A Traveller’s Guide to Feathers, Article 83 – The Value of Introduced Populations

A Traveller's Guide to Feathers

Some types of birds have an enormous distribution, and so have the opportunity to bring joy to people in many countries. These include House Sparrows, Peregrine Falcons and White Wagtails, each of which can be found over large parts of the globe. At the other end of the spectrum are those species with a distribution so small that they barely exist at all.


Mariana Swiftlet (source: hbs.bishopmuseum.org)

Mariana Swiftlets are native to just the island of Guam and the Northern Mariana islands of Rota, Aguiguan, Tinian and Saipan. They are no longer found on Tinian or Rota. These birds nest colonially within caves, and forage for small insects over forests and grasslands. The species is endangered and with a decreasing population. According to the IUCN, these swiftlets face a number of challenges, including the use of pesticide which diminishes their prey, and exotic cockroaches which degrade their nests. An introduced mud dauber wasp also creates hardships for the swiftlet; by gluing their own weighty nests to the nest of the birds, the whole assembly may detach from the cave wall and crash to the ground. Introduced predatory brown tree snakes are now abundant on Guam, and are becoming established on Saipan.

Effecting conservation programs directed toward threatened species must be based on a thorough knowledge of the natural history of the imperiled animal. This leaves biologists in a bind. How do you study a threatened creature when the act of studying it may represent a damaging disturbance? Plans to reintroduce the Mariana Swiftlet to islands from which it has been eliminated are more likely to be successful if the bird’s biology is well known.

Nathan Johnson and his colleagues recently took advantage of a unique opportunity to study the Mariana Swiftlet without causing any harm to the bird in its native range. The species was introduced to the Hawaiian island of O’ahu in 1962 and 1965, and a small population still persists there. Between 2006 and 2011, the team studied swiftlets nesting in a human-created, 55 metre long tunnel that had been constructed before WWII. Using night-vision goggles and sensitive camcorders, the group monitored active nests in the tunnel. They also trapped rats inside and outside the tunnel.

In the O’ahu tunnel, active swiftlet nests were found in all months of the year. Each nest was home to a single egg, and hatching followed 23 days of incubation. The parents had a very good average nest success of 64%. Females were capable of more than one breeding attempt in a year. Rats were found to be a significant source of swiftlet mortality.

In order for the Mariana Swiftlet to be downlisted from endangered to threatened, authorities feel that there should be established populations of at least 1000 individuals on Aguiguan, and 2000 each on Guam, Rota and Saipan. It is hoped that studies of the swiftlet at its introduced home in Hawaii might better inform reintroduction practices. Birds from O’ahu might even be used to enhance populations on their native Mariana Islands.

Johnson, N. C., S. M. Haig, S. M. Mosher and J.  P. Hellenbeck. 2017. Reproductive success of Mariana Swiftlets (Aerodramus bartschi) on the Hawaiian island of O’ahu. Journal of Field Ornithology 88:362-373.

Photo credits: Nesting Swiftlet – www.pinterest.com, originally posted on the Bishop Museum’s Hawaii Bird Survey website at hbs.bishopmuseum.org; artwork -  www.pinterest.com


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