Last but not least: nestling growth and survival in asynchronously hatching crimson rosellas

Abstract

1. Many species of altricial birds hatch their young asynchronously within broods. Although there are many potential benefits to parents, hatching asynchrony reduces the growth and often the survival of last-hatched nestlings. The consequences of hatching asynchrony on the growth, size at fledging and survival of male and female nestlings of an Australian parrot, the crimson rosella (Platycercus elegans) were examined. 2. Crimson rosella broods hatched over 1.5-7 days, creating mass hierarchies where first-hatched chicks were up to seven times larger than last-hatched chicks. Hatching asynchrony and mass hierarchies increased over the breeding season, but were not strongly correlated with brood size. 3. Male chicks grew faster and were larger at fledging than females. Growth rates did not differ between hatching ranks. Chicks of all ranks were of equal sizes at fledging, but last-hatched male chicks had lower fledging mass in pairwise analyses. Female mass at fledging did not decrease with hatching rank. Chick growth rates and size or mass at fledging were not related to hatching asynchrony, mass or size hierarchies in broods, brood size, laying date or year in mixed-model analyses. 4. Last-hatched chicks had the same post-fledging survival as other chicks, however, they were more likely to die during the nestling period. Increased mortality of last-hatched nestlings occurred only at hatching and chicks had equal mortality rates over the remainder of the nestling period. Early brood reduction was not associated with brood size or hatching asynchrony but increased over the breeding season, and in broods with high hatching success. 5. Hatching asynchrony in rosellas, unlike in most previous studies, did not lead to poor growth and subsequent survival of last-hatched chicks. This suggests that the costs of hatching asynchrony are low in this species and that selective feeding by parents may increase the growth and survival of last-hatched chicks. 6. Reduced growth and survival of later hatched chicks is not an inevitable consequence of asynchronous hatching; however, the costs to parents of overcoming competitive interactions between chicks may be higher than the benefits in most species.