Visual scoring of eggshell patterns has poor repeatability

Eggshell pattern scoring, a method to quantify the degree of surface maculation, can potentially be a quick, inexpensive and reliable method to obtain information on eggshell appearance and spot patterns. The key pigment responsible for red-brownish hues, protoporphyrin IX, is often localized as spots, either on the surface or in distinct layers within the eggshell. Heritable pigment spotting has been linked to factors such as breeding performance and eggshell strength. In this study, we investigated whether pigment scoring of eggshell patterns is repeatable within and between observers, by testing observers under standardised conditions, using the eggshells of two commonly studied passerines, Great Tits (Parus major) and Blue Tits (Cyanistes caeruleus). We found that repeatability of eggshell scores was poor, both within and between observers for both the species. We, therefore, encourage future studies to use alternative methods for quantifying spot patterns, such as digital image analysis, a technique which has already been used extensively.