (as observed for instance in great tits—*Parus major*—in Hargitai, Nagy, Herényi, et al. 2016), do not reproduce at older ages. Our study, however, is correlative and prevents us from inferring a causal relationship and pinpointing which female characteristics that change with age are responsible for the covariance between spottiness and female age. Further studies would be needed, and, for example, test for an association between a female's eggshell spottiness and her oxidative status and immune capacities.

Similar to previous studies in common terns (Gonzalez-Solis et al. 2004; Zhang et al. 2015a; Nisbet et al. 2020), we found that egg volume increased with age within females. An increased egg volume may be beneficial for chick survival, as egg volume is a strong determinant of chick mass at hatching, which can have a carry-over effect on chick fitness later on (Nisbet 1978; Reid and Boersma 1990; Williams 1994; Krist 2011; Vedder et al. 2023).

Given these benefits, the increase in egg volume with age suggests that young females have a reduced ability to invest fully in eggs, and that this ability improves with age within individuals. Such improvement with age is also reflected in the general improvement of reproductive success with age in this species (Nisbet et al. 2020; Vedder et al. 2021; Zhang et al. 2015a, 2015b) and most likely results from increased experience in foraging, or better knowledge on local food availability (Forslund and Pärt 1995).

Within clutches, egg volume, as well as the three egg shape characteristics (pointedness, elongation, and polar asymmetry) decreased with laying order. This suggests these traits are linked (also see Birkhead 2016; Stoddard et al. 2017), and that the well-known reduction of egg size with laying order in the common tern (Bollinger 1994; Garcia et al. 2011; Vedder, Zhang, and Bouwhuis 2017) is, thus, at least partly caused by a modification of egg shape toward less pointy, less long and less asymmetric eggs. In passerines, a reduction in egg volume with laying order is often attributed to a lack of calcium in females (Gosler et al. 2005; Graveland and Berends 1997; Graveland and Drent 1997). However, thanks to their piscivorous diet, seabirds should not suffer from calcium limitations (for common terns see Becker and Ludwigs 2004), such that the observed reduction in egg volume with laying order has more likely evolved as an adaptive strategy to facilitate an efficient “brood reduction strategy” when food availability during chick rearing turns out lower than anticipated during clutch formation (Slagsvold et al. 1984). Indeed, common terns typically produce an excess of chicks, of which the last-hatched chicks rapidly die after hatching when food availability is low (Vedder et al. 2021; Vedder et al. 2019; Vedder, Zhang, and Bouwhuis 2017).
