Eggshell pigmentation in the blue tit: male quality matters

Many passerines lay protoporphyrin-pigmented eggs, and the degree of spotting seems to be related to female condition and environmental characteristics. However, most studies have ignored the relationship between the male’s quality and eggshell pigmentation. Because ornaments can act as honest indicators of individual quality, spottiness could be related to the parents’ feather colouration. Using models of bird vision, we investigated whether male and female ornamentation explained variation in spotting coverage in a free-living population of blue tits (Cyanistes caeruleus). We also explored the associations between other important individual characteristics (i.e. the pair’s infection status) and spotting coverage. Females that laid more pigmented eggs suffered from higher parasitaemia by the blood parasite Leucocytozoon, had smaller clutches, more saturated yellow breasts feathers and reduced body mass. Male plumage colour and infection status explained a higher percentage of the variation in eggshell pigmentation than female characteristics. Males with more saturated white cheeks, less saturated yellow breasts, more intensely infected by the parasite Haemoproteus and less by Plasmodium, attended nests with more spotted eggs. Additionally, these males were younger and more likely to father extra-pair offspring. These results, although observational, suggest that male attractiveness, male age, extra-pair paternity, and parasitic infections could be important determinants in eggshell pigmentation. Males in poorer condition might have provided less food to laying females, which in turn laid more pigmented eggs and were also in poor condition. Alternatively, increased eggshell pigmentation could result from female differential allocation or breeding in low quality territories.