Data Zone
Justification of Red List category
This species has a small population size, with recent evidence that the stoppage of trapping for snakes on Guam has caused a reduction in population size that is precautionarily assumed to be ongoing. It therefore is assessed as Vulnerable.
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.
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.
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, Johnson et al. 2018). Threats to this species including human disturbance of caves, predation by the brown tree snake (Boiga irregularis), loss and degradation of foraging habitat and nest loss due to introduced insects are all ongoing (USFWS 2020). Currently, predation by 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) and possible disturbance by humans and feral mammals are the main threats (Wiles and Woodside 1999, G. Wiles unpubl. data).
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 (USFWS 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 (USFWS 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
Vine, J., Berryman, A.
Contributors
Derhé, M., Mahood, S., O'Brien, A., Saunders, A., Stattersfield, A., VanderWerf, E. & Wiles, G.
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.