<table><caption>**TABLE 2** | Results from general linear mixed models testing whether variation in the spottiness, Hue, Saturation, Value, volume, pointedness, elongation or polar asymmetry, of 1589 eggs laid in 687 clutches by 330 female common terns between 2017 and 2020 is explained by among- (average age) and within-individual (delta age) components of female age, and/or the combination of clutch size and laying order (CS.LO with 1.1 as a reference category).</caption><thead><tr><th rowspan="2">Egg parameter</th><th colspan="2">Spottiness</th><th colspan="2">Hue</th><th colspan="2">Saturation</th><th colspan="2">Value</th></tr><tr><th>Mean</th><th>95% CI</th><th>p</th><th>Mean</th><th>95% CI</th><th>p</th><th>Mean</th><th>95% CI</th><th>p</th></tr></thead><tbody><tr><td>Fixed effect</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Intercept</td><td>2.28 × 10<sup>-1</sup></td><td>1.99 × 10<sup>-1</sup>,<br>2.56 × 10<sup>-1</sup></td><td>&lt;0.001</td><td>1.28 × 10<sup>-1</sup></td><td>1.23 × 10<sup>-1</sup>,<br>1.33 × 10<sup>-1</sup></td><td>&lt;0.001</td><td>3.66 × 10<sup>-1</sup></td><td>3.43 × 10<sup>-1</sup>,<br>3.90 × 10<sup>-1</sup></td><td>&lt;0.001</td></tr><tr><td>Average age</td><td>-1.24 × 10<sup>-3</sup></td><td>-3.46 × 10<sup>-3</sup>,<br>9.47 × 10<sup>-4</sup></td><td>0.270</td><td>7.47 × 10<sup>-4</sup></td><td>3.35 × 10<sup>-4</sup>,<br>1.16 × 10<sup>-3</sup></td><td>&lt;0.001</td><td>-1.20 × 10<sup>-3</sup></td><td>-2.97 × 10<sup>-3</sup>,<br>6.08 × 10<sup>-4</sup></td><td>0.188</td><td>-2.51 × 10<sup>-4</sup></td><td>-1.93 × 10<sup>-3</sup>,<br>1.45 × 10<sup>-3</sup></td><td>0.769</td></tr><tr><td>Delta age</td><td>7.55 × 10<sup>-3</sup></td><td>2.91 × 10<sup>-3</sup>,<br>1.22 × 10<sup>-2</sup></td><td>0.019</td><td>3.37 × 10<sup>-4</sup></td><td>-3.63 × 10<sup>-4</sup>,<br>1.04 × 10<sup>-3</sup></td><td>0.346</td><td>-1.11 × 10<sup>-3</sup></td><td>-5.59 × 10<sup>-3</sup>,<br>3.27 × 10<sup>-3</sup></td><td>0.636</td><td>-4.54 × 10<sup>-3</sup></td><td>-1.02 × 10<sup>-2</sup>,<br>1.33 × 10<sup>-3</sup></td><td>0.166</td></tr><tr><td>CS.LO(2-1)</td><td>6.02 × 10<sup>-3</sup></td><td>-2.04 × 10<sup>-2</sup>,<br>3.28 × 10<sup>-2</sup></td><td>0.654</td><td>2.87 × 10<sup>-3</sup></td><td>-1.43 × 10<sup>-3</sup>,<br>7.16 × 10<sup>-3</sup></td><td>0.190</td><td>-4.06 × 10<sup>-3</sup></td><td>-2.50 × 10<sup>-2</sup>,<br>1.63 × 10<sup>-2</sup></td><td>0.710</td><td>1.02 × 10<sup>-2</sup></td><td>-6.51 × 10<sup>-3</sup>,<br>2.70 × 10<sup>-2</sup></td><td>0.242</td></tr><tr><td>CS.LO(2-2)</td><td>1.56 × 10<sup>-2</sup></td><td>-1.13 × 10<sup>-2</sup>,<br>4.22 × 10<sup>-2</sup></td><td>0.246</td><td>2.29 × 10<sup>-3</sup></td><td>-1.92 × 10<sup>-3</sup>,<br>6.64 × 10<sup>-3</sup></td><td>0.295</td><td>5.13 × 10<sup>-3</sup></td><td>-1.55 × 10<sup>-2</sup>,<br>2.54 × 10<sup>-2</sup></td><td>0.638</td><td>4.11 × 10<sup>-3</sup></td><td>-1.26 × 10<sup>-2</sup>,<br>2 × 10 × 10<sup>-2</sup></td><td>0.637</td></tr><tr><td>CS.LO(3-1)</td><td>1.73 × 10<sup>-2</sup></td><td>-9.09 × 10<sup>-3</sup>,<br>4.37 × 10<sup>-2</sup></td><td>0.191</td><td>3.09 × 10<sup>-3</sup></td><td>-1.09 × 10<sup>-3</sup>,<br>7.27 × 10<sup>-3</sup></td><td>0.152</td><td>-2.82 × 10<sup>-3</sup></td><td>-2.33 × 10<sup>-2</sup>,<br>1.77 × 10<sup>-2</sup></td><td>0.794</td><td>3.21 × 10<sup>-3</sup></td><td>-1.36 × 10<sup>-2</sup>,<br>1.98 × 10<sup>-2</sup></td><td>0.710</td></tr><tr><td>CS.LO(3-2)</td><td>1.98 × 10<sup>-2</sup></td><td>-6.36 × 10<sup>-3</sup>,<br>4.57 × 10<sup>-2</sup></td><td>0.135</td><td>5.20 × 10<sup>-3</sup></td><td>9.93 × 10<sup>-4</sup>,<br>9.29 × 10<sup>-3</sup></td><td>0.016</td><td>-3.74 × 10<sup>-3</sup></td><td>-2.45 × 10<sup>-2</sup>,<br>1.68 × 10<sup>-2</sup></td><td>0.728</td><td>5.74 × 10<sup>-3</sup></td><td>-1.07 × 10<sup>-2</sup>,<br>2.22 × 10<sup>-2</sup></td><td>0.504</td></tr><tr><td>CS.LO(3-3)</td><td>1.44 × 10<sup>-2</sup></td><td>-1.19 × 10<sup>-2</sup>,<br>4.03 × 10<sup>-2</sup></td><td>0.274</td><td>2.68 × 10<sup>-3</sup></td><td>-1.54 × 10<sup>-3</sup>,<br>6.83 × 10<sup>-3</sup></td><td>0.213</td><td>7.77 × 10<sup>-3</sup></td><td>-1.30 × 10<sup>-2</sup>,<br>2.83 × 10<sup>-2</sup></td><td>0.471</td><td>1.60 × 10<sup>-2</sup></td><td>-4.27 × 10<sup>-4</sup>,<br>3.24 × 10<sup>-2</sup></td><td>0.063</td></tr><tr><td>Difference<br>between average<br>age and delta age</td><td>8.79 × 10<sup>-3</sup></td><td>3.51 × 10<sup>-3</sup>,<br>1.41 × 10<sup>-2</sup></td><td></td><td>-4.09 × 10<sup>-4</sup></td><td>-1.22 × 10<sup>-3</sup>,<br>4.01 × 10<sup>-4</sup></td><td></td><td>9.57 × 10<sup>-5</sup></td><td>-4.83 × 10<sup>-3</sup>,<br>4.89 × 10<sup>-3</sup></td><td></td><td>-4.24 × 10<sup>-3</sup></td><td>-1.04 × 10<sup>-2</sup>,<br>2.09 × 10<sup>-3</sup></td><td></td></tr><tr><td>Random effect</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Female identity</td><td>7.39 × 10<sup>-3</sup></td><td>6.37 × 10<sup>-3</sup>,<br>8.48 × 10<sup>-3</sup></td><td></td><td>2.40 × 10<sup>-4</sup></td><td>2.09 × 10<sup>-4</sup>,<br>2.73 × 10<sup>-4</sup></td><td></td><td>4.66 × 10<sup>-3</sup></td><td>4.04 × 10<sup>-3</sup>,<br>5.33 × 10<sup>-3</sup></td><td></td><td>4.56 × 10<sup>-3</sup></td><td>3.94 × 10<sup>-3</sup>,<br>5.22 × 10<sup>-3</sup></td><td></td></tr><tr><td>Clutch identity</td><td>1.36 × 10<sup>-4</sup></td><td>1.19 × 10<sup>-4</sup>,<br>1.56 × 10<sup>-4</sup></td><td></td><td>1.30 × 10<sup>-5</sup></td><td>1.18 × 10<sup>-5</sup>,<br>1.52 × 10<sup>-5</sup></td><td></td><td>1.64 × 10<sup>-4</sup></td><td>1.44 × 10<sup>-4</sup>,<br>1.87 × 10<sup>-4</sup></td><td></td><td>7 × 10 × 10<sup>-5</sup></td><td>6.17 × 10<sup>-5</sup>,<br>8.08 × 10<sup>-5</sup></td><td></td></tr><tr><td>Year</td><td>6.90 × 10<sup>-6</sup></td><td>5.40 × 10<sup>-7</sup>,<br>2.06 × 10<sup>-5</sup></td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>1.03 × 10<sup>-5</sup></td><td>8.80 × 10<sup>-7</sup>,<br>2.96 × 10<sup>-5</sup></td><td></td><td>6.59 × 10<sup>-5</sup></td><td>1.65 × 10<sup>-5</sup>,<br>1.68 × 10<sup>-4</sup></td><td></td></tr><tr><td>Residual</td><td>3.58 × 10<sup>-3</sup></td><td>3.31 × 10<sup>-3</sup>,<br>3.81 × 10<sup>-3</sup></td><td></td><td>7.72 × 10<sup>-5</sup></td><td>7.19 × 10<sup>-5</sup>,<br>8.27 × 10<sup>-5</sup></td><td></td><td>2.27 × 10<sup>-3</sup></td><td>2.12 × 10<sup>-3</sup>,<br>2.43 × 10<sup>-3</sup></td><td></td><td>1.39 × 10<sup>-3</sup></td><td>1.30 × 10<sup>-3</sup>,<br>1.49 × 10<sup>-3</sup></td><td></td></tr></tbody></table>

