Justification of Red List category
The species has a small population that appears to be undergoing a continuing decline as a result of rat predation, and all individuals belong to one subpopulation. For these reasons it is assessed as Vulnerable.
There were an estimated 4,030 (95% CI 2,550–6,360) Z. tenuirostris in Norfolk Island National Park in 2009 (Dutson 2013). A more recent survey in 2016 (Director of National Parks unpublished, in Nance et al. 2021) detected little change. There are now very few birds left outside the national park (Nance et al. 2021), hence the best estimate of the population is made at 4,100 mature individuals.
Within the national park, data collected between 2009 (4,030 mature individuals; 95% CI 2,550-6,360) and 2016 ostensibly indicate no change in population size, though differences in survey techniques preclude direct comparisons of these years. There has however been a continued decline of birds outside of the national park: from 300-500 in the 1980s, including some areas where the bird was quite common (Robinson 1988) to very few now (Nance et al. 2021). Thus, while the population within the national park is assumed to be stable, an overall population decline is inferred based on reports that outside the national park, numbers continue to decline, principally as a result of black rats (Rattus rattus), which are yet to have been eradicated. These declines are therefore likely to continue into the future until either the threat has been eliminated, or the species becomes confined to the national park.
Z. tenuirostris is endemic to Norfolk Island, Australia, where its distribution has contracted to the Norfolk Island National Park, Rocky Point Reserve and adjacent private lands where suitable habitat persists (Robinson 1997). The range of breeding birds may have contracted further. There was thought to be a permanent population outside the park in 1987 when birds were seen 11/14 sites visited outside the park and were especially common in Rocky Point Reserve (Robinson 1988). Birds seen outside the boundaries of the park (BirdLife Australia 2020, eBird 2021) are now thought likely to be mostly dispersing individuals (R.H. Clarke unpublished, in Nance et al. 2021).
Parties of Z. tenuirostris occupy native hardwood forest and tall secondary growth, spending much time probing fissures in bark for insects. They also take fruit, including exotic species including guava Psidium guava (Mees 1969, Robinson 1988), an invasive shrub, the seeds of which the white-eye is likely to spread. The ecological niche differs from that of the self-introduced Silvereye Z. lateralis, which largely forages in foliage (Robinson 1988). They lay 2–6 eggs in a cup nest in the outer foliage of trees (Robinson 1988).
Since habitat clearance ceased, the main threat is from black rats Rattus rattus, which were introduced in the mid-1940s (Robinson 1988). The white-eyes may have been more vulnerable to rat predation in disturbed environments, explaining the gradual loss from agricultural settings (Robinson 1997). White-eye nests have proved difficult to find, but nearly 30% of nests of all small passerines fail because of depredation by black rats, Pacific rats R. exulans or house mice Mus musculus (Nance et al. 2021). Cats Felis catus, which were introduced before the 1830s, may also take some birds (Bell 1990, Director of National Parks unpublished in Nance et al. 2021).
Conservation Actions Underway
Most remaining habitat protected within reserves. Cat trapping and rat baiting occurs regularly. There are some localised guava controls. Restoration of native vegetation is carried out in areas from which weeds have been removed. There is preliminary work to restore habitat outside reserves. The idea of installing a predator-proof fence around the national park, Hundred Acre Reserve and other important areas of habitat was once proposed but hasn't gained traction, likely due to the costs and maintenance this would involve (M. Christian pers. comm. 2016).
Conservation Actions Proposed
Continuing monitoring the population size. Develop new techniques and strategies for eradicating or controlling rats and controlling cats on Norfolk Island, including methods for targeting rats in trees.
Rats and cats should be eradicated or controlled to reduce their impact on native species, including Z. tenuirostris. Continue management of guava, with the aim of reducing the size of the area dominated. Increase restoration of habitat outside reserves. Consider introducing the species to Phillip Island.
13-14 cm. Medium-sized, warbler-like bird with long, slightly decurved bill. Sexes similar. Greyish-brown upperparts, including head and flanks, with olive cast. White eye-ring. Black lores. Suffused olive-yellow upperwing-coverts. Yellow-tinged undertail-coverts. Grey bill, paler lower mandible. Similar spp. Silvereye Z. lateralis is grey on back and chest, has shorter, straighter bill, is less yellow overall. White-chested White-eye Z. albogularis is larger with white underparts. Voice High-pitched. Wheezier and more sibilant than Z. lateralis. Hints Tends to forage on branches and bark more than Z. lateralis.
Text account compilers
Berryman, A., Vine, J., Garnett, S.
Christian, M., Garnett, S., Holdaway, R. & Ward, R.
BirdLife International (2023) Species factsheet: Zosterops tenuirostris. Downloaded from http://datazone.birdlife.org/species/factsheet/slender-billed-white-eye-zosterops-tenuirostris on 02/12/2023.
Recommended citation for factsheets for more than one species: BirdLife International (2023) IUCN Red List for birds. Downloaded from http://datazone.birdlife.org on 02/12/2023.