Eggshell coloration is related to condition of females and offspring but not to male provisioning effort in a cup-nesting passerine

Pigments present in avian eggshells are thought to serve many different adaptive functions. According to the sexually selected eggshell coloration hypothesis, the quantity of pigments in eggshells can serve as a signal of a female’s condition or of her maternal investment into eggs that elicits a higher investment from her partner. Here, we employed a comprehensive approach to test the sexually selected eggshell coloration hypothesis using a cup-nesting passerine with protoporphyrin-pigmented eggshells, the red-backed shrike Lanius collurio, as our model species. We used calibrated digital photography and visual modelling to assess eggshells appearance from an avian point of view. We estimated the condition of chicks and adult females using the Scaled Mass Index and additionally measured average width of growth bars on rectrices of adult females. We quantified the investment of males in terms of provisioning rates to offspring. We found that females in worse condition laid eggs with redder spots and the redness of spots was positively related to the condition of chicks. Visual modelling revealed that males were likely to perceive the variation in colour and brightness among clutches in the studied population, however, the provisioning rates of males were not related to any aspect of eggshell appearance. Thus, while eggshell coloration can act as a cue of female condition, we found no support for the signalling function of eggshell coloration in post-mating sexual selection in the red-backed shrike.