The Golden White-eye (Cleptornis marchei) is endemic to the Commonwealth of the Northern Mariana Islands (to USA), occurring on Saipan (115 km2) and on the uninhabited Aguiguan (7 km2). The species is now also found on Sarigan (4.5 km2) after translocations of individuals from Saipan took place in 2011-2012, with breeding taking place in the first year (Radley 2012, MAC Working Group 2014).
The most serious current threat is considered to be habitat loss (L. Berry in litt. 2016), which is thought to have been the main cause of a recorded population decline on Saipan (Camp et al. 2012). Since the 1980s, much of the forest and agricultural habitat on Saipan has been destroyed or degraded as a result of residential, commercial and touristic development (Camp et al. 2009).
The threat that could have the most devastating consequences is the potential introduction of Brown Tree Snake (Boiga irregularis) to the species’s range islands (Rodda and Savidge 2007). If the snake were to become established on Saipan, it would likely lead to extremely rapid declines, as has been the case amongst the endemic avifauna of Guam (to USA) (A. Saunders in litt. 2003, Wiles et al. 2003, Williams 2004). Roadside surveys on Guam indicated a 90% decline in most bird species within 8.9 years (Wiles et al. 2003). Accidental introduction via cargo ships and planes has been the primary mechanism for the spread of the species from Guam to other islands, and all goods received in the Northern Mariana Islands are shipped through Guam, with the majority arriving in Saipan (Colvin et al. 2005, MAC Working Group 2014).
There have been over 70 reports of Brown Tree Snake on Saipan, including sightings away from port areas (Rodda and Savidge 2007, MAC Working Group 2014), and the island was previously thought to have an incipient population (Colvin et al. 2005). Based on this information, Golden White-eye was predicted to undergo a population reduction of 80-100% over the next three generations, and the species has been listed as Critically Endangered under Criterion A3 (and later A4) since 2004.
However, there is no evidence that the snake has become established on Saipan, and we have no evidence to suggest that there have been such rapid declines. There have been no confirmed records of Brown Tree Snake on Saipan for 20 years (L. Berry in litt. 2020) and no reports of predation on Golden White-eye (A. Tieber in litt. 2008, 2010). Although a Golden White-eye population decline has been recorded up to 2010 on Saipan (Camp et al. 2009, Ha et al. 2018), it was not rapid, and did not follow the geographic pattern that would be expected if an expanding population of snakes was the cause (Camp et al. 2012).
Additionally, the species’s generation length has recently been re-estimated. It was previously estimated to be 4.4 years, meaning that population reductions under Criteria A3 and A4 were measured over 13 years, but a new analysis has estimated it to be only 2.84 years (Bird et al. 2020)*, meaning that population reductions under Criteria A3 and A4 are now measured over ten years.
We are therefore undertaking a review of the species’s Red List Category. Our current information on the species’s conservation status will now be compared to all Red List Criteria.
Criterion A – On Saipan, surveys by Engbring et al. (1986) in 1982 using point-transect methods estimated the population at 55,500 individuals. A further survey in 2007 survey provided an estimate of 71,997 individuals (95% CI 47,586 – 106,535; Camp et al. 2009). Despite the larger population estimate in 2007, the population density was found to have declined from 1,287.3 ± 191.0 individuals/km2 in 1982 to 995.5±160.0 individuals/km2 in 1997 and to 711.8±112.1 individuals/km2 in 2007 (Camp et al. 2009). The difference between the population estimates in 1982 and 2007 was attributed to a difference in the statistical methods used to estimate population size (Camp et al. 2009). Analysis of data from roadside surveys from 1991-2010 indicated that the population has increased from 1991-2000, then decreased between 2001 and 2010 (Ha et al. 2018).
On Aguiguan, surveys using point-transect methods in 1982 resulted in a population estimate of 2,300 individuals (Engbring et al. 1986). The transects were resurveyed in 1992 (Craig et al. 1993), 1995, 2000 (Cruz et al. 2000), 2002 (Esselstyn et al. 2003) and 2008 (Camp et al. 2012, Amidon et al. 2014). All surveys followed standard point-transect methods. The 2008 surveys estimated 15,499 individuals (95% CI 10,383-22,277; Amidon et al. 2014). Density estimates were as follows: 1,094±196 individuals/km2 in 1982; 1,901±382 individuals/km2 in 1995; 2,224±396 individuals/km2 in 2000; 1,693±275 individuals/km2 in 2002, and 2,433±466 individuals/km2 in 2008 (Amidon et al. 2014). The population density increased significantly between 1982 and 2008 (Camp et al. 2012; Amidon et al. 2014).
