How Did One Snake Wipe Out Guam's Forest Birds?

One snake emptied Guam's forests of birds because it reached an island with no large snakes of its own and more prey than comparable mainland forests, where birds had never faced a hunter like it. The brown tree snake probably arrived around 1950, multiplied unseen and wiped out 10 of Guam's 12 forest bird species. Proving it took until the 1980s, and the fight now is about keeping it off other islands.

Chapters

1. 0:00Mice from the sky
2. 0:45The silence moving north
3. 2:14The snake nobody saw
4. 3:58Why the birds had no chance
5. 5:09A forest without birds
6. 5:51Fighting back

The strangest part of the story came later. In December 2013 the United States dropped 2,000 dead mice over Andersen Air Force Base on Guam, each hanging from its own little parachute of cardboard and green tissue paper. It was the fourth drop like it, and the biggest. Inside every mouse was a dose of acetaminophen (paracetamol). The mice didn't need it.

The target was the snake. Just 80 mg, about a sixth of a standard pill, kills almost any brown tree snake, though less reliably above about 200 g. A dog or a pig would have to eat about 500 of the mice to get a lethal dose. I wanted to know how one kind of snake emptied a whole island, why it took decades to prove, and why the fix comes by parachute.

How did Guam's birds disappear?

Guam sits in the western Pacific: 541 km², 45 km long and 4 km across at its narrow waist. It is the largest island in Micronesia, more than 2,000 km from Japan, the Philippines and New Guinea. As a 1997 review in BioScience tells it, World War II's bombardment levelled some forests and destroyed more than 80% of the buildings, yet the wildlife came through mostly intact.

In the 1960s, wildlife officials noticed the birds were gone from the southern third of the island, and the edge of the birdless forest was creeping steadily north. By the end of the 1970s the southern two thirds were empty. By 1985 most species survived only in small pockets at the northern tip, or not at all. Along survey routes, a decline of 90% or more took about 9 years on average, and 17 of Guam's 18 native bird species were severely affected.

There were two theories. One was pesticide, sprayed heavily after the war to control mosquitoes. The leading one was an introduced bird disease, like the one that ravaged Hawaii's birds. Guam's Division of Aquatic and Wildlife Resources hired an avian pathologist, Julie Savidge, to find it. She searched the forests and the birds and found no evidence of one.

Who found the snake, and how?

Savidge found a pattern instead: the brown tree snake's spread matched the retreat of the forest birds. Snakes raided bird-baited traps heavily where birds had declined, and little or not at all where bird numbers were stable. It was the first time a snake had been implicated as an agent of extinction. When she reported it at an American Ornithologists' Union meeting in 1983, some members were skeptical.

The snake had probably arrived around 1950, carried by accident in the postwar salvage of wrecked vehicles and equipment from the New Guinea area. It hunts at night in the trees, moves slowly and looks like a drab vine, so few visitors to Guam ever see one. Adults reach 2.4 m or more and up to 5 pounds. They are mildly venomous, and they also constrict their prey.

It eats lizards, birds, small mammals and their eggs, plus some odder things: dog food, chicken bones, raw hamburger, spareribs and paper towels. At their peak there were about 100 snakes per hectare, a square 100 m on each side.

People felt it too. The typical snakebite victim on Guam was a sleeping infant under six months old, and nine infants needed help breathing. The snakes climb power lines as well: a USGS study counted more than 1,600 outages from 1978 to 1997, and nearly 200 a year by the time it was published. One island-wide outage of 8 hours or more is estimated to cost over $3 million in lost productivity.

Why couldn't Guam's birds survive it?

Guam's native birds, bats and lizards got there by flying, or by floating for weeks on clumps of vegetation. The nearest larger island, Manus, is 1,740 km away. There are no large snakes native to Guam, so its birds had never needed a defence against something climbing to the nest at night.

The island was also full of food. Researchers removed an average of 55 rats per hectare from one forest in northern Guam in a few days, while comparable mainland forests hold 1.5 to 19 rodents per hectare. And nothing on Guam hunts the snake except humans.

The toll: 10 of Guam's 12 forest bird species gone from the island, plus 6 of its 11 native lizards and 2 of its 3 native bat species.

The bird I kept reading about is the ko'ko', or Guam rail. It can't fly, and it lived nowhere but Guam (it has since been introduced to Rota). Guam had an estimated 60,000 to 80,000 of them in the late 1960s and early 1970s. In 1985 the last wild ones were collected from the forest on Andersen Air Force Base for captive breeding.

