Mariana Swiftlet Aerodramus bartschi


Justification of Red List Category
This species qualifies as Endangered because it has undergone a rapid population decline, due to depredation by the introduced brown tree snake Boiga irregularis on Guam and perhaps pesticide use elsewhere. There is a risk of rapid decline should the brown tree snake colonise Saipan, its stronghold. Fortunately, there has been little evidence of brown tree-snakes colonising any islands outside of Guam, and this species might be down-listed after a more detailed assessment of this threat.

Population justification
The population is estimated at between 6000-7000 individuals (Johnson 2015, US Fish and Wildlife Service 2015) and so has been placed in the band 2,500-9,999.

Trend justification
Population estimates have fluctuated over the last two decades but appear to be increasing. Recent population estimates on Saipan indicate that the population has increased to over 5,000 individuals from 1985 to 2015 (US Fish & Wildlife Service 2015). The population on Guam is also believed to have increased to approximately 1500 individuals from 1983 to 2015 (A. Brooke in litt. 2006, Johnson 2015) while the population on Aguijan appears stable at around 300-400 individuals (J. Cruz et al. in litt. 2007, US Fish & Wildlife Service 2015). However, on Saipan, the species's stronghold, brown tree snake Boiga irregularis is in the process of becoming established (Rodd and Savidge 2007), which  is expected to cause a very rapid decline.

Distribution and population

This species is endemic to Guam (to USA) and the Northern Mariana Islands (to USA), and was introduced to Oahu, Hawaiian Islands (USA) in the early 1960s. On Guam, it was very abundant but, from the mid-1960s to the early 1970s, it underwent a precipitous decline (Jenkins 1983). In 1999, only three colonies remained, the largest holding c.700 birds (total population c. 800) (G. Wiles unpublished data). A 2008 report by the U.S. Navy estimated that the population on Guam had increased to around 1,150 individuals (Grimm 2008) and in 2015 numbers were reported to have increased in Guam to 1,591 (Johnson 2015). In the Northern Marianas, populations disappeared from Rota and Tinian in the 1970s, although, on Tinian, it was possibly only nomadic (Engbring et al. 1982). In 1982, estimates were 9,120 on Saipan and 1,022 on uninhabited Aguijan (Engbring et al. 1982) but, more recently, estimates are c.4,674 (eight colonies) and 267 (six colonies), respectively (Johnson 2015), with surveys on Aguiguan detecting very low numbers between 1995-2008 (Amidon 2014). Surveys on Saipan show declines between 1985-1992 and significant increases between 1998-2005, with large between year fluctuations (Cruz et al. 2008). Additional subcolonies are suspected but unconfirmed (Johnson 2015). On Oahu, it survives in a single known breeding colony in a small tunnel in the Ko`olau Mountains, although similar irrigation tunnels are common and thus other small colonies may exist (Chantler and Driessens 1995). Observations there in 1997 suggested a minimum of 17 breeding pairs (66 birds in total [Wiles and Woodside 1999]), whilst monitoring during 2009-2010 recorded up to 50 nesting pairs (E. Vanderwerf in litt. 2012). The number of nesting pairs and fledglings on Oahu was lower in 2011 because rats invaded the nesting tunnel after rat control was interrupted (E. Vanderwerf in litt. 2012). The most up to date total estimate for the Mariana Islands is  6,750 individuals (US Fish & Wildlife Service 2015).


It feeds over coastal and interior forest and grassland (and formerly mangroves), capturing small insects in flight, preferring forest on Guam and Aguijan (G. Wiles unpublished data). A study in Aguiguan found Hymenoptera to be the most commonly consumed invertebrates (Valdez et al. 2011). It breeds and roosts in colonies in caves that typically hold a few to 700 birds (G. Wiles unpublished data). Nesting occurs year-round, but is greatest from late January to September or October. One egg is laid per clutch and pairs probably lay more than one clutch per year (G. Wiles unpublished data).


The cause of the decline on Guam and the extinction on Rota may relate to loss of insect prey through pesticide-use (Kohley et al. 2006). Currently, predation by brown tree snake Boiga irregularis is the primary limiting factor on Guam and interactions with introduced mud dauber wasps Vespula sp. may interfere with recovery (the additional weight of wasps nests causes swiftlet nests to fall from cave walls). On Saipan, exotic cockroaches (which damage and destroy nests by consuming nest material and swiftlet saliva gluing them to cave walls), predation by brown tree snake (which is in the process of becoming established on the island [Rodd and Savidge 2007]) and possible disturbance by humans and feral mammals are the main threats (Wiles and Woodside 1999, A. Saunders in litt, G. Wiles unpubl. data). Climate change is also considered a threat (U.S. Fish and Wildlife Service 2015). 

Conservation actions

Conservation Actions Underway
On Guam, snakes have been trapped at the main colony, at ports of entry and military areas. Acetaminophen has been used to control snakes within a 55-hectare fenced area on the Anderson Air Force Base (AAFB) on Guam (U.S. Fish and Wildlife Service 2015). Colonies have been censused regularly, nesting has been observed and foraging ranges mapped. On Rota, surveys have been conducted confirming that there is now sufficient insect prey to allow successful reintroduction. On Saipan and Aguijan, colonies have been censused periodically and insecticide applied to kill cockroaches (G. Wiles in litt. 2000). Rat control on Oahu has resumed but is being done less often than previously because access to the site has become more difficult (E. Vanderwerf in litt. 2012). The Pacific Islands Climate Change Cooperative (PICCC) has funded research that will increase the spatial resolution in which climate change predictions can be made for this species (U.S. Fish and Wildlife Service 2015).

Conservation Actions Proposed
Continue monitoring all populations. On Saipan, eradicate the brown tree snake and limit cave disturbance. On Guam, facilitate recolonisation of caves historically used by large colonies. Reintroduce birds on Rota (G. Wiles in litt. 2000) and northern Guam  (U.S. Fish and Wildlife Service 2015). On Guam and Saipan, eradicate or control dauber wasps and cave dwelling cockroaches, respectively. On Oahu, control introduced black rats Rattus rattus near nesting tunnel entrance. Identify limiting factors for the species expansion  (U.S. Fish and Wildlife Service 2015). Continue to refine snake management approaches. Secure and manage caves currently used by the species, as well as currently uninhabited caves that could provide additional habitat. Develop methods for effective translocation  (U.S. Fish and Wildlife Service 2015).


11 cm. Swiftlet with dark greyish-brown upperparts and head. Silvery grey-white throat and upper breast. Remainder of underparts darker and greyer. Shallow fork-tail. Plumage lacks any noticeable sheen. Voice Chirps and twitterings. Makes echolocation clicks inside caves.


Text account compilers
Derhé, M., Mahood, S., O'Brien, A. & Stattersfield, A.

Cruz, J., Saunders, A. & Wiles, G.

Recommended citation
BirdLife International (2020) Species factsheet: Aerodramus bartschi. Downloaded from http://www.birdlife.org on 06/06/2020. Recommended citation for factsheets for more than one species: BirdLife International (2020) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 06/06/2020.