Data Zone
Justification of Red List category
This species has a very restricted range where there is thought to be an ongoing decline in habitat quality and population size. Additionally, given the threat of the introduction of Brown tree snake Boiga irregularis, it is estimated to occur at only two locations. It is therefore assessed as Endangered.
Population justification
In 2007 the population on Saipan was estimated at 2,742 birds (95% CI = 1,686 - 3,956) (Camp et al. 2009). The population on Alamagan was estimated to number 946 individuals (95% CI = 428-1,762) in 2010, comprising c.25% of the global population (Marshall et al. 2011, 2021). Combining these generates a global population estimate of 3,688 (95% CI = 1,019-6,356) birds (Marshall et al. 2021). Given that only a proportion likely refer to mature individuals and that the population has probably declined further following declines recorded between 1982 and 2007, the population is tentatively placed here in the band 600-5,700 mature individuals, with a best estimate of 2,200-3,300.
Trend justification
Abundance on Saipan declined from 6,658 birds (5,331-8,054) in 1982 down to 4,639 (3,669-5,689) birds in 1997, and has continued to decline to 2,742 birds (1,686-3,956) in 2007 (Camp et al. 2009). This suggests an accelerating decline equating to 40% between 1997 and 2007. Declines are thought to be driven by habitat loss in line with extensive economic development of the island, degradation due to the spread of non-native plants as well as predation from introduced rats and cats (Camp et al. 2009). The human population on Saipan is no longer rapidly expanding however and the conversion of forest to anthropogenic-dominated habitats is likely to have slowed. Population estimates for the Alamagan population show little variation but declined slightly from 1,125 (504-1,539) birds in 2000 to 946 (428-1,762) in 2010 and may decline significantly in the future as native forest continues to be overgrazed by feral ungulates (Marshall et al. 2021). However, it may be the case that the species has co-existed with ungulates on the island for several centuries (Commonwealth of the Northern Mariana Islands [CNMI] in litt. 2024). There are no more recent data on population trends, but habitat loss and degradation, as well as predation by introduced mammals, continue to be considered primary threats to the species (USFWS 2020). Declines are therefore inferred to be ongoing, though the rate has not been quantified.
This species is restricted to Saipan and Alamagan in the Northern Mariana Islands (to USA).
On Saipan, it occurs in thicket-meadow mosaics, forest edge, reed-marshes and forest openings (Craig 1996). A study on Saipan found nests in upland introduced Leucaena leucocephala forest, a native mangrove wetland and a native reed wetland (Mosher and Fancy 2002). On Alamagan, it inhabits open forest with brushy understorey and wooded ravine forest adjacent to open grasslands (Engbring et al. 1982, Reichel et al. 1992, USFWS 1998).
Habitat loss and degradation, as well as predation by introduced mammals, are considered the primary threats to the species (USFWS 2020). On Saipan, available habitat has been reduced for agriculture, home-building and tourist-related facilities as a result of extensive economic development, although in recent years this may not have had a substantial impact. During the early 20th Century the native forest habitat was devastated by agriculture such as sugar cane and the invasion of Saipan by the US military (J. E. Gourley in litt. 2012). Ivy Gourd (Coccinia grandis) became established in the mid-1990s and has effectively invaded >90% of forest stands on Saipan (S. Mosher in litt. 2012). This species covers forest canopies, effectively smothering the canopy to the point of killing trees to causing the collapse of the canopy, therefore reducing the canopy height that is important for nesting reed-warblers (S. Mosher in litt. 2012). The habitat on Alamagan is heavily degraded from grazing by feral ungulates (F. Amidon in litt. 2012, Marshall et al. 2021), though it may be the case that the species has co-existed with ungulates for several centuries (CNMI in litt. 2024). Introduced predators, including feral cats (Felis catus) and especially rats (Rattus spp.), and possibly monitor lizard (Varanus indicus), may be a large factor in the reported high proportion of nest failures (USFWS 1998, Mosher 2006). A significant future threat is the proliferation of Brown Tree Snakes (Boiga irregularis), which caused the rapid extinction of this taxon's sister species, A. luscinius, on the island of Guam. Though there have been numerous sightings of the species, the robust biosecurity measures in place are believed to have thus far prevented the snakes from becoming established on Saipan (Yackel Adams et al. 2021).
Conservation and Research Actions Underway
A recovery plan exists for the pre-split taxon (USFWS 1998) but there has been little active management for the species to date and the milestones in the plan are now out of date. However, provisions to protect habitat and mitigate loss have generally been included in major land development projects. The Saipan Upland Mitigation Bank was re-established in 2009 to maintain a baseline population and mitigate impacts of development projects (USFWS 2020). Limited habitat restoration efforts are in place (USFWS 2020). Brown tree snake interdiction efforts are ongoing on Saipan (USFWS 2020). Trap lines for snakes are maintained at ports, night searches are conducted and a sniffer dog programme has been established. Publicity campaigns have been conducted to raise the general awareness of island residents, including port workers, about the dangers of snake colonisation (USFWS 1998). The robust biosecurity measures in place are believed to have thus far prevented the snakes from becoming established on Saipan (Yackel Adams et al. 2021), however such measures are reliant on local government funds which are relatively unstable (CNMI in litt. 2024).
18 cm. Large, lanky, scruffy-looking warbler with long bill and often dishevelled feathers and erect head feathers when singing. Dingy olive-yellow above, with dull yellow eyebrow and underparts. Voice Call a loud distinctive chuck or tchack. Males sing long, loud, varied and complex song. Hints Skulks in dense thickets, more often heard than seen. Sometimes sings at night. Male most often sings from exposed perches.
Text account compilers
Vine, J.
Contributors
Camp, R., Dutson, G., Mosher, S., Amidon, F.A., Gourley, J., Tenorio, M. & CNMI, C.
Recommended citation
BirdLife International (2024) Species factsheet: Saipan Reed-warbler Acrocephalus hiwae. Downloaded from
https://datazone.birdlife.org/species/factsheet/saipan-reed-warbler-acrocephalus-hiwae on 23/12/2024.
Recommended citation for factsheets for more than one species: BirdLife International (2024) IUCN Red List for birds. Downloaded from
https://datazone.birdlife.org/species/search on 23/12/2024.