Cooperative breeding in the Noisy Miner (Manorina melanocephala): The role of genetic relatedness, sex, extra-pair paternity and acoustic signals

Abstract

<p>Vertebrates use different reproductive strategies and consequently, diverse forms of parental care to improve offspring survival and thus their lifetime reproductive success. A relatively rare form of reproduction and parental care, which is known as cooperative breeding, evolves when individuals other than breeders (i.e. helpers) provide alloparental care to offspring. Given that any form of parental care is likely costly, helping to raise the offspring of others appears to be against the expectations of natural selection theory. Hence, factors that underpin the evolution and maintenance of cooperative breeding have attracted much research attention during the last few decades.</p><p> In this thesis, I describe my research on the dynamics of cooperative breeding and the importance of various determinants for its evolution in a native Australian passerine bird, the noisy miner (<i>Manorina melanocephala</i>). Why do some individuals assist in raising the offspring of others? Do breeding females choose their nest site to maximise the amount of aid received from helpers? How does a helper's sex or the presence of extra-pair (EP) mating influence the dynamics of helping? Does acoustic communication allow helpers to aid young in pathways that don't involve provisioning? These questions are some of the important aspects of cooperative breeding that were addressed using data collected from three years of field observations of colour-marked populations, combined with molecular analyses.</p><p> Female noisy miners tended to select areas close to open and fragmented patches for their nest site, a selection that consequently may facilitate more efficient foraging, nest defence and accessibility of nests for potential helpers. Overall, in this system, kinship or relatedness between individuals appeared to be the main reason why helpers aided offspring, as they increased their provisioning rate with increased genetic relatedness to breeding pairs. Therefore helpers primarily gain indirect fitness benefits through helping their relatives. Despite this, unrelated helpers also provisioned young, so other types of direct benefits, such as group augmentation, might also be accrued by helpers and play a role in maintaining cooperatively breeding in this species.</p><p> In this system, helping behaviour seems to be closely associated with sex-related dispersal patterns and an overall male-biased population sex ratio, with helpers consisting of mainly philopatric males (93 % helpers). Given this, despite investment in male-biased broods appearing to offer higher fitness returns for both breeders and helpers, brood sex ratio did not influence the amount of food provided at a nest by either the breeding pair or their helper contingent. This finding could be due to an inability of provisioners to recognise nestling sex, or potential future costs that might be imposed upon philopatric males such as increased competition for resources and/or mating opportunities from offspring once they reach maturity.</p><p> Further, I examined the genetic mating system of this species and showed that: a) there is tendency to avoid copulation with genetically related individuals, with 86% of social breeding pairs being significantly less related to each other than the general population and b) 27% of broods contained extra-pair offspring and 14% of nestling were sired by males other than the putative breeding male. However, extra-pair mating occurred independent of the degree of relatedness between members of the social breeding pair at a given nest. In addition, extra-pair mating did not lead to greater helper recruitment at nests, as successful extra-pair males did not provision at nests after they obtained paternity.</p><p> Finally, I show that noisy miner nestlings not only respond effectively to intraspecific alarms calls by ceasing vocalisation production, but also might have the ability to differentiate between terrestrial and aerial alarm calls of conspecifics, as nestlings suppressed begging signals for longer in response to terrestrial rather than aerial alarm calls broadcast near the nest. This demonstrates a possible novel form by which helpers aid offspring, as noisy miners routinely give different alarm calls to various threats and can warn nestlings of the presence and the type of danger.</p>