Nestling sex does not influence provisioning effort of parents or helpers in a cooperative bird with a male-biased helper contingent

In cooperatively breeding species, the level of investment by both breeders and helpers may be influenced by the sex of offspring in broods that they attend, due to different fitness payoffs associated with investing in male versus female offspring. This might be expected if the sexes differ markedly in either the cost of their rearing, due to factors such as sexual size dimorphism, or the level of help that they may provide in later life that might benefit helpers in the future. Here we investigate how nestling sex influences the provisioning efforts provided by helpers and the subsequent growth and development of offspring in the cooperatively breeding noisy miner (Manorina melanocephala). In this species, helpers are predominantly philopatric males, as females disperse to other colonies to find breeding positions. We therefore predicted that helpers might invest more in helpful male-biased broods rather than those with dispersive female offspring, particularly if larger males required more resources than female offspring to successfully fledge. Male miners grew faster, fledging at the same age as their female siblings but with a heavier body mass. Male offspring therefore possibly require more resources than female offspring to raise. However, despite possible differences in the fitness returns of investing in different sexes, we were unable to detect any significant influence of brood sex ratio on the provisioning rate of pair or their helper contingent. Noisy miner helpers provision the brood irrespective of offspring sex. This could be due to an inability to recognise nestling sex, or potential future costs that might be imposed by philopatric males that subsequently compete for resources and/or mating opportunities with the helpers that reared them. From a male helper perspective, these potential costs may ultimately nullify benefits that male offspring provide in future. Male and female offspring might have different fitness benefits for helpers in the cooperatively breeding species. Given this, helpers in this breeding system might have sex-dependant preferences and direct their help towards the apparently more beneficial sex (i.e. the sex that is expected to stay at the same population and provide help in return). In this study, we showed that in the cooperatively breeding noisy miner (Manorina melanocephala), although male offspring are expected to provide higher level of care with over 90% of helpers being male, this apparent difference in the fitness return did not result in higher provisioning rate in the male-biased broods compared to female-biased broods. We argue that (a) this could be due insufficient cues for recognition of nestling sex or (b) despite the higher level of help that male offspring are expected to provide into the future, the overall inclusive fitness benefits do not vary among the sexes.