Justification of Red List Category
This species qualifies as Critically Endangered because it is present on only one island and has an extremely small range which is declining owing to habitat loss and degradation. Its small population declined very rapidly as a result of this habitat loss in combination with other factors, but since the 1990s the population has increased and returned to 1980s levels by 2012. A continuation of this trend and any confirmation of a reduction/halt in habitat loss may mean the species warrants downlisting in the near future. However, it may decline even more rapidly than previous rates if Brown Tree Snake Boiga irregularis becomes established on the island.
In 1982, the species was estimated to have a population of 8,741–18,487 individuals (Engbring et al. 1986). Since then, surveys have estimated 300-1,500 birds in 1990 (Craig and Taisacan 1994), 1,165 birds in 1996, with 94% of the population restricted to four patches of forest covering only 259 ha (Fancy and Snetsinger 2001), and 1,092 birds in 1999 (Amidon 2000, Williams 2004). Thus the population rapidly declined between 1982 and 1999, but the population has stabilised or increased since the 1990s (F. Amidon in litt. 2007, 2010, 2011, Zarones et al. 2013) and returned to 1980s levels to an estimated population of 5,620-20,961 individuals in 2012 (Camp et al. 2015). This is roughly equivalent to 3,750-14,000 mature individuals.
A decline of 50-79% over three generations (10.5 years) was estimated based on a decline from 10,763 in 1982 to 1,092 in 1999 (Williams 2004). The population has stabilised or increased since the 1990s (F. Amidon in litt. 2007, 2010, 2011, Zarones et al. 2013) and returned to 1980s levels by 2012 (Camp et al. 2015), so the current trend is assessed as increasing over the past 3 generations.
Zosterops rotensis is endemic to Rota in the Northern Mariana Islands (to USA), where it was once thought to be common and widespread, but has been restricted to the Sabana plateau since the 1980s (Camp et. al. 2015). Between 2005 and 2008, it was located in 298 ha spread over six primary areas on the Sabana tabletop and ridges between 100 and 490 m elevation (L. Zarones in litt. 2012, Zarones et al. 2013). The recovery plan presents a long-term goal of restoring the population to a stable 10,000 individuals (U.S. Fish and Wildlife Service 2007).
Its ecological requirements are poorly known. It may favour native, mature, wet limestone forest (Fancy and Snetsinger 2001), appearing to prefer Hernandia labyrinthica mixed forest and Merrilliodendron megacarpum forest, although this apparent relationship could simply be a factor of its decline in recent decades (Amidon 2000). Within the Sabana region of Rota, it was observed to favour tall limestone forest, and avoid grassland, low limestone forest, and secondary pandanus vegetation types (Zarones et al. 2013). The species favours forest with more mid-sized stems, more foliage density, more epiphytes, greater total canopy cover, and fewer overl plant species (Zarones et al. 2013). It feeds on insects, seeds, and fruit, and perhaps nectar (Amidon 2000). Adults have been observed feeding young and incubating partners on the nest with moths and caterpillars, as well as possibly snails, spiders, beetles, mayflies and katydids (S. Faegre in litt. 2009). Its absence from some areas of apparently suitable native forest (J. Lepson in litt. 1999) is unclear, but may relate to prey availability (Amidon 2000) and/or forest structure (Zarones et al. 2013). Nesting has been recorded from December to August (although nest building has been recorded in September), and clutch-size is generally one to two eggs (Amidon et al. 2004, Berry and Taisacan 2008); although a nest was recently observed containing three eggs (F. Amidon in litt. 2007, 2010, 2011). The average number of fledglings from successful nests is 1.5 (n = 4) (Berry and Taisacan 2008).
The recent rapid declines and the current localised distribution are most likely to be primarily a result of habitat loss and degradation owing to agricultural activities, development, typhoons (Fancy and Snetsinger 2001) and use of pesticides (USFWS 2001). The recent introduction of the Brown Tree Snake Boiga irregularis on Saipan is a serious concern, and if the snake were to become established on Rota, serious future declines would be likely (A. Saunders in litt. 2003). Predation by introduced rats Rattus spp. and Black Drongo Dicrurus macrocercus has been implicated in its decline (Craig and Taisacan 1994, Craig 1999). Among eight nests observed during a nest success/predation study, one was predated by Mariana Crow Corvus kubaryi during the nestling stage, and a nest with eggs was predated by a rat Rattus spp., although in the latter case the nest may have already been abandoned (Berry and Taisacan 2008). The native Micronesian Starling Aplonis opaca is also a nest predator, and a 2009 study of 18 nests found that 33% of the nests were depredated (F. Amidon in litt. 2007, 2010, 2011).
Conservation Actions Underway
The species was listed as endangered under the US Endangered Species Act in 2004 and in October 2006 c.1,600 ha of critical habitat was designated for the species (U.S. Fish and Wildlife Service 2004, 2006). The US Fish and Wildlife Service published a recovery plan in 2007 (U.S. Fish and Wildlife Service 2007). Funding was acquired in 2003 to start a nest predation study and experimental trapping of predators observed raiding nests, but additional work is still needed (T. de Cruz in litt. 2003). In addition, funding in 2005 was provided to begin a foraging ecology study on the species to help determine reasons for its limited distribution and further work on nesting ecology was conducted in 2009 (F. Amidon in litt. 2007, 2010, 2011). Funding for a snake barrier at the Rota port has also been provided, but planning and construction are still in their early phases (F. Amidon in litt. 2007, 2010, 2011).
10 cm. Small, warbler-like bird that moves about in small groups. Yellowish-olive above, saffron-yellow lores and underparts. Bold white eye-ring. Voice Calls and song not well differentiated, include harsh, rolling, low-pitched tsheip, buzzing zee-zee-zeeee, and two-note see-tseep. Hints Quieter and less conspicuous than most white-eyes.
Text account compilers
Butchart, S., Mahood, S., O'Brien, A., Bird, J., Shutes, S., Dutson, G., Stattersfield, A., Westrip, J., Calvert, R., Derhé, M., Symes, A., Harding, M.
Amidon, F.A., Lepson, J., Saunders, A., Wiles, G., de Cruz, T., Zarones, L. & Faegre, S.
BirdLife International (2022) Species factsheet: Zosterops rotensis. Downloaded from http://www.birdlife.org on 26/09/2022. Recommended citation for factsheets for more than one species: BirdLife International (2022) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 26/09/2022.