Guam, a small island territory in the western Pacific, has a globally recognized ecological tragedy concerning its avian population. The island’s history illustrates how a single invasive species can completely unravel a natural environment developed over millennia. This situation has effectively silenced Guam’s native forests, requiring decades of intensive conservation efforts to preserve the last remnants of its unique wildlife.
The Introduction of the Brown Tree Snake
The devastation of Guam’s birdlife began accidentally with the arrival of the brown tree snake (Boiga irregularis). This mildly venomous snake likely stowed away in military cargo ships from the South Pacific shortly after World War II. The island lacked the snake’s natural predators or competitors, allowing its population to explode unchecked. Densities reached staggering levels, estimated to be as high as 30,000 snakes per square mile in some forested areas.
The native birds were defenseless due to a lack of anti-predator behaviors, having evolved in an environment where ground predators were absent. The brown tree snake, which can grow up to 8 feet long, is an agile nocturnal and arboreal hunter. It easily navigated the forest canopy, preying on eggs, nestlings, and roosting adult birds while they slept.
Native Species Lost to Extinction
The snake’s impact was swift and catastrophic, driving an estimated nine to ten of the island’s twelve native forest bird species to local extinction on Guam. Two species endemic to the island were among the most widely known losses: the Guam rail (Gallirallus owstoni), known locally as the Ko’ko’, and the Guam kingfisher (Todiramphus cinnamominus), or Sihek.
The Guam rail, a flightless, ground-dwelling bird, was particularly vulnerable and was declared extinct in the wild on Guam by the 1980s. The Guam kingfisher, a medium-sized bird with striking cinnamon and blue plumage, was last seen in the wild on Guam in 1986. These species now exist solely in managed captive breeding programs established in Guam and in zoos across the United States.
Birds Currently Residing on Guam
Guam’s forests are now famously quiet, contrasting sharply with a healthy tropical island. The majority of birds encountered today are introduced or migratory species. The most successful avian residents are non-native species that possess different nesting habits or a greater resistance to the snake.
Commonly seen introduced birds include the Philippine collared dove, the Black francolin, and the Eurasian tree sparrow, which have successfully colonized the island’s varied habitats. These species often thrive in urban environments or have shorter incubation periods that reduce vulnerability. Only two native forest birds have managed to persist in the wild on Guam in precarious numbers.
The Mariana swiftlet (Aerodramus bartschi) survives by nesting high inside deep, protected limestone caves that are inaccessible to the snake. The Micronesian starling (Aplonis opaca) has also managed to cling to survival, mainly in small populations restricted to urbanized areas like the Andersen Air Force Base.
Efforts to Restore the Bird Population
Conservation efforts focus on a two-pronged approach: rigorous snake control and safeguarding the last remaining native birds. The U.S. government and local agencies commit millions of dollars annually to snake management. This includes trapping and the aerial distribution of toxicant baits, often dead mice containing acetaminophen, to reduce snake numbers.
The captive-bred populations of the Guam rail and Guam kingfisher are being used to establish “insurance populations” on nearby snake-free islands. The Guam rail has been successfully translocated to Rota and Cocos Island (Islan Dåno’), where wild populations are now breeding. The kingfisher is also being released on remote, predator-free islands like Palmyra Atoll as a prelude to possible future reintroduction to a secured area on Guam.