Darker eggshell spotting indicates lower yolk antioxidant level and poorer female quality in the Eurasian Great Tit (Parus major)

Protoporphyrin pigment causes the red-brown eggshell colors; however, for many species, the function of this pigment is unknown. It has been proposed that eggshell pigmentation may strengthen the shell or that it may be a sexually selected signal, which advertises the quality of the female and that of her offspring to the male parent. In this study, we aimed to discover whether protoporphyrin-based eggshell pigmentation patterns of Eurasian Great Tits (Parus major) were related to female or egg quality. Additionally, we tested whether different methods of eggshell pigmentation estimation could be reliable predictors of eggshell protoporphyrin levels. We found that spot intensity, spot size, spotting coverage, and brown spot chroma indicated the protoporphyrin pigment concentration of the eggshell. Our results revealed that Eurasian Great Tit females that laid eggs with darker pigmentation had more lymphocytes in their circulation and had paler yellow breast and lower UV plumage reflectance, possibly indicating poorer health and individual quality. However, we did not find evidence that eggshell pigmentation patterns indicated the body condition, body size, or plasma oxidative status of females. Furthermore, we found that eggs with darker spots contained lower concentrations of antioxidants in the yolk. High protoporphyrin levels may be detrimental to females as they may cause oxidative damage, and this may be why birds that laid eggs with darker spots deposited lower amounts of antioxidants into the egg yolk. Shell spot darkness may also indicate territory quality, as females that laid smaller clutches also laid eggs with higher eggshell pigmentation levels. Thus, our results suggest that shell spot darkness may indicate the state of health of the female, egg yolk antioxidant level, and possibly also the quality of the territory.