Re-Colonisation Dynamics of a Highly Social and Aggressive Bird, the Noisy Miner (Manorina melanocephala): Variations in Sex Ratio, Age Composition and Physiological Condition

Abstract

<p> The characteristics of populations such as sex and age structure are of ecological importance in wild animals, as they determine vital population traits such as growth rate and movement dynamics. In highly social species, population structure can also impact social aspects of populations, such as interactions between individuals within or between groups, territory selection and defence, and interspecific interactions with other taxa. The impact of these traits on population viability is even more important for social species with more complex population dynamics, especially if a taxa has negative impacts on ecological communities with increasing population size. For these species, population structure can determine the dynamics of interspecific interactions and the consequent level of ecological impact of taxa presence. Despite their importance, these aspects of the ecology of despotic species have largely remained unclear.</p> <p>One of the species that is characterised by both population expansion and potential negative impacts on ecological communities in south-eastern Australia is the Noisy Miner (</i>Manorina melanocephala</i>). This species has benefited from human-induced landscape changes in the eucalypt-dominated habitats of this region, colonising new habitats and undergoing rapid range expansion through following anthropogenic habitat modification to these woodlands. This expansion, coupled with the aggressive behaviour of Noisy Miners towards other species, particularly small native avifauna, has negatively impacted ecological communities through a reduction in biodiversity. Removal and target culling have been considered as one of the management and control practices for this species. Challenging this intervention strategy has been rapid recolonisation by miners into newly culled areas, and the dynamics of these recolonisation events in this obligately social species are not well known. Population management methods will be more efficient when there is enough information about social behaviour and the response of the target species to the environmental changes. Also, a better understanding of the factors stimulating dispersal will be gained, when there is more knowledge on details of species behaviour and the drivers of animal movement. </p> <p>This thesis investigates mentioned areas and ideas, focusing on pivotal aspects of Noisy Miner recolonisation dynamics that can be informative for management strategies and our understanding about the impact of sociality on these behaviours, including age structure, sex composition and the influence of an individual`s body condition at the time of dispersal. This project had access to the large sample size (2036 individuals) of Noisy Miners removed from six sites in fragmented woodlands of Bundarra region in NSW of Australia during 2015-2016. Below, I briefly outline how these samples were analysed and used to answer the key questions of each chapter:</p> <p>Chapter 1 introduces the influences of anthropogenic changes in the landscape and its impacts on wild populations. This chapter discusses the influences of habitat fragmentation and potential shifts in interspecific interactions, such as aggression and competition on avian movements. Especially, habitat fragmentation increases ‘edge’ habitat thus, impacts physical and biotic characteristics associated with patch boundaries. Further, this chapter introduces the study species, its habitat requirements and its complex social systems. This chapter also summaries what is known about the negative impacts of Noisy Miner on the ecological communities, in particular on the native avifauna. </p> <p>Accurately aging individuals and therefore the population is necessary for the study of population dynamics and social behaviour. However, this is not always a straightforward process for species which do not have different clear physical markers. Thus, in chapter 2, I established a methodology that can reliably be used for grouping Noisy Miners to different age categories according to the criteria of skull ossification level. Skull ossification level increased with age and was also found to be correlated with other multiple physiological and morphometric characteristics known to be age-specific. Thus, this ‘skulling’ provides a non-invasive method which allows the sampling of living individuals so that they can be accurately aged. </p> <p>During the recolonisation stages, sex and age structures might vary between resident and recolonising populations. In this study, `resident` birds are defined as individuals that occupy study sites prior to the first removal, and the term `recolonisers` refers to birds which move to vacant sites after resident birds are removed. It is important to understand differences in sex ratio between newly established and previously existing colonies or resident birds, and the resultant ecological and social consequences of any differences. In chapter 3, I examined how sex and age structures vary among resident and recolonising populations of the Noisy Miner. According to the results from this chapter, population sex ratio of Noisy Miners colonies was significantly male-biased in undisturbed-resident colonies with 61% of the population being male. In the period following recolonisation, the proportion of males in the population dropped to ~ 48% during the first month postremoval, and stayed around the same level for next five months. At all examined sites, the resident birds were primarily adult birds with about 60% of birds being adult. The proportion of adult birds declined one month post-removal to as low as 35% but slightly increased five months post-removal reaching 48% of adults in the population. Overall, the results suggested that newly recolonised populations have a sex ratio that is closer to parity than resident colonies. Also, recolonised populations have more young individuals than established colonies. Together, these results suggest that recolonising birds may have left their natal colony to seek better breeding opportunities in new areas and the reduced proportion of males in the population might have ecological significance</p> <p> How important was the body condition and fat resources of birds during recolonisation? Are Noisy Miners with a better body condition more likely to occupy newly vacated areas? These questions were examined in chapter 4 of this thesis. Adult female Noisy Miners were in significantly better body condition in terms of fat deposit levels when compared to adult males, although the scaled mass and fat deposit level of resident females did not differ significantly from those of recolonising birds, nor did scaled mass and fat deposit levels vary significantly during the recolonisation process. Newly recolonising populations did not have the same social composition in terms of age structure or sex ratio to that of the established-resident colonies. This might have an ecological consequence, such as a reduced effectiveness of newly established colonies in excluding other birds from habitats, perhaps as a result of reduced social cohesion leading to decreased levels of aggression. Further, lack of differences between the body condition of birds during recolonisation suggests that factors other than high body condition drive the initiation of recolonisation movement in this species. This has important implications for population management of this despotic species, as it shows that factors other than just habitat quality or body condition drive recolonisation. </p> <p>Chapter 5 of the thesis sums up the main findings of the thesis and introduces further directions. Briefly, the outcome from this study provides useful and practical information concerning the recolonisation of areas by Noisy Miners. However, further research on recolonisation and movement dynamics, most easily assessed through population connectivity and gene flow, could enhance our understanding of the ecology of this species. In addition, habitat structure and quality and also resource availability should also be incorporated into future research in study sites.</p>