Data from: Carotenoid-based coloration correlates with the hatching date of Blue Tit Cyanistes caeruleus nestlings

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 Tit Cyanistes caeruleus
nestlings. 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
between UV chroma of breast feathers and hatching date. The amount of UV
reflectance is thought to be negatively related to carotenoid content in feathers. The
observed increase of UV chroma 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.