If you visit the island of Guam, you won't see many of the birds illustrated on this page. The Mariana Fruit Dove, Guam Flycatcher, Rufous Fantail, Cardinal Honeyeater, and other native forest birds of Guam have become extinct since the late 1940s, when the brown tree snake (Boiga irregularis) is believed to have first colonized the island.
These snakes are native to New Guinea, northern Australia, eastern Malaysia, and the Solomon Islands. They may have arrived as accidental stowaways in lumber shipments from New Guinea as islanders rebuilt from the devastating effects of World War II.
On Guam, brown tree snakes found dense forests that were teeming with the birds and small lizards that are their natural foods. No enemies slowed their spread, and their numbers mushroomed. The snake populations moved steadily north at a rate of a mile per year from their southern landfall. The forests fell silent in their wake.
Forests and second growth, prefers to remain high in the forest canopy.
The Mariana Fruit-dove is endemic to the Mariana Islands of Guam, Rota, Agiguan, Tinian and Saipan.
The Mariana Fruit-dove was last seen on Guam in 1985.
The only snake species native to Guam is a small harmless creature the size of an earthworm. The local birds had no experience with snakes as predators, or with any animal that hunts by night as these snakes do. They showed no fear of the invaders and had no innate defenses against them. The result has been a catastrophe.
By 1983, the only original forest community, which held all ten native woodland species, clung precariously to life in a 160-hectare woodland on the extreme northern tip of the island. Scientists recognized the dangers and began attempts to establish captive breeding populations of the endangered forest birds, but it was too late.
Of 18 species of native Guamanian birds, seven are extinct, two are extinct in the wild (the Guam Rail, pictured left, and the Micronesian Kingfisher survive only in captivity), six are rare, and three are uncommon. Birds were not the only Guamanian animals to suffer; small mammal and reptile faunas have also been reduced in numbers. Five native lizard species have become extinct locally. In some local surveys of small mammals, the only species that were found were the introduced house mouse and the black rat. Shrews have virtually disappeared.
Captures insects in mid-air
Though they were most common in forests and thickets, Guam Flycatchers were found in most habitats on the island.
The Guam Flycatcher, also known as the Guam Broadbill, was found only on the island of Guam.
It was last seen in 1984.
The smaller bird species that were restricted to the forest habitats were the first to disappear. Although Brown Tree Snakes may grow to more than eight feet in length, smaller specimens are more common and prefer smaller prey items. The tiny Bridled White-eye had an unfortunate habit of sleeping shoulder to shoulder in its nocturnal roosts. A single snake could consume several individuals in one night. The Bridled White-eye disappeared from the wild in 1983. Larger birds such as the Mariana Fruit-dove declined more slowly, but even the smallest tree snake found their eggs and nestlings to be easy prey. With their reproduction virtually shut down, the medium-sized species were not long to follow their smaller neighbors to extinction.
There is grave danger that the snakes will spread to other Pacific islands with vulnerable bird populations. Guam is a major hub for sea and air traffic. Electric barriers at port and airport freight depots act as a deterrent, but offer no long-term solution. brown tree snakes have successfully established themselves on the Northern Marianas island of Saipan. USDA inspectors at the Honolulu International Airport use specially trained beagles to sniff out snakes in flights arriving from Guam. Their efforts have turned up several snakes concealed in cargo or hiding in wheel wells. Even more alarmingly, seven live brown tree snakes were captured on a military base miles away from the airport. A single pregnant female slipping through the net is all it would take to establish a breeding colony in the Hawaiian islands. Then the already beleaguered native birds of Hawaii would meet the same fate as the forest birds of Guam.
Captures insects in mid-air
Forest undergrowth. On the island of Guam, the Rufous Fantail was found in most habitats except savanna.
The Rufous Fantail is widely distributed throughout the southern Pacific from the Mariana Islands and Yap south to Australia.
The Guam subspecies, Rhipidura rufifrons uraniae, was last seen in 1984.
Might another predator be introduced to control the snakes? No. Using one predator to control another has never been very successful because most predators will hunt any suitable animal. For example, mongooses that were introduced to control rats and snakes in the Caribbean and in Hawaii have killed far more birds than rats or snakes.
Yes, and this is being done. The United States Department of Agriculture traps more than 6,000 snakes a year around commercial and military airfields on Guam. But the island is too large and there are too many snakes for trapping to do more than temporarily reduce a local population. There are close to two million brown tree snakes on the entire island. Prime snake habitat may hold 14,000 snakes per square mile. This is one of the highest snake densities in the world.
Only to a limited degree. The last Micronesian Kingfishers and Guam Rails are being bred at the National Zoo and other zoos. Captive propagation of the Guam Rail has been very successful. The Zoo's Conservation and Research Center in Front Royal, Virginia, houses several breeding pairs of rails and is one of the institutions supplying rails for reintroduction efforts.
The Micronesian Kingfisher has proven to be more difficult to breed in captivity. The Species Survival Program began with 29 individuals captured from the wild. The population grew to 65 birds by 1991, dropped to 59 birds in ten U.S. institutions in late 1999, but climbed back to 65 birds in 12 zoos by summer 2004. By 2007, the population reached nearly 100.
