Eggshell Patterning in the Red-Backed Shrike Lanius collurio: Relation to Egg Size and Potential Function

Despite numerous studies on avian eggs, the function of eggshell patterning remains largely unknown. Using a museum collection of Red-backed Shrike clutches, we estimated the repeatability of characteristics describing the intensity and contrast of egg pigmentation, as well as egg size and shape. Digitalized photographs of eggs were used for all measurements. Repeatability was highest for overall pigmentation intensity and overall pigmentation contrast (R = 0.81 and R = 0.82 respectively). Lower values were recorded for egg breadth (R = 0.66) and shape (R = 0.57). These findings suggest that eggshell patterning, as well as the variables describing egg size and shape, may respond to natural selection. Among the possible functions of this trait, condition signalling within species and mimicry are the most probable. To obtain more evidence of this, however, further studies in wild populations are required.