Mariana Gray Swiftlet

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Mariana Gray Swiftlet

Aerodramus vanikorensis bartschi

ListedAugust 27, 1984
DescriptionMarked by sooty black upper parts, slightly paler rump, some white is present at base of feathers.
HabitatGrasslands, limestone forest, ravine forest, coconut groves.
FoodInsects captured in flight.
ReproductionClutches contain single eggs laid between January and July.
ThreatsTyphoons, pesticides, disease, introduced predators.
RangeGuam, Marianas Islands


The Aerodramus vanikorensis bartschi (Mariana gray swiftlet) has a long, narrow body with sooty black upper parts and a slightly paler rump. The underparts are dark gray but with a brownish tinge. Some white is present at the base of the feathers in the loreal region. The tarsi are naked and the irises are dark hazel. This species does not display sexual dimorphism in coloration. The taxonomy of this group of swiftlets is in much debate. Virtually all current authorities recognize bartschi as either a subspecies (usually of vanikorensis ) or as its own monotypic species. The generic name continues to be debated as well. Several common names have been applied to this species. These include edible nest swiftlet, Guam edible nest swiftlet, gray swift-let, uniform swiftlet, Mauritus swiftlet, Caroline swiftlet, Carolines swiftlet, Guam cave swiftlet, mossy-nest swiftlet, island swiftlet, and the lowland swiftlet. The species was previously listed pursuant to the Endangered Species Act of 1973, as amended under the common name Vanikoro swiftlet. "Yayaguak" is the local name for swiftlets used by Chamorro residents in the Mariana Islands.


A. v. bartshi is a highly colonial rooster and nester. They typically form colonies numbering from a few individuals to several thousand birds. A colony on Guam contains 50-300 birds while on Saipan there are at least 200 birds. Swiftlets are not known to be migratory. Their former presence on Tinian is described as being nomadic. Clutches of A. v. bartschi contain single eggs laid between January and July. Nesting possibly occurs during the remainder of the year but has not been reported previously. Nesting is not synchronous within colonies. Double clutches or renesting attempts have not been observed. The incubation period of A. v. bartschi is at least 12 days. Other breeding information is not available for these swiftlets. A. v. bartschi roosts exclusively in caves. They inhabit natural and man-made caves, such as abandoned Second World War gun emplacements on Micronesia. Caves used by A. v. bartschi for roosting and nesting are formed from limestone rock and occur in limestone forest and ravine forest. Colonies of swiftlets are also known to roost in at least three limestone sinkholes. There do not appear to be any seasonal changes in habitat use by A. v. bartschi. A. v. bartschi feeds on insects captured in flight. Detailed food studies have not been conducted and insect species that are eaten are not known. A. v. bartschi roosts in caves at night. Some birds may return to caves during the day to rest or for nesting. Swiftlets are diurnal and crepuscular feeders. It was noted on Saipan that large flocks of A. v. bartschi fed for only a few weeks in any one part of the island. It was believed their movements may have been seasonal.


Species of A. v. bartschi have been reported to forage over a wide variety of habitats including grasslands, limestone forest, ravine forest, and coconut groves. These habitats on Guam have been described in detail. On Saipan, they also feed over tangan-tangan (Leucaena leucocephala ) forests. Clifflines, grassy hills, and grassy ravines are among the preferred foraging habitats. On Guam, the colony inhabiting the eastern side of the Naval Magazine feeds over savannah grasslands dominated by Miscanthus and ravine forest. Birds from this colony also feed over several types of seasonally-flooded wetlands. These areas are designated in the National Wetlands Inventory as palustrine, forested, broad-leaved evergreen and palustrine, emergent, persistent. No specific information exists on the habitat use of A. v. bartschi on Saipan and Aguijan. On Oahu, swiftlets occur over forested hills and valleys.


A. v. bartschi presently occurs in southern Guam, Saipan, and Aguijan, and in the Mariana Islands. It may possibly still be present on the Mariana Islands of Rota and Tinian. The past distribution of this swiftlet included all of Guam and the islands of Rota, Aguijan, Tinian and Saipan. A colony of 250-300 birds roosts in a cave on the eastern side of the U.S. Naval Magazine. Birds from this colony are believed to forage over a wide area including the Naval Magazine, Talofofo and Inarajan. A second, much smaller group of swiftlets inhabits the Geus River Valley north of Merizo. The Naval Magazine is the only federally-owned land on Guam on which A. v. bartshi occur.


In the past, A. v. bartschi was considered abundant on Guam and Rota. Reasons for the decline of A. v. bartschi is unknown but hypothesized causes include pesticide poisoning, introduced predators, disease, typhoons, and disturbance or permanent loss of nesting caves. To date, no infectious diseases have been isolated on Guam that could account for the decline. Feral dogs, cats, and rats probably have little or no effect on swiftlets, which are aerial feeders and cave nesters. It is not known if the brown tree snake, an introduced snake present only on Guam, is a serious predator. The swiflet's cave nest in not readily accessible to predation by snakes, and the bird has disappeared from islands which are free of brown tree snakes. Pesticide residues are apparently not a current problem for swiftlets or their prey on Guam but pesticides may have had some impact on egg fragility and breeding success in the past. Typhoons are a regular occurrence under which all species of native wildlife in the Marianas have evolved. Typhoons might cause temporary declines in the swiftlets' populations but presumably would not cause permanent losses. Caves were used for a variety of defensive purposes by the Japanese military in the Second World War. During the invasion of Saipan, Tinian, and Guam, a number of caves were destroyed or highly disturbed. These activities probably affected swiftlets for at least a short period of time before reuse. Guano has been mined from swiftlet caves on Rota, Aguijan, Tinian, and Saipan, and this activity may have disturbed birds that were roosting or nesting.

Conservation and Recovery

In order to maintain effective conservation of this species the swiftlet caves must be preserved and managed. A crucial element of conserving swiftlet populations involves protection and management of their caves. The caves prove shelter and roosting and nesting habitats for the swiftlets. With this in mind, 12 caves have been identified as high priority for recovery. To promote the expansion of the population into suitable historical habitat many key factors must be determined. First, it must be ascertained if natural re-colonization of unoccupied caves is possible. Second, reproductive success must be improved. Third, techniques must be developed for the reintroduction process. These three factors are crucial and must be seriously overlooked. It is not yet known if it is feasible to reestablish a swift-let population in unoccupied habitat. Techniques must be developed. Sites should be chosen and methods developed to hasten the establishment of populations.


U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Building
911 N. E. 11th Ave.
Portland, Oregon 97232-4181
Telephone: (503) 231-6121


U.S. Fish and Wildlife Service. 1991. Recovery Plan for the Mariana Islands Population of the Vanikoro Swiftlet, Aerodramus vanikorensis bartschi. U.S. Fish and Wildlife Service, Portland, OR. 49 pp.

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Mariana Gray Swiftlet

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