Hatching asynchrony and brood reduction in the jackass penguin: an experimental study

Abstract

The brood reduction hypothesis for asynchronous hatching was tested for the jackass penguin, Spheniscus demersus, by comparing the reproductive success of two-chick broods with different degrees of asynchrony, and by identifying the behavioural basis of competitive asymmetries between siblings. By interchanging newly hatched chicks between nests, broods of synchronous and extended asynchronous siblings were created. There was no significant difference in fledging success between unmanipulated normal asynchronous (mean number of chicks per brood=1·14) and synchronous (1·44) broods; however, extended asynchronous (0·6) broods lost significantly more chicks than normal asynchronous ones. Last-hatched chicks in normal asynchronous broods were more likely to die than their siblings, and all last-hatched chicks in extended asynchronous broods failed to fledge. Chicks in normal asynchronous broods were not significantly heavier at fledging, but did attain higher maximum body weights before fledging, and fledged sooner than chicks in synchronous broods. This appeared to be due to higher begging success and lower sibling competition during feeding by asynchronous chicks. In terms of chick survival, the brood reduction hypothesis does not provide an explanation for hatching asynchrony in the jackass penguin. Asynchrony may, however, provide an advantage with respect to fledgling quality and post-fledging survival, as predicted by extensions of both the brood reduction and the sibling rivalry reduction hypotheses.