(Continues)
<table><caption>TABLE 2 | (Continued)</caption><thead><tr><th rowspan="2">Egg parameter</th><th colspan="3">Volume</th><th colspan="3">Pointedness</th><th colspan="3">Elongation</th><th colspan="3">Polar asymmetry</th></tr><tr><th>Fixed effect</th><th>Mean</th><th>95% CI</th><th>p</th><th>Mean</th><th>95% CI</th><th>p</th><th>Mean</th><th>95% CI</th><th>p</th><th>Mean</th><th>95% CI</th><th>p</th></tr></thead><tbody><tr><td>Intercept</td><td>18.56</td><td>18.09, 19.03</td><td>&lt;0.001</td><td>5.73 &times; 10<sup>-1</sup></td><td>5.68 &times; 10<sup>-1</sup>,<br>5.78 &times; 10<sup>-1</sup></td><td>&lt;0.001</td><td>1.43</td><td>1.41, 1.45</td><td>&lt;0.001</td><td>2.32</td><td>2.17, 2.47</td><td>&lt;0.001</td></tr><tr><td>Average age</td><td>4.78 &times; 10<sup>-2</sup></td><td>1.23 &times; 10<sup>-2</sup>,<br>8.12 &times; 10<sup>-2</sup></td><td>0.006</td><td>-3.17<br>&times; 10<sup>-4</sup></td><td>-6.56 &times; 10<sup>-4</sup>,<br>2.87 &times; 10<sup>-5</sup></td><td>0.065</td><td>-2.33<br>&times; 10<sup>-3</sup></td><td>-3.88 &times; 10<sup>-3</sup>,<br>-8 &times; 10<br>&times; 10<sup>-4</sup></td><td>0.003</td><td>-7.23<br>&times; 10<sup>-3</sup></td><td>-1.69 &times; 10<sup>-2</sup>,<br>2.27 &times; 10<sup>-3</sup></td><td>0.139</td></tr><tr><td>Delta age</td><td>1.44 &times; 10<sup>-1</sup></td><td>1.56 &times; 10<sup>-2</sup>,<br>2.76 &times; 10<sup>-1</sup></td><td>0.054</td><td>2.84 &times; 10<sup>-4</sup></td><td>-8.18 &times; 10<sup>-4</sup>,<br>1.37 &times; 10<sup>-3</sup></td><td>0.624</td><td>-2.58<br>&times; 10<sup>-3</sup></td><td>-7.49 &times; 10<sup>-3</sup>,<br>2.25 &times; 10<sup>-3</sup></td><td>0.361</td><td>2.48 &times; 10<sup>-2</sup></td><td>-7.26 &times; 10<sup>-3</sup>,<br>5.64 &times; 10<sup>-2</sup></td><td>0.163</td></tr><tr><td>CS.LO(2-1)</td><td>2.29 &times; 10<sup>-1</sup></td><td>-1.49 &times; 10<sup>-1</sup>,<br>6.03 &times; 10<sup>-1</sup></td><td>0.232</td><td>-2.42<br>&times; 10<sup>-3</sup></td><td>-6.63 &times; 10<sup>-3</sup>,<br>1.91 &times; 10<sup>-3</sup></td><td>0.269</td><td>-2.89<br>&times; 10<sup>-2</sup></td><td>-4.66 &times; 10<sup>-2</sup>,<br>-1.14 &times; 10<sup>-2</sup></td><td>0.001</td><td>3.16 &times; 10<sup>-2</sup></td><td>-1.08 &times; 10<sup>-1</sup>,<br>1.71 &times; 10<sup>-1</sup></td><td>0.664</td></tr><tr><td>CS.LO(2-2)</td><td>-3.27<br>&times; 