Egg colouration and male parental effort in the pied flycatcher Ficedula hypoleuca

A recent hypothesis posits that the bright colours of many avian eggs may act like signals of female genetic quality or condition to males in species with biparental care, inducing them to work harder for their offspring. We measured the colour of blue eggs of pied flycatchers Ficedula hypoleuca in Central Spain with a spectrophotometer on the day of laying, and also quantified the provisioning effort by males and females during the last days of the nestling period. Both chromaticity coordinates in the CIELAB colour space (blue to yellow, green to red) showed significant associations with male provisioning rates, explaining more than 20% of variation in male parental effort. Male provisioning rates were positively correlated with nestling condition, thereby potentially contributing to female fitness. This evidence is only tentative until experimental confirmation, but suggests that males are affected by the colour of their mates’ eggs, a possibility not considered hitherto in the study of sexual selection.