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Offshore islands have also been heavily modified. Nevertheless, many islands escaped invasions by exotic predators such as ship rats and possums (Parker, 2008). These islands provided refuges for native species which cannot survive in the mainland due to the presence of introduced mammals (Veitch & Bell, 1990). Most offshore islands in New Zealand have been unoccupied by humans for several decades. As a result, regeneration and vegetation occurred naturally. Since 1980s, there has been a major drive to restore island ecosystem by replanting vegetation, eradicating exotic predators, and reintroducing native species (Armstrong et al., 1990; Veitch & Bell, 1990). At the beginning, there has been high failure rate of reintroduction attempts, yet an increase in monitoring for all types of conservation management has improved the reintroduction success for some species. Now, Reintroductions of New Zealand birds to islands become the major component of conservation management in New Zealand (Veitch & Bell, 1990). As of March 2002, there had been 224 documented reintroductions or introductions of native species to New Zealand islands, 188 involving animals and 36 involving plants (Armstrong et al., 1990).

In the 1000 years since people first settled in New Zealand, 32 percent of indigenous land and freshwater birds and 18 percent of sea birds became extinct (Cullena et al., 2005). The most recent extinction was the h in 1907 and the rarest bird is fairy tern(36). There are other rare birds, such as kakapo, takahe, Campbell Island teal (50-100), taiko (50-100), black stilt (80), Chatham Islands oystercatcher (100), and black robin (around 250) (Cullena et al., 2005). DOC (Department of Conservation) in New Zealand works to protect and improve the status of these species and other populations which are small and suffer from introduced predators and competition for breeding sites. Primarily, nesting sites have been protected from disturbance by humans and introduced predators. As hands-on assistance, staffs from DOC are involved in more specific projects about how to intervene with different species to ensure their survival. Birds like saddleback have been transferred to various offshore islands to build up new populations in predator-free habitats. Since saddlebacks have been successfully relocated to nine different islands, the population has increased tenfold, which is around 5000 (Taylor et al., 2008).

New Zealand Saddleback(Philesturus carunculatus) is medium sized forest passerine(c. 25cm, 90g males, 70g females) and a member of the endemic New Zealand wattlebird family(Callaeidae) (Armstrong et al., 1990; Hooson & Jamieson, 2003). Saddlebacks have two distinct geographical subspecies: Philesturnus carunculatus rufusater in the North Island and P. c. carunculatus in the South Island. They usually form monogamous pairs that last until one member dies (Armstrong et al., 1990).They were originally found throughout the North and South Islands as well as many islands within 50km of the New Zealand mainland (Armstrong et al., 1990; Hooson & Jamieson, 2003). However, they are highly vulnerable by introduced mammals such as stoats and rats. As a result, they became extinct everywhere except three small offshore islands following European colonisation (Love groove, 1996). By the late 1880s, saddleback species was “extremely rare” and “very irregular” in its distribution and virtually extinct both in the North and South Island by 1905 (Buller, 1888).

The first translocation of saddleback to offshore islands began in 1964 (Hooson & Jamieson, 2003). There were two factors that limited the early recovery of saddlebacks, predators and insufficient land area for population expansion (Hooson & Jamieson, 2003; Love groove, 1996). The eradication of predators and continuous control in offshore islands has been achieved by poison drop. Right after the poison operations, there was a population drop in Mokoia Island which the reintroduction of saddleback has been processed since 1992. Based on mark-recapture analysis, 40% of birds died as a result of poison drop. However, the population had recovered by following breeding seasons (Davidson & Armstrong, 2002). Along with the predator control by poison drop, nest boxes were placed around islands, especially, in protected sites such as bases of bushes and crevices under tree roots. Placing nest boxes around islands aided to monitor individuals as well as supply habitats for population expansion (Armstrong et al., 1990). Since reintroduction of saddleback to offshore islands, South Island saddlebacks have gone from 36 birds on one island to over 1,200 birds spread among 15 islands, with the present capacity to increase to a maximum of 2,500 birds and North Island saddlebacks have gone from a remnant population of 500 birds on one island to over 6,000 birds on 12 islands with the capacity to increase to over 19,000 individuals (Hooson & Jamieson, 2003)

Bird reintroductions in New Zealand have had a high success rate compared to the rest of the world (Armstrong & McLean, 1995). Replanting vegetation and eradication of predators have produced the favourable habitat for saddlebacks in offshore islands. The favourable habitat led to high pairing success in the closed island system. Besides, the sedentary nature of endemic birds was expected to produce high initial growth (Taylor et al., 2005). On the other hand, there was a high risk of extinction due to allele effects, inbreeding and reduced genetic variation as populations established with a small number of founders (Taylor et al., 2005). Thus, it was very critical to monitor the persistence in reintroductions of saddlebacks to New Zealand offshore islands.

