• Summary
• Text account
• Data table and detailed info
• Distribution map
• Reference and further resources

Taxonomic source(s)
del Hoyo, J., Collar, N.J., Christie, D.A., Elliott, A., Fishpool, L.D.C., Boesman, P. and Kirwan, G.M. 2016. HBW and BirdLife International Illustrated Checklist of the Birds of the World. Volume 2: Passerines. Lynx Edicions and BirdLife International, Barcelona, Spain and Cambridge, UK.

Red List criteria met

Critically Endangered	Endangered	Vulnerable
-	B1ab(iii)+2ab(iii)	B1ab(iii)+2ab(iii); D1+2

Red List history

Year	Category	Criteria
2023	Endangered	B1ab(iii)+2ab(iii)
2016	Critically Endangered	B1ac(iv)
2015	Critically Endangered	B1ac(iv)
2012	Critically Endangered	B1ac(iv)
2010	Critically Endangered	B1a+c(iv)
2009	Critically Endangered	B1a+c(iv)
2008	Critically Endangered
2004	Critically Endangered
2000	Critically Endangered
1996	Vulnerable
1994	Vulnerable
1988	Threatened

Migratory status	not a migrant	Forest dependency	does not normally occur in forest
Land-mass type		Average mass	18 g

	Estimate	Data quality
Extent of Occurrence (breeding/resident)	2,100 km2	medium
Area of Occupancy (breeding/resident)	24 km2
Number of locations	2	-
Severely fragmented?	no	-

	Estimate	Data quality	Derivation	Year of estimate
Population size	250-999 mature individuals	good	estimated	2016
Population trend	unknown	medium	-	2000-2010
Generation length	2.99 years	-	-	-
Number of subpopulations	2	-	-	-
Percentage of mature individuals in largest subpopulation	1-89%	-	-	-

Population justification: The naturally occurring population on Nihoa was estimated at 489  ± 70 (SD) in 2010, 699 ± 78 in 2011, 610 ± 210 in 2012, 468 ± 166 in 2013, and 893 ± 303 in 2014 (Gorresen et al. 2016). The most recent estimate of the population on Laysan was 164 in September 2014 (Dalton et al. 2014, Freifeld et al. 2016). Given that the Nihoa subpopulation is thought to fluctuate widely, the population is tentatively placed in the band 250-999 mature individuals. This estimate equates to 375-1,499 individuals in total, rounded here to 350-1,500 individuals.

Trend justification: Although there is a lack of precision over the monitoring methods, the existing data suggest that Millerbird numbers on Nihoa have experienced pronounced fluctuations and have likely ranged between fewer than 50 and more than 800 individuals (VanderWerf et al. 2011). Although it is still thought to undergo substantial fluctuations, the species may not currently be as at risk as in the late 1980s (Gorresen et al. 2016). Numbers on Laysan have greatly increased since translocations in 2011 and 2012 (C. Farmer in litt. 2022). In 2016, population trends were considered inconclusive on Nihoa due to high within-year variance (Gorresen et al. 2016). Although an updated population estimate is not currently available (C. Farmer in litt. 2022), raw detections per effort on Nihoa in 2010-2021 are not indicative of a decline overall (Plentovich et al. 2021). However, detections were reportedly very low in 2022 (R. Rounds in litt. 2022). This may be due to several different reasons including the time of year, however severe drought driven by climate change may be a contributing factor (R. Rounds in litt. 2022, S. Plentovich in litt. 2022). It is currently unclear whether this is a natural fluctuation or if dry conditions from 2022 are indicative of longer-term trends (J. Vetter in litt. 2022), and as such, the population trend is precautionarily considered to be unknown overall.

