Saipan White-eye Zosterops saypani


Justification of Red List Category
This species is restricted to three very small islands and is currently abundant. However, it is projected to undergo a very rapid population decline owing to the establishment of brown tree-snake Boiga irregularis on Saipan. It therefore qualifies as Endangered.

Population justification
The population was estimated to be 534,029 (95% CI: 427,858-650,667) individuals on Saipan in 2007 (Camp et al. 2009), 225,360 (95% CI: 192,080-283,200) individuals on Tinian and 3,388 (95% CI – 2,492-4,909) individuals on Aguijan in 2008 (Camp et al. in press, Amidon et al. in prep), giving an estimated global population of 620,000-940,000 individuals.

Trend justification
Preliminary analysis of survey data from 1982, 1997, 2007, 2008, 2009 and 2010 on Saipan indicates that percent occurrence at survey stations has decreased slightly from 1982 and 1997 to 2007 and fluctuated from 2008 to 2010, but Bridled White-eye detections per station have almost doubled during this time period (USFWS unpublished data, Saracco et al. 2008, Pyle et al. 2009, Pyle et al. 2010). However, this data has not been analyzed to determine if this trend is significant. In addition, Boiga irregularis may be in the process of becoming established on Saipan, an island which constitutes over 50% of the Bridled White-eye's total archipelago wide population. If bird populations on Saipan decline as rapidly in response to B. irregularis as those reported on Guam, then a very rapid decline in Bridled White-eyes on Saipan is expected in the future.

Distribution and population

Zosterops conspicillatus is restricted to the Northern Mariana Islands (to U.S.A.) where it occurs on the islands of Saipan, Tinian and Aguijan. It is very abundant (it was the second most abundant breeding landbird caught in mist-nets during the TMAPS survey on Saipan in 2010 [Pyle et al. 2010]) and populations currently appear stable or increasing. On Saipan, the population was estimated to be 534,029 (95% CI: 427,858-650,667) birds in 2007, which was similar to estimates from 1982 and 1997 (Camp et al. 2009). Surveys of Tinian and Aguijuan were conducted in 2008. The population on Tinian (225,360 [95% CI: 192,080-283,200]) appears stable while the population on Aguijan (3,388 [95% CI – 2,492-4,909]) increased from estimates in 1982 (Camp et al., in press, Amidon et al., in prep). The population on Saipan is estimated to be more abundant in native limestone forest (c. 5,950 birds/km2) than in disturbed habitats (c. 4,000 birds/km2) (Craig 1996).


The species is found in a wide range of habitats from native limestone forest to scrubby secondary growth of disturbed habitats and even urban areas. It is less common in swordgrass savannah (Craig 1996). It commonly forages in large flocks in the upper canopy of native limestone forest, but also feeds in other habitats (Engbring et al. 1982). Its food consists of insects, seeds, fruits, caterpillars and berries and it is not strongly nectarivorous (Engbring et al. 1982). It seemingly competes for food with the larger, dominant Golden White-eye Cleptornis marchei, probably because of the extremely high densities (Craig 1996). Breeding occurs in January, February and August, October; though on Saipan there is a distinct peak in February/March (Craig 1996, Sachtleben 2005). It nests predominately in Leucaena leucocephala thickets (Sachtleben 2005). It is not territorial but birds remain in the home range (Craig 1996).


The biggest threat on Saipan comes from the brown tree snake Boiga irregularis which may be in the process of becoming established there (Rodd and Savidge 2007). The proposed development of Tinian for military training (US Dept. of Defence 2007) and tourism requires the importation of large amounts of building materials and increases the likelihood of accidental introduction of brown tree snake (J. Lepson in litt. 1999).

Conservation actions

Conservation Actions Underway
A feasibility study completed in 2006 revealed that Sarigan island (5km2) is a suitable site for a benign introduction (Martin and Kremer 2006), and so a conservation introduction plan was development and birds were transferred in Spring 2008 and 2009 as part of the Marianas Avian Conservation (MAC) Project (Division of Fish and Wildlife in litt. 2008). Breeding of the introduced Sarigan population has been observed and the population appears to have increased, but the population is not yet thought to be self-sustaining (F. Amidon in litt. 2012). A brown tree-snake barrier has been constructed at the port on Tinian to support interdiction efforts on the island (N. Hawley in litt. 2007). Breeding surveys are planned for Spring 2012.

Conservation Actions Proposed
Implement stringent measures such as traps and monitors to prevent the spread of B. irregularis onto Tinian, particularly around the airport and the harbour (J. Lepson in litt. 1999). Avoid introduction of B. irregularis onto Aguijan. Control the spread of B. irregularis on Saipan. Continuously monitor the population on Saipan. Monitor the introduction of the species to Sarigan.


12 cm. Small green and yellow warbler-like forest bird. Bright, olive-green upperparts, olivaceous ear-coverts, a white orbital ring and pale yellowish-white underparts. Voice Call a high-pitched tszeeip, often rapidly uttered and organized into a loose song.


Text account compilers
Benstead, P., Derhé, M., Mahood, S., O'Brien, A., Khwaja, N., Martin, R & Westrip, J.

Amidon, F., Hawley, N., Lepson, J., Saunders, A. & Radley, P.

Recommended citation
BirdLife International (2018) Species factsheet: Zosterops saypani. Downloaded from http://www.birdlife.org on 19/12/2018. Recommended citation for factsheets for more than one species: BirdLife International (2018) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 19/12/2018.