the blue-green eggshell pigment biliverdin is an efficient antioxidant, the deposition of which may be costly and act as a handicap to the female’s antioxidant capacity. thus, it is hypothesized that there exists a positive correlation between the degree of eggshell coloration and the condition and health state of the mother. in this study, we measured the blue-green chroma of collared flycatcher ficedula albicollis eggs, and investigated the associations between egg colour and parental characteristics, laying date, clutch size and laying order. we also analysed the possible correlations between egg size, yolk carotenoid and antibody levels and eggshell pigmentation. our results revealed no significant relationships between egg colour and female age, morphological and serological variables. blue-green colour intensity of eggs increased throughout the breeding season, which could be the result of either a proximate constraint or an adaptive strategy. we found a positive association between egg colour saturation and yolk lutein concentration. this relationship could be the result of the seasonal improvement of caterpillar supply, which could influence the deposition of both components into the egg. furthermore, we found that females mated to lower-quality males produced eggs with higher biliverdin pigmentation. it is possible that deposition of biliverdin is correlated to some unmeasured aspect of egg quality that could enhance the viability of offspring of lower-quality fathers. we found that intensity of blue-green coloration increased with laying order between the first and fifth eggs.
variation in avian eggshell coloration has traditionally been interpreted as a response to selection pressures imposed by nest predators and brood parasites, although other functions, e.g. filtering solar radiation or strengthening the eggshell have also been proposed (for recent review see underwood and sealy 2002, kilner 2006). the blue egg pigmentation of many thrushes and other songbirds has been hypothesized to be the result of crypsis, as it blends in with the sunspots on the green leaves around the nest (lack 1958, oniki 1979, 1985). however, no difference in predation frequencies on nests with differently coloured eggs could be demonstrated, suggesting that blue eggs are not cryptic (go¨tmark 1992, kim et al. 1995, weidinger 2001).