A third population was established on the uninhabited island of Sarigan through translocations of 24 and 50 individuals in 2011 and 2012, respectively, with breeding detected within the first year (Radley 2012, MAC Working Group 2014).
Based on the relative population densities recorded in Saipan in 1997 and 2007, the relative population densities recorded in Anguiguan in 1995 and 2008, the abundance estimates from 2007 (Saipan) and 2008 (Aguiguan), and assuming that the population on Sarigan has no more than 100 individuals and that rates of population change have remained constant, the total population is suspected to have undergone a reduction of 10-19% over the past ten years. Assuming the trend continues over the next decade, the population is also suspected to undergo a reduction of 10-19% over the next three generations. These magnitudes of population size reduction do not approach the thresholds for listing the species as threatened under Criterion A. The species is assessed as Least Concern under this Criterion.
Criterion B – The extent of occurrence (EOO) is inferred to be 3080 km2, based on a minimum convex polygon around the three range islands (including Sarigan). This meets the initial threshold for Endangered under Criterion B1.Based on the area of a grid of 2km x 2 km squares overlaying the mapped range, the maximum possible Area of Occupancy (AOO) is 268 km2. As it is unlikely that the entire mapped range is occupied, the AOO is here placed in the range 10-268 km2. This meets the threshold for Endangered under Criterion B2.To list the species as threatened on the Red List under Criterion B, two of conditions a-c must also be met.
The species is not severely fragmented, since >50% of its total area of occupancy is not in habitat patches that are smaller than would be required to support a viable population, and separated from other habitat patches by a large distance. According to the IUCN Red List Categories and Criteria, ‘the term ‘location’ defines a geographically or ecologically distinct area in which a single threatening event can rapidly affect all individuals of the taxon present’ (IUCN 2012). Since the establishment of Brown Tree Snake could rapidly devastate the subpopulation on each of the Golden White-eye’s three range islands, the species is judged to have three locations. Condition a is met at the level of Endangered.
Although the population size appeared to be increasing on Aguiguan (Camp et al. 2012; Amidon et al. 2014) and a new population has been established on Sarigan (Radley 2012, MAC Working Group 2014), the largest subpopulation on Saipan was found to be decreasing up to 2010 (Camp et al. 2009, Ha et al. 2018). Therefore overall the species is inferred to be undergoing a continuing decline in population size. Loss of forest has been observed on Saipan from the 1980s-2000s (Camp et al. 2009) and so a continuing decline in the extent and quality of habitat is also inferred. Condition b(iii,v) is met.There is no evidence that the species’s population or range size are undergoing extreme fluctuations. Condition c is not met.
The species’s EOO and AOO fall beneath the thresholds for listing the species as Endangered, condition a is met at the level of Endangered, and condition b is met. The species qualifies for listing as Endangered under Criterion B1ab(iii,v)+2ab(iii,v).
Criterion C – Based on the suspected population sizes described under Criterion A, the current total population on Saipan and Aguiguan is suspected to be within the range 43,806-96,857 (best estimate: 66,014) individuals, which roughly equates to 29,204-64,571 (44,009) mature individuals. There is now an additional small population on Sarigan. The total population size is therefore placed in the band 29,000 – 70,000 mature individuals. This does not approach the threshold for listing the species as threatened under Criterion C. The species is therefore assessed as Least Concern under this criterion.
Criterion D – Based on the estimates described above, the population size does not meet or approach the threshold for Vulnerable under Criterion D1. As described under Criterion B, the species is considered to have three locations. The largest subpopulation on Saipan could very rapidly be depleted if the Brown Tree Snake became established, driving the taxon to Critically Endangered within a very short time. The species therefore qualifies for listing as Vulnerable under Criterion D2.
