Species translocations result in demographic bottlenecks that may produce inbreeding depression and reduce genetic variation through random sampling and drift, an outcome that could decrease long-term fitness and adaptive potential of many New Zealand species. Despite considerable evidence for costs associated with inbreeding and reduced genetic variation, some species have recovered from a small number of individuals and are thriving, perhaps via high growth rates, differential survival of heterozygous individuals or inbreeding avoidance. I examined the genetic consequences of species translocations in saddlebacks (Philesturnus carunculatus) with additional data provided for robins (Petroica australis) where possible. I first assessed whether contemporary genetic variation represented historical levels or a decline following demographic bottlenecks. I then examined whether sequential demographic bottlenecks caused sequential genetic bottlenecks and reviewed whether populations founded with a small number of birds were likely to go extinct. This analysis was followed by an investigation of two mechanisms that may maintain or reduce fitness costs, differential survival of heterozygous individuals and mate choice to avoid genetically similar individuals. Evidence from museum specimens suggests that low levels of genetic variation in contemporary saddlebacks is no different to historical genetic variation in the only source population, Big South Cape Island. An ancient founding event to Big South Cape Island is probably the cause of severe genetic bottlenecking rather than the demographic bottleneck caused by rats in the 1960s. In robins, genetic variation decreased slightly between museum and contemporary samples suggesting that recent population declines and habitat fragmentation have caused reductions in current levels of genetic variation. Serial demographic bottlenecks caused by sequential translocations of saddlebacks did not appear to decrease genetic variation. Loss of genetic variation due to random sampling was probably minimized because the low level of genetic variation remaining in the species was probably represented in the number of birds translocated to new islands. Models assessing future loss of genetic variation via drift showed that high growth rates combined with high carrying capacity on large islands would probably maintain existing genetic variation. In contrast, low carrying capacity on small islands would probably result in considerable loss of genetic variation over time. Saddleback populations on small islands may require occasional immigrants to maintain long-term genetic variation. Saddleback and robin populations established with a small number of founders did not have an increased risk of failure, suggesting that inbreeding was not substantial enough to prevent populations from growing and recovering. However, modelling showed that translocated saddleback and robin populations grow exponentially even when egg failure rates (a measure of inbreeding depression) are extremely high. Although inbreeding depression may be considerable, populations may be judged healthy simply because they show strong growth rates. Discounting the problem of inbreeding depression may be premature especially under novel circumstances such as environmental change or disease. Finally, two mechanisms proposed to avoid or delay the costs of inbreeding depression and loss of genetic variation do not appear to be important in saddlebacks or robins. Heterozygosity was not related to survivorship in saddlebacks that successfully founded new populations, and neither saddlebacks nor robins chose genetically dissimilar mates to avoid inbreeding. In conclusion, most saddleback populations should not require genetic management, although populations on small islands will probably need occasional immigrants. In robins, large, unfragmented populations should be protected where possible.

Inbreeding depression is one of the factors that can increase the risk of extinction of small populations, and therefore understanding its effects is currently an important issue in conservation biology. Until recently, few studies on inbreeding depression were carried out in wild populations. These recent studies have highlighted the variability in detecting inbreeding depression among natural populations and the multitude of factors that can influence its expression. Many of the factors affecting inbreeding depression in wild populations remain largely unexplored and most of the recent studies in this area have tended to focus on incidents of inbreeding in populations with a history of large population size. The aim of this study is to investigate the relative importance inbreeding depression has had on individual fitness parameters in a population of New Zealand's Stewart Island robins Petroica australis rakiura introduced to Ulva Island. This island population has historically gone through several population bottlenecks. Four main factors that potentially influence the rate of inbreeding and the extent of inbreeding depression, were investigated: environmental variability, life history stage, genetic load and dispersal. Generalized Linear Mixed Modelling was first used to determine how weather affected nest survival. Weather effects were then incorporated into models containing demographic factors to control for environmental variability, and finally parental, maternal and paternal inbreeding co-efficients (=f) were added to models to determine the relative importance of inbreeding depression. Interactions between inbreeding depression and environmental factors were explored. Three different life history stages were compared to determine the differences in inbreeding depression at each stage as well as cumulative effects over time. The genetic load of the population was estimated using lethal equivalents allowing for standardised comparison of inbreeding depression with other species. The likelihood of inbreeding in the population was also explored by investigating the factors affecting dispersal patterns and evaluating evidence for inbreeding avoidance. Inbreeding depression was found to be mild in the robin population. Weather did not have strong effects on nest survival or interactions with inbreeding. Female age was the only factor interacting with inbreeding, with younger inbred females experiencing significantly reduced offspring juvenile survival. Parental and paternal f did not significantly affect brood survival at any life history stage, however, maternal f showed significant effects on nest juvenile survival with the strongest effect occurring when survival was examined cumulatively over all life history stages. The Stewart Island robin had a relatively low lethal equivalent value compared to the closely related North Island robin and other avian species. This difference was associated with the Stewart Island robin having a low genetic load, most likely due to historical genetic purging during periods of population bottleneck. The Ulva Island robin population did not appear to be avoiding inbreeding through dispersal. Dispersal distance was most strongly influenced by the location of the natal nest of the dispersing offspring. In conclusion, the genetic history of the population was likely to have had the strongest impact on the severity of inbreeding depression in the Ulva Island robin population. The results of the thesis highlight the need to examine a number of factors to be able to explain variability in inbreeding depression among populations.