Do parents and helpers adjust their provisioning effort in relation to nestling sex in a cooperatively breeding bird?

In cooperatively breeding vertebrates, investment strategies of breeders and helpers may be affected by the sex of offspring that they provision because the fitness benefits gained from caring for sons or daughters will depend upon the fitness impacts of positive (i.e. local resource enhancement) and negative (i.e. local resource competition) social interactions between relatives. We investigated the relationship between provisioning effort by breeders and helpers and the sex of nestlings that they provision, using long-term data over 16 years and detailed behavioural observation of interactions between carers and offspring in a cooperatively breeding species, the long-tailed tit, Aegithalos caudatus. We predicted preferential care for male nestlings because males are the more helpful sex in the kin-selected social system of long-tailed tits. However, we found that despite the significant size difference between male and female nestlings, there was no evidence of facultative adjustment of provisioning effort by either breeders or helpers in relation to brood sex ratio. In addition, we found no evidence for sex-biased provisioning by carers, and no bias in the feeds received by nestlings according to their sex. Thus, we conclude that the provisioning strategy of long-tailed tits does not incorporate differential investment according to sex. We suggest that the unpredictable nature of helping in the atypical cooperative breeding system of long-tailed tits makes random provisioning effort in relation to nestling sex the best option for carers.