Parent-Offspring Interaction over Brood Size: Cooperation or Conflict?

Abstract

Results from studies of parent-offspring interactions over brood size have been interpreted both as being the result of cooperation and conflict. The key question is if brood reduction is adaptive or non-adaptive from the parents' point of view. This can be linked to hypotheses suggested as explanations of hatching asynchrony. One set of hypotheses states that, by starting to incubate before clutch completion and so creating a size hierarchy in the brood, parents can later trim the brood to the optimal size. Thus, brood reduction is adaptive and parents and surviving offspring cooperate over the size of the brood. Other hypotheses view brood reduction as an unselected side-effect and cost of an early start of incubation within the laying sequence. The cost will be in the form of non-adaptive loss of small nestlings through sibling competition even though feeding conditions may be adequate for raising the whole brood. Thus, parents and offspring disagree over brood size. To test this, the published experimental manipulations of hatching asynchrony were used. Higher fledging success in asynchronous control broods than in experimentally synchronized broods would indicate parent-offspring cooperation, whereas the opposite result would indicate parent-offspring conflict over brood size. The result of the analysis showed that 83% of all species with adequate data had higher fledging success when the broods were made synchronous compared to asynchronous control broods. Even when differences in mass between nestlings in synchronous and asynchronous broods were considered, the majority of studies indicated the existence of a parent-offspring conflict over brood size. The reduction of asynchronous broods is a consequence of the offspring being in a position to exert their will, which is possible due to the presence of one or more easily outcompeted runts in such broods.