Country/Territory	Presence	Origin	Resident	Breeding visitor	Non-breeding visitor	Passage migrant
USA	extant	native	yes

Country/Territory	IBA Name
USA	Northwestern Hawaiian Islands

Habitat (level 1)	Habitat (level 2)	Importance	Occurrence
Shrubland	Subtropical/Tropical Moist	major	resident
Altitude	0 - 270 m	Occasional altitudinal limits

Threat (level 1)	Threat (level 2)	Impact and Stresses
Climate change & severe weather	Droughts	Timing		Scope		Severity		Impact
		Ongoing		Whole (>90%)		Causing/Could cause fluctuations		Medium Impact: 7
		Stresses Ecosystem degradation, Species mortality
Climate change & severe weather	Habitat shifting & alteration	Timing		Scope		Severity		Impact
		Future		Whole (>90%)		Unknown		Unknown
		Stresses Indirect ecosystem effects, Ecosystem degradation
Climate change & severe weather	Storms & flooding	Timing		Scope		Severity		Impact
		Future		Whole (>90%)		Rapid Declines		Medium Impact: 6
		Stresses Ecosystem degradation, Species mortality
Invasive and other problematic species, genes & diseases	Invasive non-native/alien species/diseases - Anoplolepis gracilipes	Timing		Scope		Severity		Impact
		Future		Minority (<50%)		Unknown		Unknown
		Stresses Reduced reproductive success, Species mortality
Invasive and other problematic species, genes & diseases	Invasive non-native/alien species/diseases - Cenchrus echinatus	Timing		Scope		Severity		Impact
		Ongoing		Majority (50-90%)		Unknown		Unknown
		Stresses Ecosystem degradation
Invasive and other problematic species, genes & diseases	Invasive non-native/alien species/diseases - Named species	Timing		Scope		Severity		Impact
		Future		Whole (>90%)		Rapid Declines		Medium Impact: 6
		Stresses Reduced reproductive success, Species mortality
Invasive and other problematic species, genes & diseases	Invasive non-native/alien species/diseases - Oryctolagus cuniculus	Timing		Scope		Severity		Impact
		Past, Unlikely to Return		Whole (>90%)		Rapid Declines		Past Impact
		Stresses Ecosystem degradation
Invasive and other problematic species, genes & diseases	Invasive non-native/alien species/diseases - Schistocerca nitens	Timing		Scope		Severity		Impact
		Ongoing		Majority (50-90%)		Causing/Could cause fluctuations		Medium Impact: 6
		Stresses Indirect ecosystem effects, Ecosystem degradation
Invasive and other problematic species, genes & diseases	Invasive non-native/alien species/diseases - Unspecified species	Timing		Scope		Severity		Impact
		Ongoing		Minority (<50%)		Unknown		Unknown
		Stresses Ecosystem degradation
Invasive and other problematic species, genes & diseases	Problematic native species/diseases - Telespiza cantans	Timing		Scope		Severity		Impact
		Ongoing		Minority (<50%)		Negligible declines		Low Impact: 4
		Stresses Reduced reproductive success
Invasive and other problematic species, genes & diseases	Problematic native species/diseases - Telespiza ultima	Timing		Scope		Severity		Impact
		Ongoing		Minority (<50%)		Negligible declines		Low Impact: 4
		Stresses Reduced reproductive success
Invasive and other problematic species, genes & diseases	Problematic species/disease of unknown origin - Unspecified species	Timing		Scope		Severity		Impact
		Future		Majority (50-90%)		Rapid Declines		Low Impact: 5
		Stresses Reduced reproductive success, Species mortality
Natural system modifications	Fire & fire suppression - Trend Unknown/Unrecorded	Timing		Scope		Severity		Impact
		Past, Likely to Return		Majority (50-90%)		Rapid Declines		Past Impact
		Stresses Ecosystem degradation

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Recommended citation
BirdLife International (2024) Species factsheet: Millerbird Acrocephalus familiaris. Downloaded from https://datazone.birdlife.org/species/factsheet/millerbird-acrocephalus-familiaris on 22/12/2024.
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 22/12/2024.