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

Taxonomic note
Aerodramus bartschi (del Hoyo and Collar 2014) was previously placed in the genus Collocalia. Collocalia inquieta (Sibley and Monroe 1990, 1993) was previously split into C. bartschi following Browning (1993) and AOU (1998) and C. inquieta following Browning (1993).

Taxonomic source(s)
del Hoyo, J., Collar, N.J., Christie, D.A., Elliott, A. and Fishpool, L.D.C. 2014. HBW and BirdLife International Illustrated Checklist of the Birds of the World. Volume 1: Non-passerines. Lynx Edicions BirdLife International, Barcelona, Spain and Cambridge, UK.

Red List criteria met

Critically Endangered	Endangered	Vulnerable
-	-	C1

Red List history

Year	Category	Criteria
2023	Vulnerable	C1
2016	Endangered	A2e+3e+4e
2012	Endangered	A2e+3e+4e
2008	Endangered	A2e; A3e; A4e
2004	Endangered
2000	Endangered
1994	Not Recognised
1988	Not Recognised

Migratory status	not a migrant	Forest dependency	medium
Land-mass type		Average mass	-

	Estimate	Data quality
Extent of Occurrence (breeding/resident)	4,400 km2	medium
Area of Occupancy (breeding/resident)	1,004 km2
Number of locations	11-20,15	-
Severely fragmented?	no	-

	Estimate	Data quality	Derivation	Year of estimate
Population size	3000-5000 mature individuals	good	estimated	2022
Population trend	decreasing	poor	estimated	2016-2026
Rate of change over the past & future 10 years/3 generations (longer of the two periods)	15-20%	-	-	-
Generation length	2.6 years	-	-	-
Number of subpopulations	3	-	-	-
Percentage of mature individuals in largest subpopulation	1-89%	-	-	-

Population justification: Population size estimates for this species vary substantially depending on the method used. On Saipan, Radley (2009) estimated 6,111 birds in 10 caves in April 2009 but 4,891 swiftlets at eight caves in April 2013 (Radley 2013). Most recently, in 2016, 3,817 birds were counted in nine colonies (Liske-Clark et al. 2017a). On Aguiguan, the species has declined from the 1980s and the population was, in August 2016, estimated to number at least 338 birds across three colonies (Liske-Clark et al. 2017b) however at least four caves previously occupied by the species could not be surveyed due to inclement weather (Liske-Clark et al. 2017b, Johnson et al. 2018). On Guam, surveys at Mahlac Cave (the stronghold) between November 2010 and March 2016 yielded a mean population estimate of 1,145 (887-1,418) and 1,252 individuals were counted there in March 2016 (Johnson et al. 2018); in total, 1,549 birds were counted across three colonies in 2016 (Liske-Clark et al. 2017b) but were thought to have dropped to c.1,000 in 2020 (USFWS 2020). Populations on Rota and Tinian are considered extinct (Johnson et al. 2018). A smaller population, comprising 142 birds in a single tunnel, was introduced to O'ahu in the 1960s (Johnson et al. 2018).
Excluding the O'ahu population, the best global estimate available, made in 2016, is that there are least 5,700 birds in the Mariana Islands (Johnson et al. 2018), corrected here to c.5,200 to account for the apparent reduction in the Guam population. This is thought to be equivalent to c.3,400 mature individuals. There remains a likelihood that additional populations will be discovered (Johnson et al. 2018, USFWS 2020), however these undiscovered colonies are unlikely to host large numbers of birds. Consequently, the population size is placed in the band 3,000-5,000 mature individuals.

