All around the egg: consistency of spottiness and colouration across an avian eggshell

Birds have evolved an astonishing diversity of egg pigmentation and this variation is driven by a complex interplay of proximate and ultimate factors. In ecological research, egg pigmentation is often measured at randomly selected points or regions of an eggshell, but it remains poorly explored whether and how spottiness and colouration are spatially consistent within a single egg. Here, we aimed to quantify consistency (repeatability) of spottiness patterns and background/spot colouration traits among the opposite lateral regions of an avian eggshell. For this purpose, we collected digital images of eggshells from three non-passerine birds showing different patterns of egg spottiness and colouration, i.e. Eurasian Coot Fulica atra, Black-headed Gull Chroicocephalus ridibundus, and Common Tern Sterna hirundo. We found an excellent within-egg repeatability (mean R > 0.90) of both background and spot colouration in all three species. In contrast, within-egg repeatability of spottiness traits was significantly lower, but the patterns of spatial variability were trait- and species-specific. While some spottiness traits showed relatively high within-egg repeatability (e.g. the total spot area), others were either poorly or moderately repeatable, depending on the species. Our results may have important practical implications for future research on eggshell colouration and spottiness, since measurements of reflectance spectra within a single region (side) are likely to be representative for the entire eggshell, while reliable characterisation of maculation may require replicate measurements across different eggshell regions. We recommend that the adaptive significance of spatial (within-egg) heterogeneity in avian eggshell pigmentation should be subject to further investigation under broader comparative framework.