In the 1940s, a few snakes arrived in Guam hidden in military cargo; decades later, nine of the island’s 11 native forest bird species were gone

An island that had evolved without native snakes suddenly faced a predator its wildlife had never encountered before. In the late 1940s, brown treesnakes are believed to have reached Guam aboard military cargo ships or aircraft following World War II operations, as per the Archive for the U.S. Department of State, Diario AS, and U.S. Geological Survey.

The consequences unfolded over decades. The nocturnal, tree-dwelling snakes found an island where native birds had developed without snake predators and had few defenses against them. As the snakes spread, they preyed on birds and their eggs, eventually being linked to the disappearance of nine of Guam’s 11 native bird species.

Today, the brown treesnake remains one of Guam’s most difficult invasive species problems, while conservationists are trying new ways to reduce its numbers and give native wildlife a chance to recover.

How did the brown treesnake reach Guam?

Brown treesnakes are native to Australia, Papua New Guinea and Indonesia. They were unintentionally introduced to Guam in the 1940s, with the supplied material identifying military transports and cargo following World War II as the route by which they arrived.

Guam originally had no native snakes or large predators capable of controlling this new species. The brown treesnake therefore encountered an environment where it could establish itself and spread.


The snake is arboreal and nocturnal, and it feeds on rats, shrews and lizards while preferring birds. Guam’s native birds had evolved in a snake-free environment, leaving them particularly vulnerable. Many nested low or on the ground and lacked the protective behavior that might have helped them avoid the new predator.

As the brown treesnake population expanded, its effect on native wildlife became severe.

The snakes have been linked to the extirpation of nine of the island’s 11 native bird species, along with significant declines in other native bird, reptile and bat populations.

What happened to Guam’s native wildlife?

The disappearance of native birds affected more than the birds themselves.

Without native birds dispersing seeds and eating insects, Guam’s forest vegetation changed considerably. Spider populations also soared, reaching more than 40 times the levels recorded on neighboring islands, according to the supplied material.

The brown treesnake population itself became remarkably dense. Estimates in the material put peak densities at up to 12,000 snakes per square mile, while another section gives densities of up to 30,000 per square mile.

The consequences also reached Guam’s human communities.

Brown treesnakes have entered houses through air ducts and sewage lines, and approximately 200 people have been treated for snakebites. The snakes also climb electrical lines and transformer boxes, causing frequent power outages and costing more than $1 million annually.

Can Guam’s native species recover?

Reducing brown treesnake numbers is now an important part of efforts to restore Guam’s native wildlife.

At War in the Pacific National Historical Park’s Asan Beach Unit, the National Park Service, U.S. Geological Survey, U.S. Fish and Wildlife Service and community citizen scientists are working together on brown treesnake removal.

Asan Beach covers approximately 44 hectares. Its position makes it a useful place to test control and restoration methods, with the ocean on one side, dense forest along a ridge and a large road bordering another side.


The site could provide a semi-permeable barrier to snake movement while also offering habitat suitable for native birds.

Visual surveys along established trails detected 47 snakes between May and August 2024, with 31 removed by hand. Researchers believe hundreds more may still need to be trapped before wildlife restoration projects can begin.

Why are citizen scientists joining the effort?

Removing brown treesnakes takes considerable time and requires people who know how to locate and capture them.

That has led project leaders to bring community members into the effort through citizen science. The National Park Service, U.S. Geological Survey and U.S. Fish and Wildlife Service are organizing the program to examine whether members of the public can contribute effectively to snake removal.

The project will evaluate several approaches, including citizen science, semi-impermeable barriers that could limit snake re-entry, suppression in difficult habitats such as cliffs, and restoration of native species after substantial snake reductions.

Future control work at Asan Beach will use several ground-based methods, including hand capture during visual surveys, toxicant baits and traps using live lures.

How is Guam trying to stop the snakes from spreading?

The brown treesnake problem has also prompted efforts to prevent the species from reaching other islands.

The snakes can survive for long periods without food, allowing them to remain alive inside cargo holds on ships or aircraft. Six brown treesnakes have been intercepted at Honolulu airport, although Hawaii has not been colonized.

The supplied material notes that a pregnant snake escaping into Hawaii could threaten native wildlife as well as the tourism industry, which is why efforts have focused on detection and control.

Controls have included fumigating cargo with methyl bromide, toxicants, baits and traps. Dog teams have also been used in Honolulu for snake detection.

In Guam, conservationists have used an unusual method involving dead mice containing acetaminophen, dropped by parachute to target brown treesnakes.

The broader effort has also received federal support. In 1992, the Department of Defense received $1 million in new funding for brown treesnake research and control, while the 1990 Nonindigenous Aquatic Nuisance Prevention and Control Act directed development of a program to control the species in Guam.

The cost of brown treesnake detection and control has been estimated at $2.5 million per year.

At Asan Beach, the latest citizen science effort represents another attempt to tackle a problem that began with a small number of snakes arriving decades ago.

What followed was a dramatic ecological shift. A predator that Guam’s native wildlife had never evolved alongside spread across the island, contributing to the loss of native bird species and altering the wider ecosystem.

The current effort is aimed at changing that trajectory by reducing the invasive snake population and creating an opportunity for native birds and reptiles to return.

Editorial input: The strongest element of this story is the enormous ecological consequence that followed an accidental introduction. The brown treesnake did not arrive as part of a deliberate environmental change, yet its spread transformed Guam’s wildlife. The ongoing work at Asan Beach shows how conservationists are now trying to reverse some of those effects through snake removal, citizen science and native species restoration.

FAQs:

How did brown treesnakes reach Guam?

They were unintentionally introduced to Guam in the 1940s, with the supplied material linking their arrival to military cargo and transport following World War II.

How many native bird species were lost?

Brown treesnakes have been linked to the extirpation of nine of Guam’s 11 native bird species.