Trend justification: On Guam, the population had slowly been increasing to c.1,500 in 2015/16 (Johnson 2015, Johnson et al. 2018); this is thought to have fallen since to c.1,000 due to the stoppage of baiting for Brown Tree Snake Boiga irregularis which are postulated to have been a substantial contributor to the species' initial rapid decline. On Saipan the trend is uncertain but comparisons between higher counts in 2010 and 2015 (vs data from 2016) are thought to be explainable methodologically, rather than necessarily reflect a reduction in population size over this time period (Johnson et al. 2018). Although there are uncertainties, the population on Saipan is thought to be stable and previous certainty that Brown Tree Snakes would establish on Saipan (Rodda and Savidge 2007) appears to have been pessimistic and unrealised (Yackel Adams et al. 2021) and the island now has robust biosecurity measures. The most recent data from Aguiguan (where the population previously is suspected to have declined) indicate there has been no change to the species' population size since 2010 (USFWS 2015, 2020; Johnson et al. 2018).
Combining data, in the ten years to 2022, the population is estimated to have remained relatively stable, with increases on Guam between 2010 and 2015 offset by later decreases 2016 to 2020 (USFWS 2020). Between 2016 and 2020, the global population is estimated to have fallen by 8.8%, equivalent to 18% over ten years assuming a constant exponential rate. A decline in swiftlet numbers between 2020 and 2021 suggests that this decline is continuing (G. Wiles in litt. 2023). The rate of decline over the next ten years is unknown and depends heavily on the scale of conservation interventions on Guam.

Country/Territory	Presence	Origin	Resident	Breeding visitor	Non-breeding visitor	Passage migrant
Guam (to USA)	extant	native	yes
Northern Mariana Islands (to USA)	extant	native	yes
USA	extant	introduced	yes

Country/Territory	IBA Name
Guam (to USA)	Guam National Wildlife Refuge
Guam (to USA)	Mahlac Caves
Northern Mariana Islands (to USA)	Aguiguan Island and Naftan Rock
Northern Mariana Islands (to USA)	Northern Saipan
Northern Mariana Islands (to USA)	Topachau-Susupe-Kagman

Habitat (level 1)	Habitat (level 2)	Importance	Occurrence
Artificial/Terrestrial	Pastureland	suitable	resident
Caves and Subterranean Habitats (non-aquatic)	Caves	major	breeding
Forest	Subtropical/Tropical Mangrove Vegetation Above High Tide Level	suitable	resident
Forest	Subtropical/Tropical Moist Lowland	major	resident
Altitude	0 - 600 m	Occasional altitudinal limits

Threat (level 1)	Threat (level 2)	Impact and Stresses
Human intrusions & disturbance	Recreational activities	Timing		Scope		Severity		Impact
		Ongoing		Minority (<50%)		Negligible declines		Low Impact: 4
		Stresses Species disturbance
Invasive and other problematic species, genes & diseases	Invasive non-native/alien species/diseases - Boiga irregularis	Timing		Scope		Severity		Impact
		Ongoing		Majority (50-90%)		Rapid Declines		Medium Impact: 7
		Stresses Species mortality
Invasive and other problematic species, genes & diseases	Invasive non-native/alien species/diseases - Capra hircus	Timing		Scope		Severity		Impact
		Ongoing		Minority (<50%)		Slow, Significant Declines		Low Impact: 5
		Stresses Species disturbance
Invasive and other problematic species, genes & diseases	Invasive non-native/alien species/diseases - Named species	Timing		Scope		Severity		Impact
		Ongoing		Minority (<50%)		Slow, Significant Declines		Low Impact: 5
		Stresses Reduced reproductive success
Invasive and other problematic species, genes & diseases	Invasive non-native/alien species/diseases - Named species	Timing		Scope		Severity		Impact
		Ongoing		Minority (<50%)		Slow, Significant Declines		Low Impact: 5
		Stresses Reduced reproductive success
Pollution	Agricultural & forestry effluents - Herbicides and pesticides	Timing		Scope		Severity		Impact
		Ongoing		Minority (<50%)		Unknown		Unknown
		Stresses Reduced reproductive success

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Recommended citation
BirdLife International (2024) Species factsheet: Mariana Swiftlet Aerodramus bartschi. Downloaded from https://datazone.birdlife.org/species/factsheet/mariana-swiftlet-aerodramus-bartschi on 25/11/2024.
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 25/11/2024.