Data Zone
Justification of Red List Category
This species qualifies as Critically Endangered because the very rapid rate of decline in its very small global population observed over the past three generations is expected to increase owing to habitat loss and degradation combined with the impact of introduced predators including the possible arrival of Brown Tree Snake (Boiga irregularis) on Saipan. This snake is a likely factor in the reed-warblers sister species' extirpation from Guam.Should the recorded rate of decline be lower than suggested then the species may become eligible for downlisting in the future, though the number of extinctions within this species complex and the potential existential threat posed by accidental introductions would suggest that extreme caution be employed prior to any lowering of the threat status.
Population justification
Two island populations exist: Alamagan (946 individuals [Marshall et al. 2011]) and Saipan (2,742 individuals [Camp et al. 2009a]). This equates to a global population of c.3,700 individuals. However, given the rapid decline occurring on Saipan and that a proportion of the population will be immature birds this is cautiously interpreted as 2,000-2,499 mature individuals.
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. 2009a). Over three generations this equates to an overall population decline of 47%, but this rate of decline has been increasing such that the rate recorded between 1997 and 2007 corresponds to a three generation decline of 60%. Given the projected increase in military personnel on Saipan and the possible introduction of Brown Tree Snake (Boiga irregularis) projected declines above 80% over three generations in the future are expected.
This species is restricted to Saipan and Alamagan in the Northern Mariana Islands (to USA). A population exists on Alamagan, that numbered 1,125 (95% CI = 504 - 1,539) individuals in 2000 and 946 (95% CI = 428-1,762) individuals in 2010 (Marshall et al. 2011). The majority of the population occurs on Saipan. Abundance (based on a density estimate extrapolated across the area of Saipan, 115.39 km2) 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 the 2007 survey (Camp et al. 2009a). This represents a 59% decline in the species since 1982. Over three generations this equates to an overall population decline of 47%, but this rate of decline has been increasing such that the rate recorded between 1997 and 2007 corresponds to a three generation decline of 60%. The rapid human expansion on Saipan in the 1990s has been slowing since c. 1998, but the U.S. military is expanding its presence and operations which could lead to further habitat conversion and degradation, if this expansion occurs on Saipan or Alamagan. Furthermore, persistent reports from the island of Saipan suggest that Boiga irregularis may be in the process of becoming established there (Rodd and Savidge 2007). This is cause for concern given the catastrophic declines it caused on Guam's birds.
On Saipan, it occurs in thicket-meadow mosaics, forest edge, reed-marshes and forest openings (Craig 1996). A recent 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).
On Saipan, available habitat has been reduced for agriculture, home-building and tourist-related facilities although in recent years these have not 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). Persistent reports from the island of Saipan suggest that the Brown Tree Snake (Boiga irregularis) may be in the process of becoming established there (Rodd and Savidge 2007) and unless it can be controlled, the reed-warbler population is likely to be extirpated rapidly once snake numbers have reached the point where they impact bird populations. 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, N. Johnson in litt. 2012). Introduced predators, including feral cats (Felis catus) and 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).
Conservation and Research Actions Underway
A recovery plan exists for the pre-split taxon (U.S. Fish and Wildlife Service 1998) but there has been little or no 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. Trap lines for snakes are maintained at ports, night searches are conducted and a sniffer dog programme has recently been established. Publicity campaigns were conducted to raise the general awareness of island residents, including port workers, about the dangers of snake colonisation (USFWS 1998). Repeat surveys have been conducted on Saipan in 1982, 1997 and 2007 and on Alamagan in 2000 and 2010 (Marshall et al. 2011).
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
Bird, J., O'Brien, A., Benstead, P., Stattersfield, A., Derhé, M., Symes, A., Martin, R., Khwaja, N., Wright, L, Mahood, S., Westrip, J., Calvert, R.
Contributors
Camp, R., Dutson, G., Freifeld, H., Saunders, A., Radley, P., Mosher, S., Amidon, F.A. & Gourley, J.
Recommended citation
BirdLife International (2023) Species factsheet: Acrocephalus hiwae. Downloaded from
http://datazone.birdlife.org/species/factsheet/saipan-reed-warbler-acrocephalus-hiwae on 29/05/2023.
Recommended citation for factsheets for more than one species: BirdLife International (2023) IUCN Red List for birds. Downloaded from
http://datazone.birdlife.org on 29/05/2023.