10<sup>-1</sup></td><td>-7.05 &times; 10<sup>-1</sup>,<br>5.93 &times; 10<sup>-2</sup></td><td>0.088</td><td>-6.19<br>&times; 10<sup>-3</sup></td><td>-1.04 &times; 10<sup>-2</sup>,<br>-1.90 &times; 10<sup>-3</sup></td><td>0.005</td><td>-3.24<br>&times; 10<sup>-2</sup></td><td>-5.02 &times; 10<sup>-2</sup>,<br>-1.52 &times; 10<sup>-2</sup></td><td>&lt;0.001</td><td>-1.15<br>&times; 10<sup>-2</sup></td><td>-1.52 &times; 10<sup>-2</sup>,<br>1.24 &times; 10<sup>-1</sup></td><td>0.875</td></tr><tr><td>CS.LO(3-1)</td><td>4.16 &times; 10<sup>-1</sup></td><td>3.76 &times; 10<sup>-2</sup>,<br>7.94 &times; 10<sup>-1</sup></td><td>0.029</td><td>-1.07<br>&times; 10<sup>-3</sup></td><td>-5.39 &times; 10<sup>-3</sup>,<br>3.18 &times; 10<sup>-3</sup></td><td>0.621</td><td>-3.25<br>&times; 10<sup>-2</sup></td><td>-4.99 &times; 10<sup>-2</sup>,<br>-1.52 &times; 10<sup>-2</sup></td><td>&lt;0.001</td><td>8.01 &times; 10<sup>-2</sup></td><td>-5.93 &times; 10<sup>-2</sup>,<br>2.23 &times; 10<sup>-1</sup></td><td>0.268</td></tr><tr><td>CS.LO(3-2)</td><td>1.68 &times; 10<sup>-2</sup></td><td>-3.56 &times; 10<sup>-1</sup>,<br>3.98 &times; 10<sup>-1</sup></td><td>0.930</td><td>-2.51<br>&times; 10<sup>-3</sup></td><td>-6.75 &times; 10<sup>-3</sup>,<br>1.84 &times; 10<sup>-3</sup></td><td>0.246</td><td>-4.62<br>&times; 10<sup>-2</sup></td><td>-6.36 &times; 10<sup>-2</sup>,<br>-2.88 &times; 10<sup>-2</sup></td><td>&lt;0.001</td><td>6.05 &times; 10<sup>-2</sup></td><td>-7.73 &times; 10<sup>-2</sup>,<br>2.01 &times; 10<sup>-1</sup></td><td>0.401</td></tr><tr><td>CS.LO(3-3)</td><td>-6.30<br>&times; 10<sup>-1</sup></td><td>-1.01, -2.56<br>&times; 10<sup>-1</sup></td><td>0.001</td><td>-1.24<br>&times; 10<sup>-2</sup></td><td>-1.65 &times; 10<sup>-2</sup>,<br>-8.08 &times; 10<sup>-3</sup></td><td>&lt;0.001</td><td>-1.76<br>&times; 10<sup>-2</sup></td><td>-3.51 &times; 10<sup>-2</sup>,<br>-1.90 &times; 10<sup>-4</sup></td><td>0.042</td><td>-1.97<br>&times; 10<sup>-1</sup></td><td>-3.34 &times; 10<sup>-1</sup>,<br>-6.25 &times; 10<sup>-2</sup></td><td>0.006</td></tr><tr><td>Difference<br>between<br>average age<br>and delta age</td><td>9.66 &times; 10<sup>-2</sup></td><td>-4 &times; 10<br>&times; 10<sup>-2</sup>, 3.36<br>&times; 10<sup>-1</sup></td><td></td><td>5.98 &times; 10<sup>-4</sup></td><td>-5.61 &times; 10<sup>-4</sup>,<br>1.76 &times; 