Oahu Elepaio Chasiempis ibidis


Justification of Red List Category
This species qualifies as Endangered because it has a very small and severely fragmented population, occupying a very small and declining range. Its population is suspected to be in decline owing to the effects of introduced species and declines in the extent and quality of habitat.

Population justification
The total population was estimated at 1,261 mature individuals (95% CI=1, 205 - 1, 317) following surveys conducted from 2011-2012 (VanderWerf et al. 20013), consisting of about 477 breeding pairs and 307 single males. This total includes 592 males (95% CI= 554-630) and 369 females in the Ko'olau Mountains and 192 males and 84 females in the Wai' anae Mountains (VanderWerf et al. 2011, 2013).

Trend justification
Surveys conducted in the 1990s produced evidence that the species had declined by more than 75% since 1975 and by approximately 96% since the arrival of humans (VanderWerf et al. 2001). The species's population is still in decline owing to the effects of disease, introduced species and ongoing habitat loss and degradation. Bayesian modelling demonstrate rapid population collapse for this species (Aagaard et al. 2016). Surveys in 2006-2010 showed that the number of birds in the Waianae Mountains, which had supported half the population, had declined to only 300 birds (VanderWerf et al. 2011a).

Distribution and population

Chasiempis ibidis is endemic to O`ahu in the Hawaiian Islands (USA) (VanderWerf et al. 2009). It had an estimated population of 1,261 birds (95% CI=1, 205 - 1, 317), restricted to c.52 km2 in the Ko`olau and Wai`anae mountains, consisting of about 477 breeding pairs and 307 single males (Vanderwerf et al. 2013). This is only 25% of the range occupied in c.1975, and less than 4% of the presumed prehistoric range (VanderWerf et al. 2001). The O'ahu 'Elepaio has declined in abundance by about 50% since the 1990s, when the population was estimated to be about 1, 974 birds (VanderWerf et al. 2001). The genetically effective population size may be even lower because of the fragmented distribution (VanderWerf et al. 2001, 2013). The species has continued to decline over much of its range, particularly in the Wai`anae mountains, where only 300 birds were found in 2006-2010 (VanderWerf et al. 2011a). Surveys are underway in the Koolau Mountains to provide a complete update of current status (E. VanderWerf in litt. 2012).


The species is most abundant in mesic forest in valleys (VanderWerf et al. 2001), preferring mixed-species forest with a tall canopy and well-developed understorey, at 200-800 m (VanderWerf et al. 2001). The species is less common in drier forest (Vanderwerf 1998). It feeds on insects and other invertebrates (VanderWerf 1998).


Habitat loss to development has been extensive, with 56% of its former range zoned for agricultural or urban development (VanderWerf et al. 2001). Diseases such as avian pox and malaria, spread by mosquitoes, are a problem throughout its range, increasing mortality of adults by c.10-25%, and possibly preventing birds from nesting (E. VanderWerf in litt. 1999, VanderWerf et al. 2006). Malaria prevalence in the species has been recorded at 87%, with 36% of birds showing signs of avian pox (VanderWerf et al. 2006). High prevalence of mosquito-borne diseases and local declines in the species's population are associated with high rainfall (USFWS 2006, VanderWerf et al. 2006). Nest-predation by black rats Rattus rattus is the most serious current problem (VanderWerf et al. 1997, VanderWerf 1998, VanderWerf et al. 2001, VanderWerf 2009), leading to a male-biased sex ratio (VanderWerf et al. 2001). Fires are known to destroy key habitat and promote the spread of alien plants on O`ahu (USFWS 2006) which promote high rat abundance, with negative consequences for nest success (VanderWerf 2009.)

Conservation actions

Conservation Actions Underway
Rat control in 1996-2008 resulted in a 25% increase in survival of breeding females and a 100% increase in reproductive success, stabilising studied populations (VanderWerf and Smith 2002, VanderWerf 2009). Further rat control is currently taking place at other populations, but efficacy has been variable (VanderWerf et al. 2011b). Critical habitat has been designated on O`ahu (USFWS 2001). Captive breeding has been suggested (E. VanderWerf in litt. 1999). Recent research has shown that nesting height of Oahu Elepaios is increasing through evolution by selection from rat predation, which has lead to an overall increase in nest success in at least one population (VanderWerf 2012).

Conservation Actions Proposed
Continue to monitor population trends. Closely monitor the extent and quality of habitat. Conduct research into disease resistance (VanderWerf 2009). Implement further measures to control introduced rats (VanderWerf 2009). Protect forest from fires and development (USFWS 2006). Investigate whether nesting height is increasing in other parts of the island.


14 cm. Small monarch flycatcher that often cocks its tail. Adults have white tips to tail feathers, white rump, and white wing bars. Immatures have buffy or rufous wingbars (VanderWerf 2001). Adult brown above, white below with brownish streaks on breast, and with black chin irregularly blending into white throat. Similar spp. Introduced Japanese Bush-warbler Cettia diphone duller with prominent eyebrow and no white markings. Introduced juvenile White-rumped Shama Copsychus malabaricus larger, darker above, with dark spots on breast. Voice Song a lively whistled eh-leh-PYE-o, often given in series (del Hoyo et al. 2006); calls include sharp chup, two-note squeak-it like dog's toy, and raspy chatter.


Text account compilers
Benstead, P., Derhé, M., Stattersfield, A., Stuart, T., Taylor, J. & North, A.

Camp, R., Fretz, S., Gorresen, M., Pratt, T., VanderWerf, E. & Woodworth, B.

Recommended citation
BirdLife International (2022) Species factsheet: Chasiempis ibidis. Downloaded from http://www.birdlife.org on 08/12/2022. Recommended citation for factsheets for more than one species: BirdLife International (2022) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 08/12/2022.