Both hatching sequence and sex influence the early life development of young cooperatively breeding noisy miners Manorina melanocephala (#889)
Asynchronous hatching is an almost ubiquitous phenomenon among altricial birds, where earlier hatched offspring may gain access to greater resources over their younger siblings. Further, nestling sex may also influence growth patterns, as investment in one sex over the other may yield competitive advantages to the individuals providing care, be they parents or, in the case of cooperative species, non-breeding helpers. We investigated how hatching sequence and nestling sex influenced the growth and development of nestlings in the cooperatively breeding Noisy Miner Manorina melanocephala, a bird that lives in large, complex social groups with female-biased dispersal of offspring. Overall, offspring growth rates, expressed as body mass gained during the linear growth phase prior to fledging, were best explained by hatching order and offspring sex. In contrast, skeletal growth was primarily affected by other factors such as the brood's sex ratio, with nestlings raised with smaller female siblings having a faster growth rate than those raised with larger male siblings. Significant sex differences were apparent in nestling growth; male nestlings had a higher growth rate and a larger pre-fledging body size, potentially indicating a greater cost to raise. Earlier-hatched nestlings also grew at a faster rate, regardless of their sex, and achieved a greater pre-fledging body mass than later hatched young. Overall, our findings suggest that early life development in noisy miners is heavily influenced by both hatching sequence and nestling sex, probably through within-brood competition and higher investment in philopatric males than dispersive females, leading to potential conflict in brood allocation patterns between helpers and breeders in this obligately cooperatively breeding system.