Egg quality and parental ornamentation in the blue tit Parus caeruleus

Components of avian egg quality often show remarkable variation between females and yet the causes of variation in this fitness-related trait remain poorly understood. We investigated the egg size and yolk carotenoid investment of blue tit Parus caeruleus females in relation to multiple parental traits. We show that females produce eggs with more colourful yolks when mated to ultraviolet (UV) attractive males. Since yolk carotenoids may enhance offspring fitness, but may be available to females in limiting amounts, the correlation we found suggests adaptive egg quality adjustment in response to mate attractiveness, but an experimental approach is required to rule out alternative explanations. Yolk colour was also correlated with laying date and yolk mass. Though the effect of laying date could not be explained by ambient temperature during egg formation, it suggests a proximate constraint of general carotenoid availability on yolk composition. Egg size was not affected by male attractiveness; however, females with a brighter crown produced larger eggs, suggesting that crown coloration might indicate individual quality in females too.