Hatching Asynchrony in Birds: The Hypothesis of Sexual Conflict Over Parental Investment

For animals with parental care, a sexual conflict may exist over the degree of parental investment involved. This may explain why, in birds, the eggs in a clutch do not hatch together but over a period of one or more days (asynchronous hatching). By adjusting the onset of incubation during the egg-laying period, the female is able to regulate the hatching times of the young and thereby the parental-investment pattern of the male. Some of the field data from the pied flycatcher support this sexual-conflict hypothesis. Females rearing asynchronous broods had higher body weights at the end of the nestling period than those rearing synchronous broods. However, it remains to be shown whether the post-fledging survival of the young is improved if the females are in good condition, and thus whether asynchronous hatching also benefits males.