Why did Guam's native forest birds have little protection against the brown tree snake?

Guam’s native forest birds had little protection against the brown tree snake primarily because they had no prior evolutionary experience with such a predator.1

In ecological terms, islands like Guam are particularly vulnerable to invasive species because their native wildlife often evolves in isolation, lacking the defensive adaptations necessary to survive encounters with new, aggressive predators. When the brown tree snake was accidentally introduced to the island—likely as a stowaway on military transport following World War II—it encountered a naive bird population that was unprepared for its hunting behavior.123

The snake is a highly effective, arboreal predator that can easily access nests to consume eggs and young birds. Because the native birds had not developed mechanisms to recognize or evade this specific threat, the snake was able to systematically decimate their populations, leading to the extinction of numerous endemic species.213