10<sup>-3</sup></td><td></td><td>-3.18<br>&times; 10<sup>-4</sup></td><td>-5.64 &times; 10<sup>-3</sup>,<br>4.92 &times; 10<sup>-3</sup></td><td></td><td>3.18 &times; 10<sup>-2</sup></td><td>-1.18 &times; 10<sup>-3</sup>,<br>6.60 &times; 10<sup>-2</sup></td><td></td></tr><tr><th>Random effect</th><th>Mean</th><th>95% CI</th><th>Mean</th><th>95% CI</th><th>Mean</th><th>95% CI</th><th>Mean</th><th>95% CI</th><th>Mean</th><th>95% CI</th><th>Mean</th><th>95% CI</th></tr><tr><td>Female identity</td><td>1.81</td><td>1.58, 2.06</td><td>1.56 &times; 10<sup>-4</sup></td><td>1.35 &times; 10<sup>-4</sup>,<br>1.79 &times; 10<sup>-4</sup></td><td></td><td>3.43 &times; 10<sup>-3</sup></td><td>2.98 &times; 10<sup>-3</sup>,<br>3.94 &times; 10<sup>-3</sup></td><td>1.15 &times; 10<sup>-1</sup></td><td>9.64 &times; 10<sup>-2</sup>, 1.28 &times; 10<sup>-1</sup></td><td></td><td></td><td></td></tr><tr><td>Clutch identity</td><td>2.80 &times; 10<sup>-2</sup></td><td>2.44 &times; 10<sup>-2</sup>,<br>3.20 &times; 10<sup>-2</sup></td><td>5.56 &times; 10<sup>-6</sup></td><td>4.85 &times; 10<sup>-6</sup>,<br>6.30 &times; 10<sup>-6</sup></td><td></td><td>1.01 &times; 10<sup>-4</sup></td><td>8.78 &times; 10<sup>-5</sup>,<br>1.14 &times; 10<sup>-4</sup></td><td>2.02 &times; 10<sup>-3</sup></td><td>1.76 &times; 10<sup>-3</sup>, 2.30 &times; 10<sup>-3</sup></td><td></td><td></td><td></td></tr><tr><td>Year</td><td>3.74 &times; 10<sup>-2</sup></td><td>4.05 &times; 10<sup>-3</sup>,<br>1.06 &times; 10<sup>-1</sup></td><td>1.31 &times; 10<sup>-6</sup></td><td>2.09 &times; 10<sup>-7</sup>,<br>3.24 &times; 10<sup>-6</sup></td><td></td><td>3.36 &times; 10<sup>-5</sup></td><td>5.82 &times; 10<sup>-6</sup>,<br>8.81 &times; 10<sup>-5</sup></td><td>7.92 &times; 10<sup>-4</sup></td><td>8.89 &times; 10<sup>-5</sup>, 2.07 &times; 10<sup>-3</sup></td><td></td><td></td><td></td></tr><tr><td>Residual</td><td>7 &times; 10 &times; 10<sup>-1</sup></td><td>6.62 &times; 10<sup>-1</sup>,<br>7.61 &times; 10<sup>-1</sup></td><td>9.53 &times; 10<sup>-5</sup></td><td>8.88 &times; 10<sup>-5</sup>,<br>1.02 &times; 10<sup>-4</sup></td><td></td><td>1.46 &times; 10<sup>-3</sup></td><td>1.36 &times; 10<sup>-3</sup>,<br>1.57 &times; 10<sup>-3</sup></td><td>1.20 &times; 10<sup>-1</sup></td><td>1.12 &times; 10<sup>-1</sup>, 1.29 &times; 10<sup>-1</sup></td><td></td><td></td><td></td></tr></tbody></table>