What happens to a forest without birds?

Five of the lost species ate fruit and spread the seeds of about 70% of Guam's forest trees. Now seeds fall directly under the parent tree, where they are less likely to sprout or survive. For the two tree species studied, that means 61 to 92% fewer seedlings. Guam is becoming a different forest.

Haldre Rogers, the Iowa State University scientist behind that work, says Guam is the only place in the world that has lost all its fruit-eating birds, which makes the island an accidental experiment. Nobody asked the trees.

The other surprise: without birds, the spiders boomed. Guam's jungles have up to 40 times more spiders than nearby islands such as Saipan.

How is Guam fighting the brown tree snake?

You can't search every tree on 541 km² for an animal that looks like a vine, so the bait has to find the snake. The bait is a dead baby mouse with an acetaminophen tablet inside.

A newer aerial system fires 3,600 biodegradable bait cartridges at four a second, each designed to tangle in the canopy. In a trial, 66.6% of the properly opened baits caught in the trees, and snakes were confirmed to have taken 5.9% of the radio-tagged ones, about 1 in 17. Given the snake's numbers, some people on Guam wonder whether the drops are little more than a bandage on a very big wound.

The bigger job is keeping the snake on Guam. By 1997 it had been carried to Hawaii, 6,100 km away, on at least seven known occasions, and more than 40 had been seen in six years on Saipan, about 175 km north. Dozens of flights leave Guam every day, including at least one nonstop to Honolulu.

The ko'ko' got a second chance. In 2010, 16 birds were released on Cocos Island, 1,600 m long, 1.5 miles off Guam's southwest coast. By early 2020 there were 60 to 80. At the end of 2019 the IUCN had moved the ko'ko' from extinct in the wild to critically endangered, only the second bird ever to recover from extinct in the wild. The first was the California condor.

Then, in 2020, a breeding population of brown tree snakes was confirmed on Cocos. A 2026 local report counts 130 removed since, including a large female carrying 13 eggs in June 2025, and an estimated 24 to 36 ko'ko' left on Cocos, Guam's only wild ones. In two searches in 2025, 90 volunteers found no snakes. As numbers fall, the snakes get harder to detect, so the searching goes on.

Listen to the next tree

How did one snake empty Guam's forests? It reached an island with no large snakes, more rats than similar mainland forests and birds that had never met a hunter like it, and it hid so well that proving it took decades.

Next time you hear birds in a tree, they may be planting the next one.

Is there an animal that used to be everywhere near you, and now you hardly see it?

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Sources

1. Two thousand mice dropped on Guam by parachute to kill snakes (NBC News, December 2013)
2. Rodda, Fritts and Chiszar, The Disappearance of Guam's Wildlife (BioScience 47(9), October 1997)
3. Wiles et al., Impacts of the Brown Tree Snake: Patterns of Decline and Species Persistence in Guam's Avifauna (Conservation Biology, 2003)
4. Savidge, Extinction of an Island Forest Avifauna by an Introduced Snake (Ecology, 1987)
5. Brown treesnake and ko'ko' pages (Guam Department of Agriculture, Division of Aquatic and Wildlife Resources)
6. Fritts, Economic costs of electrical system instability and power outages caused by snakes on the Island of Guam (USGS, 2002)
7. Conserving the Last of Guam's Avifauna: The Recovery of the Guam Rail (Smithsonian's National Zoo and Conservation Biology Institute, January 2020)
8. Guam: Can Invasive Species Be Stopped? (National Geographic, November 2013)
9. ISU scientist details the indirect effects of an invasive species in Guam (Iowa State University, March 2017)
10. Spiders run amok on Guam: without birds (ScienceDaily, September 2012)
11. Siers, Goetz, Volsteadt and Nafus, Evaluation of lethal doses of acetaminophen for brown treesnakes (Management of Biological Invasions, 2021)
12. Siers, Pitt et al., In situ evaluation of an automated aerial bait delivery system for landscape-scale control of invasive brown treesnakes on Guam (USDA National Wildlife Research Center, 2019)
13. Endangered species on Cocos Island facing threat from brown tree snakes (Pacific Island Times, 6 November 2020)
14. Snake Blitz Returns (Jason Salas, 20 March 2026)