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 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.