A recent hypothesis proposes that the bright colors, especially blue and green, of many avian eggs may function as signals of female or offspring phenotypic quality or condition to males in species with biparental care, inducing them to allocate more effort to their offspring. The pigment determining blue and green egg colors is an antioxidant whose availability for eggshell coloring may be limited. To test the signaling function on a species with blue eggs, the pied flycatcher Ficedula hypoleuca, we measured egg color with a spectrophotometer on the day of laying and obtained two principal components from their reflectance spectra that together explained 99% of variation and represented shell lightness, and hue and saturation, respectively. We also measured female immunocompetence during the nestling period through the response to phytohemagglutinin as a measure of cell-mediated immunity and the response to a tetanus vaccination as a measure of Immoral immunity. The total amount of immunoglobulins in blood of females and of nestlings before fledging was also estimated. Mean within-clutch egg darkness was positively associated with both measures of female immunocompetence, while better female condition was associated with colors tending away from intermediate and toward short wavelengths. Ageing female laid lighter eggs. The mean within-brood level of nestling IgY was also associated with mean within-clutch egg colors tending away from intermediate and toward short wavelengths. Mean egg darkness decreased linearly during the laying sequence, suggesting pigment limitation. Males were observed frequently visiting nests during the laying period, allowing them to observe eggs before the start of incubation. These results support the signaling hypothesis for explaining bright colors of avian eggs.
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