A standardized method for quantifying eggshell spot patterns

Spottiness is an important component of the morphology of bird eggs and a number of methods have been developed for characterizing variation in spottiness. We developed a quantitative method for measuring and comparing eggs to determine if female European Cranes (Grus grus) lay eggs with individually distinct color and spotting patterns. We used photographs taken under standard conditions and developed a computer program (ESPANA) to quantify egg‐spot patterns. The goal of the analysis was to create a “fingerprint” of each eggshell by measuring reflection along virtually drawn lines (transects) on an egg's image. Values measured in the same positions along transects can be compared among eggs by considering them as separate variables defining the pattern. Data were analyzed using cluster analyses and by performing analyses of similarity (ANOSIM). We found that the eggs of female European Cranes (N = 11) had individually distinct color patterns, with eggs laid by a given female more similar to each other than to the eggs of other females. Beyond its potential use for identifying the eggs of specific females, we believe our method could also be useful for investigators quantifying differences in egg‐spot patterns for other reasons, for example, examining possible relationships between egg‐spot patterns and female quality. The program ESPANA is implemented using the Java programming language and is available as supporting information on the Journal of Field Ornithology website.