Population Characteristics of the Introduced Brown Tree Snake (Bioga irregularis) on Guam

Population characteristics of Boiga irregularis, a vertebrate predator introduced to Guam in the late 1940s or early 1950s, were studied. Size class distributions reflected prey availability. Large snakes (> 150 cm) were rare in forest habitat, where their preferred food (birds and mammals) were scarce or nonexistent, yet were commonly found in urban locations that supported a variety of vertebrate prey. Males reached larger body sizes than females in both urban and nonurban habitats. The sex ratio was about equal in the smaller size classes (<120 cm), but became increasingly male-biased with increasing snout-vent length. Snakes colonizing a previously snake-free area were significantly larger than individuals present in an area that had been colonized at least a decade earlier. Night searches in forest habitat indicated snake densities in northern Guam were about 1.6 times greater than those in southern Guam. A conservative density estimate of approximately 16 snakes/ha was obtained by grid trapping in an area where birds had been essentially eliminated by snakes. THE BROWN TREE SNAKE, BOIGA IRREGULARIS, was introduced to Guam in the late 1940s or early 195 Os, possibly as a stowaway on military cargo ships traveling between the Solomons or New Guinea and Guam. B. irregularis was first reported at Naval Magazine in south central Guam in the early 1950s and then spread throughout the rest of the island (Savidge 1987). Within the past several decades, its population has increased dramatically and research suggests B. irregularis has been responsible for the virtual extinction of Guam's native forest avifauna (Savidge 1986). The snake has few predators on Guam; other than humans, Monitor lizards (Varanus indicus) have been observed killing B.