Since reintroduction to offshore islands, intensive monitoring has been conducted for 5 years. In case of Mokoia Island, data were collected to develop models of population dynamics under different management scenarios. Stochastic models were used to predict the probabilities of successful reintroduction by measuring reproduction, survival, immigration and emigration. At the beginning and the end of the breeding season, reproduction was monitored in terms of number of fledgling produced per female per year. Surveys walked through all parts of island and recorded all individuals seen in nest boxes. In addition, they recorded individual status such as weight, height, and behaviours and collected blood sample to assess their condition with respect to food supply, abundance of fruits and flowers and soil, and air levels of pathogenic fungus. Population modelling determined the number of adults and juveniles that survived to the next breeding season (Armstrong et al., 2002). Beyond the simply monitoring, possible experiments were also conducted, which manipulated the levels of familiarity in founder groups for reintroductions of saddlebacks (Armstrong & McLean, 1995).

Monitoring aimed mainly to test for density-dependence in parameters as the population grew and this provided strategies for harvesting saddleback populations for further translocation. The second objective was to provide a baseline model for assessing the amount of predation which saddlebacks could tolerate. Pertaining to the amount of predation, the effect of potential rat invasion on the Mokoia population was estimated. There was strong evidence for density-dependence in both survival and reproduction. Since thirty-six birds were released in 1992, the population grew rapidly, reaching approximately 217 birds in 1996. The simulation model predicted that the population would grow to approximately 250. As guidance for reintroductions to the mainland, the predator control should be accompanied using permanent bait stations (Armstrong & McLean, 1995; Davidson & Armstrong, 2002).

Although small island populations of New Zealand native birds, including Saddlebacks, have persisted with a relatively high rate of success, repeated population bottlenecks, in the form of sequential translocations can lead to any loss of fitness or genetic variation due to inbreeding in populations (Taylor & Jamieson, 2008). According to the analysed success/failure of reintroductions versus released number, population experienced a high level of egg failure. Since conservative definition of a failed population was considered as the initial release population declined by 50% or more, the simulation models suggested that the impact of egg failure would be still negligible for extinction risk. However, the low level of egg fertility and hatchability was considered as the negative effects of inbreeding depression. Hence, newly introduced populations have greater risk to invading pathogens or environmental perturbations over the long term (Taylor et al., 2005).

In 2008, there was a study related to any loss of genetic variation following sequential translocation in extant populations of saddlebacks with historically low level of genetic variation. The population of saddlebacks have been restored in resulting of frequent translocations to offshore islands (Lambert et al., 2005). The Study showed that genetically depauperate and threatened species including Saddlebacks may be less sensitive than more genetically diverse species. Indeed, there was a further loss of genetic variation during further translocation or bottleneck effects (Taylor & Jamieson, 2008; Taylor et al., 2008).

Conservation managers and many researchers not only establish monitoring programme to obtain baseline data that facilitates early detection of increasing stress levels and declining health, but also put more emphasis on preventing pathogen introduction and protocols for dealing with outbreaks of pathogen (Derraik et al., 2008). In 2002, there was a dramatic drop in population of saddlebacks on Motuara Island, New Zealand. The population dropped from 130 birds to 60 birds by avian malaria. Despite the population had restored over 130 birds next three years, avian malaria is concerned as one of the biggest current issues on the island conservation in New Zealand. Small size of island populations of endangered birds, together with their high density, low genetic diversity and naivety to introduced pathogens, may render them vulnerable to disease epidemics (Hale & Briskie, 2009). Even at this time in New Zealand, investigation of epidemiology is being studies in the most recently confirmed cases. Also, analysing previous case studies provide guidance for reintroductions to the mainland. The constant efforts on island studies will result having potentially important implications for not only the future native bird management but also evolution, biogeography and community ecology.
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