Hatching asynchrony, sibling competition and siblicide in nestling birds: Studies of swiftlets and bee-eaters

The consequence of hatching asynchrony for fledging success and sibling competition in white-bellied swiftlets, Collocalia esculenta, and blue-throated bee-eaters, Merops viridis, was studied by observation and manipulation of nest contents at breeding colonies in Malaysia. A greater synchrony of hatching tended to increase fledging success amongst swiftlets, a result consistent with most other similar manipulative studies to date. Sibling competition in blue-throated bee-eaters invariably led to nest fatalities, which were induced by sibling attacks with an apparently unique, but developmentally temporary, hook at the tip of the upper mandible. Measurements of energy expenditure using the doubly labelled water technique showed that synchrony also reduced the energy cost of nestling competition in bee-eaters, thereby providing a possible mechanism for improved growth and survival of synchronous broods. No universal adaptive function for hatching asynchrony was identified, so it is proposed that a primary function is to encourage distribution of food amongst the brood in a way that permits resource tracking, but can also allow brood reduction under extreme adversity (i.e. a sustained food ‘crash’, parental death or desertion).