Carotenoid-based coloration correlates with the hatching date of Blue TitCyanistes caeruleusnestlings

Carotenoid-based coloration occurs predominantly in adult birds, yet in some species from the family Paridae, this trait is also present at the nestling stage. One of the factors proposed to affect the expression of this trait in immature birds is hatching date. Here, using the avian tetrahedral colour space model, we examined the influence of hatching date on the breast carotenoid-based plumage coloration of the Blue TitCyanistes caeruleusnestlings. Because Blue Tits are sexually dichromatic, we also investigated the potential interaction between hatching date and sex that could arise from differences in condition dependence of this trait between males and females. We found a positive relationship betweenUVchroma of breast feathers and hatching date. The amount ofUVreflectance is thought to be negatively related to carotenoid content in feathers. The observed increase ofUVchroma through the breeding season might therefore be caused by a seasonal decline in the availability and quality of Lepidoptera larvae - the main source of carotenoids in food of the Tits. We also observed a sex difference in the relationship between brightness of breast feathers (achromatic, structural component) and hatching date, which in males was negative and in females not significant. Our study provides further evidence that the timing of breeding is related to the expression of nestling carotenoid-based coloration, a potentially meaningful element of offspring-parent communication, and suggests a sex-specific effect of hatching date on its structural component.