When the program began, there was little information on the nutrition and behavioral ecology of wild Micronesian Kingfishers. The kingfishers had very specific nest log requirements. When successful nesting did occur, propagators were discouraged to discover that newly hatched chicks were disappearing from the nest. Most zoos were feeding newborn mice to their kingfishers, and scientists fear that some parents were confusing their blind and naked hatchlings with food.
Micronesian Kingfishers are now fed a more natural diet featuring small lizards called anoles. Researchers are hopeful that the population will begin to increase once again as our understanding of their behavior grows. The Micronesian Kingfisher chick, pictured at the right, was hatched at the National Zoo's Rock Creek facility in 1998. He has since matured into a beautiful adult male. The long-term goal of the SSP is to have 100 to 200 birds in 25 zoological institutions.
Possibly, but not easily. Until the snakes are controlled, birds cannot be reintroduced in Guam. The island of Rota in the Commonwealth of Northern Marianas Islands lies 31 miles to the north of Guam and is rich in rail habitat. It is also, so far, snake-free. The Species Survival Program had enough captive rails by 1989 to begin releasing them on Rota. Though the earliest releases were plagued by vehicle collisions and predatory feral cats, survival rates have improved steadily. They hope to be able to release up to 100 rails a year while maintaining a captive population of 150 to 175 in mainland zoos. If the snakes on Guam are ever brought under control, rails from the wild population on Rota may form the nucleus of a recolonization of their home island.
Flowers and nectar
Found in forests and second-growth, especially near flowering trees
The Cardinal Honeyeater is widely distributed throughout the islands of the southern Pacific.
The subspecies endemic to Guam, Myzomela cardinalis saffordi, was last seen in 1984.
Prior to the discovery of its involvement in Guam extinctions, the brown tree snake was hardly a popular subject of herpetological research. It was not until 1983 that the snake that was overrunning Guam was even properly identified. Residents had been under the mistaken impression that their new neighbor was the relatively harmless Philippine Rat Snake. Local naturalists even lauded the snake for its "beneficial" role in controlling introduced rat populations.
Today the brown tree snake is the focus of intense research from the forests of Guam to mainland university research laboratories. Every aspect of the snake's ecology, behavior and physiology is being investigated as we search for ways to restrict its population. Studies of the snake's sensory perception help explain its hunting behavior and help improve trapping success. Reproductive studies may provide clues to developing a strategy for interfering with the natural reproductive cycles.
Fruits, seeds, and flowers
Forests, clearings, and second-growth
The White-throated Ground Dove is endemic to the Mariana Islands from Guam to Saipan.
The Guam subspecies, Gallicolumba xanthonura xanthonura , was last seen in 1986.
Unfortunately, the more herpetologists and behaviorists discover about the brown tree snake, the more respect they have for its potential as a colonizer and its enormous adaptability as a generalist predator. It is probably impossible to eliminate all brown tree snakes from the island of Guam. The best we can hope for is a strong reduction in the snake population that would allow the native species to strike a balance with their new chief predator.
At the National Zoo, scientists are looking for a virus or other infectious agent that is specific to the Brown Tree Snake. Great care must be taken so that these agents would not infect any other form of life, including humans. Zoo pathologist Don Nichols is working on two promising paramyxoviruses. These viruses cannot survive in animals with body temperatures above 95 degrees, making them no threat to mammals or birds. A pathogen such as these, that can consistently kill half of the snakes it infects may be sufficient to bring the brown tree snake back under control.
Insects found in foliage
The Nightingale Reed-warbler is found only in the Marianas Islands.
The subspecies endemic to Guam, Acrocephalus luscinia luscinia, was extinct by 1970. The remaining subspecies are all considered endangered.
The snake is the cause of the destruction, but it has simply behaved in a way that is normal for snakes. In its natural range, it must compete with other predators for a prey base that has evolved along with it for millions of years and it must avoid predators of its own. In Guam, it is out of place. In the forest canopy of the Solomon Islands or New Guinea it is just one of many snake species in a balanced forest community.
Human activities often have unexpected results. No one imported the snakes deliberately. They reached Guam because wartime sea and air traffic provided an opportunity for accidental introduction. Although the brown tree snake has had the most obvious impact on the ecology of Guam, they now share the radically altered island ecosystems with numerous other introduced species including birds, mammals, other reptiles, invertebrates, and plants. Each of these plant and animal species plays a role in altering the habitat and all have an impact on the native survivors. Our world is closely tied together and all of us must realize that only by using forethought can we avoid more ecological disasters such as that on Guam.
Jaffe, M. 1994. And No Birds Sing: The Story of an Ecological Disaster in a Tropical Paradise. Simon & Schuster. New York.
Savidge, J.A. 1987. "Extinction of an Island Forest Avifauna by an Introduced Snake." Ecology 68(3):660-668.
Illustrations by Debi Talbott, NZP
Photos by Jessie Cohen, NZP