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November 5, 2020

For the first time, an invasive brown treesnake population has been found on Cocos Island, an 83.1 acre atoll located 1.5 miles off the southwest coast of Guam.  

Scientist holding brown treesnake
A USGS scientist holds an invasive brown treesnake.

The brown treesnake was a major contributor to the loss of nine of 11 native forest birds and significant population declines of several native lizards, bats and other bird species on Guam. They now pose a threat to the wildlife of Cocos Island.

Guam Department of Agriculture Division of Aquatic and Wildlife Resources is working with partners to better understand how many brown treesnakes are on the island and the best way to remove them.

“Thanks to the diligence of the members of the public who reported the brown treesnake sighting, we were able to quickly respond to the threat that the brown treesnake poses to the wildlife on Cocos Island,” said DAWR Wildlife Supervisor Diane Vice.

A local fisherman reported potential sightings of brown treesnakes to DAWR in September 2020, who then worked with the U.S. Geological Survey’s (USGS) Brown Treesnake Rapid Response Team to take quick action. If it were not for the careful attention and reporting of local residents, this population of snakes would remain unknown and its expansion unchecked. Surveys to better understand the number of brown treesnakes on the island are ongoing and the Rapid Response Team is also capturing and removing any snakes they find. To date, four adult snakes capable of breeding, as well as five juveniles, have been removed.

DAWR is working with USGS, the U.S. Fish and Wildlife Service and the U.S. Department of Agriculture’s Animal Plant Health Inspection Service Wildlife Services to develop a long-term plan for controlling and eradicating brown treesnakes on Cocos Island. These territorial and federal agencies, along with the private and territorial landowners, have a long history of collaboration that will be vital to the successful eradication of brown treesnakes from Cocos Island.

Cocos Island is a special area of wildlife conservation for DAWR because it is home to the only wild population of the endangered ko’ko’ (Guam rail) on Guam. One of the few bird species endemic to Guam remaining, ko’ko’ was successfully introduced on the island in 2010 by DAWR to save the species from extinction in the wild. Like many native bird species, the ko’ko’ was once abundant across Guam before the introduction of brown treesnakes contributed to their disappearance.

Other native wildlife has survived and thrived on the island, creating a natural home for animals that Guam residents and tourists enjoy. Cocos Island is a diversity hotspot for Mariana lizards, including the endangered Mariana skink. The island also provides habitat for såli (Micronesian starlings), fahang (brown noddy), chunge (white terns) and nesting beaches for endangered haggan (green sea turtles). The presence of invasive brown treesnakes could negatively impact all wildlife that call the island home. Many animals are prey for the snakes and their presence threatens Guam’s endemic ko’ko’once again.

The Department of Agriculture is asking all visitors and workers to please inspect vessels for snakes and other invasive species before leaving for Cocos Island. If you see a snake on locations other than Guam, please report it immediately to the Brown Treesnake Rapid Response Team on Guam at (671) 777-HISS (4477). Report any other invasive species, such as mice, rats, cats or dogs on Cocos Island to 488-Rail (7245). 

Should you have any questions, please contact the DAWR office at

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