Within‐ and between‐season repeatability of eggshell colouration in the great reed warbler Acrocephalus arundinaceus

Abstract

While avian eggshell colouration has attracted biologists for decades, little is known about its variation within individuals. The main goal of this study was to explore within‐ and between‐season repeatability of eggshell appearance in the great reed warbler Acrocephalus arundinaceus. To achieve this, we measured eggshell reflectance of first and replacement clutches of individual females within one breeding season, and the reflectance of their first clutches in two following breeding seasons. As environmental conditions may affect egg colouration, repeatability was estimated from linear mixed‐effects models where we initially included temperature and rainfall during egg laying, as well as year, clutch order and egg‐laying date as fixed effects, and clutch identity nested in females as random effects. Eggshell appearance within clutches showed moderate repeatability, while both within‐ and between‐season repeatability in clutch colouration for individual females was low. Our findings indicate that low intra‐clutch variation may have a function within the context of brood parasitism, i.e. in facilitating host recognition of alien eggs. With variable eggs between successive clutches, however, host females may need to relearn the appearance of their eggs with every clutch they lay. This could represent a significant constraint for host egg‐discrimination abilities. Yet, whether environmental or intrinsic physiological factors are responsible for the variation in eggshell colouration between successive clutches of the same females still remain to be discovered.