Egg characteristics of female common terns are repeatable, and vary with maternal age and laying order

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 the 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.