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 speciesfrom the family Paridae, this trait is also present at the nestling stage. One of the factorsproposed 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 hatchingdate on the breast carotenoid-based plumage coloration of the Blue Tit Cyanistes caeruleusnestlings. Because Blue Tits are sexually dichromatic, we also investigated the potentialinteraction between hatching date and sex that could arise from differences incondition dependence of this trait between males and females. We found a positive relationshipbetween UV chroma of breast feathers and hatching date. The amount of UVreflectance is thought to be negatively related to carotenoid content in feathers. Theobserved increase of UV chroma through the breeding season might therefore be causedby a seasonal decline in the availability and quality of Lepidoptera larvae – the mainsource of carotenoids in food of the Tits. We also observed a sex difference in the relationshipbetween brightness of breast feathers (achromatic, structural component) andhatching date, which in males was negative and in females not significant. Our studyprovides further evidence that the timing of breeding is related to the expression of nestlingcarotenoid-based coloration, a potentially meaningful element of offspring–parentcommunication, and suggests a sex-specific effect of hatching date on its structural component.