Criterion E – To the best of our knowledge no quantitative assessment of the probability of extinction has been conducted for this species, and so it cannot be assessed against this criterion.
Based on the above assessment, it is proposed to list the Golden White-eye (Cleptornis marchei) as Endangered under Criterion B1ab(iii,v)+2ab(iii,v). Please note that species should only be downlisted to a lower category of threat on the IUCN Red List when they have not qualified for the higher category for at least five years.We welcome any comments to the proposed listing. Information is particularly requested on the species’s population size and trends over the past ten years; and projected trends and the likely impact of threats over the next ten years.
Please note that this topic is not designed to be a general discussion about the ecology of the species, rather a discussion of the species’s Red List status. Therefore, please make sure your comments are about the proposed listing. By submitting a comment, you confirm that you agree to the Comment Policy.
*Bird generation lengths are estimated using the methodology of Bird et al. (2020), as applied to parameter values updated for use in each IUCN Red List for birds reassessment cycle. Values used for the current assessment are available on request. We encourage people to contact us with additional or improved values for the following parameters; adult survival (true survival accounting for dispersal derived from an apparently stable population); mean age at first breeding; and maximum longevity (i.e. the biological maximum, hence values from captive individuals are acceptable).
An information booklet on the Red List Categories and Criteria can be downloaded here and the Red List Criteria Summary Sheet can be downloaded here. Detailed guidance on IUCN Red List terms and definitions and the application of the Red List Categories and Criteria can be downloaded here.
Amidon, F., Camp, R.J., Marshall, A., Pratt, T.K., Williams, L., Radley, P. and Cruz, J.B. 2014. Terrestrial bird population trends on Aguiguan (Goat Island), Mariana Islands. Bird Conservation International 24(4): 505-517.
Bird, J. P., Martin, R., Akçakaya, H. R., Gilroy, J., Burfield, I. J., Garnett, S. G., Symes, A., Taylor, J., Şekercioğlu, Ç. H. and Butchart, S. H. M. 2020. Generation lengths of the world’s birds and their implications for extinction risk. Conservation Biology online first view.
Camp, R. J., Pratt, T. K., Marshall, A. P., Amidon, F., Williams, L. L. 2009. Recent status and trends of the land bird avifauna on Saipan, Mariana Islands, with emphasis on the endangered Nightingale Reed-warbler (Acrocephalus luscinia). Bird Conservation International 19(4): 323-337.
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Craig, R. J., Chandran, R. and Ellis , A. 1993. Bird populations of Aguiguan: a ten year update. In: R. J. Craig (ed.), Mariana Research Symposium, pp. 2-25. Northern Marianas College.
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Esselstyn, J., J. B. Cruz, L.L. Williams, and N. Hawley. 2003. Wildlife and vegetation surveys: Aguiguan 2002.
Ha, J.; Cruz, J. B.; Kremer, S.; Camacho, V. A.; Radley, P. 2018. Trends in avian roadside surveys over a 20-year period on Saipan, Commonwealth of the Northern Mariana Islands. Pacific Science 72(1): 81-93.
IUCN. 2012. IUCN Red List Categories and Criteria: Version 3.1. Second edition. Gland, Switzerland and Cambridge, UK: IUCN. Available at www.iucnredlist.org/technical-documents/categories-and-criteria
MAC Working Group. 2014. Marianas Avifauna Conservation (MAC) plan: long-term conservation plan for the native forest birds of the Northern Mariana Islands. CNMI Division of Fish and Wildlife, Saipan, & U.S. DOI Fish and Wildlife Service, Honolulu, Hawaii. 152 pp.
Radley, P.M. 2012. Marianas Avifauna Conservation (MAC) project. A Second and First Translocation of Golden White-eyes (Cleptornis marchei) and Mariana Fruit Doves (Ptilinopus roseicapilla), Respectively, from Saipan to Sarigan, and an Assessment of Bridled White-eyes (Zosterops conspicillatus) on Sarigan, 1-8 May 2012. Division of Fish and Wildlife, Saipan, CNMI.
Rodda, G.H., and Savidge, J .A. 2007. Biology and impacts of Pacific Island invasive species. 2. Boiga irregularis, the brown tree snake (Reptilia: Colubridae). Pacific Science 61: 307-324.
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