Note: Random effects included in the models were female identity, clutch identity (nested in female identity), and year. The table presents posterior mean estimates and associated 95% credible intervals (95% CI) for fixed and random effects. Significant fixed effects (p value <0.05 and a 95% CI that does not overlap with zero) are presented in bold.
<figure><img src="image_15.png" /><figcaption>**FIGURE 4** | (a) Egg volume, (b) pointedness, (c) elongation, and (d) polar asymmetry as a function of the combination of clutch size (CS) and laying order (LO) assessed using 1589 eggs from 687 clutches laid by 330 individual female common terns in the years 2017–2020. Circles represent the means of the egg shape characteristics for each CS-LO category, bars represent the standard errors. Significant differences between CS-LO categories are indicated by the lines and stars at the top of the plots (\*\*p < 0.01, \*\*\*p < 0.001).</figcaption></figure>

In sum, we found that egg characteristics were moderately to highly repeatable between female common terns, both within clutches, and among clutches laid in different years. Yet, specific aspects of egg characteristics were variably related to different aspects of the common tern's general life history, and may thus result from different selective pressures. We suggest further studies to identify the specific agents of selection that determine the adaptive function of the specific egg variables, to fully understand the eco-evolutionary significance of this extended phenotype of the female.

## Author Contributions

**Coraline Bichet:** conceptualization (equal), data curation (lead), formal analysis (lead), methodology (lead), writing – original draft (lead).

**Maria Moiron:** conceptualization (equal), formal analysis (supporting), methodology (supporting), writing – original draft (supporting).

**Nathalie Kürten:** data curation (supporting), methodology (supporting), project administration (supporting), writing – original draft (supporting). **Oscar Vedder:** conceptualization (equal), formal analysis

(supporting), methodology (supporting), writing – original draft (supporting). **Sandra Bouwhuis:** conceptualization (equal), formal analysis (supporting), funding acquisition (lead), methodology (supporting), project administration (lead), supervision (lead), writing – original draft (supporting).

## Acknowledgments

We are indebted to Peter H. Becker for setting up the long-term common tern study. We warmly thank Timo Ubben for building the photobox, and Götz Wagenknecht for assisting with the management of fieldwork and maintenance of the long-term dataset. We are very grateful to all field workers and students who helped with data collection, and to Mark Hase, Alessio Parisi, Hannah H. Kurz, Kristin Piening, Jasmine Sommerville and Choon W. Fuller for helping take the egg pictures. The study was performed under licences from the city of Wilhelmshaven and the Lower Saxony State Office for Consumer Protection and Food Safety, Germany. M.M. was funded by an Alexander von Humboldt fellowship for postdoctoral researchers.

## Conflicts of Interest

The authors declare no conflicts of interest.
# Data Availability Statement

Our data is available from the Dryad Digital Repository (DOI: https://doi.org/10.5061/dryad.6m905qgdk).

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## Supporting Information

Additional supporting information can be found online in the Supporting Information section. **Figure S1:** Intra-specific variability in egg characteristics in our common tern colony. **Figure S2:** Correlation matrix for eight egg characteristics assessed for 1589 common tern eggs from 687 clutches laid by 330 females in the years 2017–2020. **Figure S3:** Common tern nest containing two fake eggs and one original egg. Despite the fake eggs roughly resembling the real egg, common terns do not hesitate to incubate the clutch, neither when only a single original egg is replaced by a fake egg, nor when the whole clutch is replaced. **Table S1:** Estimates of the repeatability of measurements of eight egg characteristics obtained by analysing two pictures taken of different sides of a subset of 1430 eggs. **Table S2:** Proportion and cumulative proportion of variance explained by principal component axes loading variation in eight egg characteristics (spottiness, Hue, Saturation, Value, volume, pointedness, elongation, and polar asymmetry) assessed using 3019 pictures of 1589 eggs from 687 clutches laid by 330 female common terns in the years 2017–2020. **Table S3:** Factor loadings for eight egg characteristics assessed for 1589 eggs from 687 clutches laid by 330 female common terns in the years 2017–2020 onto the first two axes of the principal component analysis described in Table S2. **Table S4:** Estimates of the repeatability of eight egg characteristics for female common terns (a) across years and (b) within clutches, assessed using 1589 eggs laid in 687 clutches by 330 individual female common terns in the years 2017–2020.