Mass provisioning to different-sex eggs within the laying sequence:: consequences for adjustment of reproductive effort in a sexually dimorphic bird

1. In birds, the potential maternal ability to adjust resource allocation to different eggs in the clutch might have a major effect on the survival expectancies of particular nestlings or entire broods. We assessed whether sexually size-dimorphic Eurasian kestrels Falco tinnunculus (Linnaeus) are able to adjust their reproductive effort by adopting different strategies of egg mass provisioning according to egg sex and laying order. 2. Initial eggs bearing male embryos were heavier than initial eggs bearing female embryos, but no differences in egg mass associated to sex were detected for eggs laid subsequently. Furthermore, in clutches started with a male egg, egg mass declined in subsequent eggs, while in clutches started by a female egg the opposite trend in within-clutch egg-mass variation was found. This suggests differential deposition of resources invested in initial eggs of different sex leading to saved or depleted resources for subsequent eggs. 3. Daughters from initial eggs hatched earlier than sons from initial eggs, which may enhance survival of smaller siblings hatched later. These contrasting strategies of egg provisioning and hatching patterns depending on the sex of the first-laid egg were associated, respectively, with marked mass hierarchies and a lack of mass hierarchies at fledgling in broods initiated with eggs bearing sons and daughters. 4. Parental kestrels may allocate reproductive effort by promoting favouritism towards early hatched chicks or by avoiding any favouritism by producing siblings of each sex with similar mass. This may be achieved depending on the female ability both to identify egg sex and to partially reabsorb or differentially allocate resources to eggs accordingly to adjust reproductive investment. This may be a key mechanism to control sibling competition in birds with sexual dimorphism in mass.