Justification of Red List Category
This species has been downlisted to Near Threatened because although it has a very restricted range, it is abundant and the population is estimated to be stable despite the presence of several invasive species. It is not classified as Vulnerable as there is not thought to be any plausible threat which is likely to cause rapid future declines.
The species is abundant and the population has been estimated at 20,000 mature individuals (Garnett and Crowley 2000, Garnett et al. 2011), and as high as high as 80,000-170,000 (Beeton et al. 2010); the conservative figure of 20,000 is maintained pending more accurate estimates.
The population is suspected to be stable (S. Garnett in litt. 2005, Garnett et al. 2011).
Zosterops natalis was confined to Christmas Island (Australia), in the Indian Ocean, until some time between 1885 and 1900, when it was introduced to Cocos Keeling Islands (to Australia). On Christmas Island, it is the most abundant bird species, numbering c.20,000 individuals, and is believed to be stable (S. Garnett in litt. 2005, Garnett et al. 2011). On Cocos-Keeling Islands, it apparently persists only around the settlement. On Christmas Island it forages over virtually all of the 137 km2 (D. James in litt. 2007). Surveys in 2005 and 2006 confirmed the species's high abundance, with the species recorded during 99% of counts and at 100% of survey sites (James and Retallick 2007).
It is found in all forested habitats on Christmas Island up to 360 m (D. James in litt. 2007). It also occurs in suburban gardens and weeds fields in abandoned mine sites (D. James in litt. 2007). There is no other species competing for food in its niche between the canopy and the lower-bole zone.
Approximately a third of forested habitat has been destroyed on Christmas Island as a result of mining operations. The species appears to tolerate a degree of habitat modification. In 2007, significant patches of mature secondary forest were cleared for mining (D. James in litt. 2007). Also in 2007, a new application to mine a 250 ha area of rainforest (P. Green in litt. 2007) was turned down (J. Hennicke in litt. 2007), but has since gone to appeal (D. James in litt. 2007). The introduced yellow crazy ant Anoplolepis gracilipes, which formed super-colonies during the 1990s and spread rapidly to cover about 25% of the island or about 3,400 ha, but was controlled over about 2,900 ha in September 2002, was thought to be a potential threat. In 2006, the ants were regarded as widespread and patchily common (T. Low in litt. 2006). It was thought that ants might prey directly on nestlings, and that they would alter island ecology by killing the dominant life-form, the red crab Gecaroidea natalis, which otherwise inhibits understorey plant growth and the spread of weeds by eating the seeds and seedlings of both native and invasive species (P. Green and D. O'Dowd in litt. 2003, S. Garnett in litt. 2003, D. James in litt. 2007). Fears that the white-eye would be affected by the ants appear to have been proved wrong however: counts and foraging success of the white-eye were higher in places where ants were present, because of an increase in scale insects (Garnett et al. 2011). Black Rats Rattus rattus and feral cats Felis catus are present but are not currently thought to be causing declines (Garnett et al. 2011).
Conservation Actions Underway
The Christmas Island National Park was established in 1980, and has since been extended to cover more than 60% of the island (D. James in litt. 2007). The introduced population on the Cocos (Keeling) Island might serve as an insurance population. A control programme for A. gracilipes has been successfully initiated since 2000 and eliminated the ant from 2,800 ha of forest (95% of its former extent) (P. Green and D. O'Dowd in litt. 2003). Monitoring of the problem continues and hand-baiting measures have been ongoing. Aerial baiting commenced in September 2002 and proved to be successful against major colonies (Olsen 2005), eliminating over 98% of ants (D. James in litt. 2007), however the ants have since recovered (D. James in litt. 2007) and perpetual baiting may be the only means of controlling them (T. Low in litt. 2006). The bait used so far is known to be toxic to invertebrates, including crabs, and although alternatives have been trialled, an effective replacement has not been found (D. James in litt. 2007). Control of the scale bugs that the ants tend for their sugar secretions has been suggested, in order to reduce this food supply (T. Low in litt. 2006, D. James in litt. 2007). Control of the ants may have allowed the species's population to stabilise (S. Garnett in litt. 2005).
11-13 cm. Small, warbler-like bird lacking bright yellow colouring. Sexes alike. Dull green above, greyish-white below. White eye-ring. Dull brownish primaries, secondaries and tail, edged green. Black lores, continuing to halfway under eye-ring. Bright chestnut iris in adults, grey in juveniles. Pale yellow undertail-coverts. Black bill, pale grey base to lower mandible. Similar spp. Vagrant Arctic Warblers Phylloscopus borealis are possible. Voice Variable, includes chirping, twittering. Hints Smallest bird in range. Flocks forage in all levels of vegetation.
Text account compilers
Benstead, P., Garnett, S., McClellan, R., Pilgrim, J., Symes, A. & Taylor, J.
Blyth, J., Garnett, S., Green, P., Hennicke, J., James, D., Low, T. & O'Dowd, D.
BirdLife International (2019) Species factsheet: Zosterops natalis. Downloaded from http://www.birdlife.org on 21/07/2019. Recommended citation for factsheets for more than one species: BirdLife International (2019) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 21/07/2019.