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