Why do parents produce small broods of offspring that have lower body mass, survival, and lifetime reproductive success? A case study in a long-lived bird

Numerous studies have examined the correlation between offspring quantity and quality, and many have found that the most common brood size is often smaller than broods with the highest offspring quality or production. However, the reasons why these small broods with lower offspring quality are produced are still poorly explained. Using data spanning 29 years, we investigated the effects of brood size on nestlings’ body mass and the lifetime fitness for those offspring as adults (as proxies of offspring quality) in the Crested Ibis ( Nipponia nippon ). We also examined the temporal variation of brood size. We found that overall offspring quality increases with brood size and that individuals from broods of three had the highest quality, as quantified by larger body mass, higher adult survival, and lifetime reproductive success. Furthermore, the brood size of an individual pair significantly varied across years, and the proportion of broods containing two offspring increased while broods of three decreased after 2000 when the population dispersed to low-quality habitat. These findings indicate that spatiotemporal variation in resources may impact variation in brood size and subsequent fitness consequences, and that small broods are more common in resource-poor years or low-quality habitats. In contrast, parents with access to high-quality resources produce larger broods of nestlings that achieve higher body mass and subsequently experience higher adult survival and lifetime fitness. This study highlights how variation in life history traits can be influenced by resource condition and provides an insight into particular habitats that need conservation for Crested Ibis. Significance statement Although life history theory predicts a trade-off between offspring quantity and quality, and that fewer, high-quality offspring are expected to be more common to prolong one’s own survival prospect in long-lived species, birds, mammals, and humans often show a positive correlation for these traits. Why do parents produce small broods with lower offspring quality? Here, we found that offspring quality—such as nestlings’ body mass, survival, and the lifetime reproductive success of offspring as adults—increased overall with brood size, up to broods of three of Crested Ibis. Brood size varied across years; in particular, pairs appear to produce smaller broods of nestlings that have lower body mass and lifetime fitness in resource-poor years or lower-quality habitats. This long-term study helps to advance our understanding of the fitness consequences and ecological mechanisms that impact offspring quantity and quality in long-lived animals.