Laying-sequence variation in yolk carotenoid concentrations in eggs of yellow-headed blackbirds (Xanthocephalus xanthocephalus)

Asynchronous hatching creates a size hierarchy among siblings and a survival disadvantage for last-hatched nestlings. Female birds can influence this disadvantage by differentially investing maternal resources, such as carotenoids, across the laying sequence. We studied intraclutch variation in carotenoid concentrations in Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) and predicted that yolk concentrations would decrease across the laying sequence, because nestling mortality is inversely related to hatching order in this species. We quantified intraclutch variation in the concentrations of total and individual identifiable carotenoids (β-carotene, β-cryptoxanthin, lutein, and zeaxanthin) in Yellow-headed Blackbird eggs collected from five breeding colonies, and correlated these concentrations with egg mass, yolk mass, and yolk water content. Carotenoid concentrations were not related significantly to any of the egg metrics measured. The concentration of total identifiable carotenoids increased across the laying sequence in Yellow-headed Blackbird eggs, which is the opposite of what has been found in most other passerines. Concentrations of the two most abundant carotenoids, β-carotene and β-cryptoxanthin, and zeaxanthin, all increased across the laying sequence, whereas the concentration of less-abundant lutein decreased. If yolk carotenoid concentrations contribute to increased survival of older over younger Yellow-headed Blackbird nestlings, concentrations of specific carotenoids, such as lutein, may be important. The differential change in concentrations of the various carotenoids across Yellow-headed Blackbird clutches may be attributable to the availability of these compounds in the diet combined with differences in absorption and antioxidant function.