Growth of yellow-headed blackbird Xanthocephalus xanthocephalus nestlings in relation to maternal body condition, egg mass, and yolk carotenoids concentrations

Condition-dependent resource allocation to eggs can affect offspring growth and survival, with potentially different effects on male and female offspring, particularly in sexually dimorphic species. We investigated the influence of maternal body condition (i.e., mass-tarsus residuals) and two measures of female resource allocation (i.e., egg mass, yolk carotenoid concentrations) on nestling mass and growth rates in the polygynous and highly size dimorphic yellow-headed blackbird Xanthocephalus xanthocephalus. Egg characteristics and carotenoid concentrations were obtained from the third-laid egg of each clutch and were correlated with the mass and growth rates of the first two asynchronously hatched nestlings. Maternal body condition was associated with the growth of first-hatched, but not second-hatched nestlings. Specifically, females in better body condition produced larger and faster growing first-hatched nestlings than females in poorer body condition. As predicted for a polygynous, size-dimorphic species, females that fledged first-hatched sons were in better body condition than females that fledged first-hatched daughters. Associations between egg mass, yolk carotenoid content, and nestling growth were also specific to hatching-order. Egg mass was positively correlated with the mass and growth rates of second-hatched nestlings, and yolk concentrations of beta-carotene were positively correlated with second-hatched nestling mass. Surprisingly, the relationship between yolk lutein and hatchling growth differed between the sexes. Females with high concentrations of yolk lutein produced larger and faster growing first-hatched sons, but smaller first-hatched daughters than females with lower lutein concentrations. Mass and growth rates did not differ between first- and second-hatched nestlings of the same sex, despite asynchronous hatching in the species. Results from this study suggest that maternal body condition and the allocation of resources to eggs have carotenoid-, sex-, and/or hatch-order-specific effects on yellow-headed blackbird nestlings.