<P> However, due to the Guam and CNMI visa waiver program for certain countries, an eligibility pre-clearance check is carried on Guam for flights to the States . For travel from the Northern Mariana Islands to Guam, a pre-flight passport and visa check is performed before boarding the flight to Guam . On flights from Guam to the Northern Mariana Islands, no immigration check is performed . Traveling between Guam and the States through a foreign point, however, does require a passport . </P> <P> Most residents travel within Guam using personally owned vehicles . The local government currently outsources the only public bus system (Guam Regional Transit Authority), and some commercial companies operate buses between tourist - frequented locations . </P> <P> Believed to be a stowaway on a U.S. military transport near the end of World War II, the brown tree snake (Boiga irregularis) was accidentally introduced to Guam, which previously had no native species of snake . It nearly eliminated the native bird population . The problem was exacerbated because the snake has no natural predators on the island . The brown tree snake, known locally as the kulebla, is native to northern and eastern coasts of Australia, Papua New Guinea, and the Solomon Islands . It is slightly venomous, but relatively harmless to human beings; it is nocturnal . Although some studies have suggested a high density of these serpents on Guam, residents rarely see them . The United States Department of Agriculture has trained detector dogs to keep the snakes out of the island's cargo flow . The United States Geological Survey also has dogs that can detect snakes in forested environments around the region's islands . </P> <P> Before the introduction of the brown tree snake, Guam was home to several endemic bird species . Among them were the Guam rail (or ko'ko' bird in Chamorro) and the Guam flycatcher, both common throughout the island . Today the flycatcher is entirely extinct and the Guam rail is extinct in the wild but bred in captivity by the Division of Aquatic and Wildlife Resources . The devastation caused by the snake has been significant over the past several decades . As many as twelve bird species are believed to have been driven to extinction . According to many elders, ko'ko' birds were common in Guam before World War II . </P>

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