Read this and answer the question. If the question is unanswerable, say "unanswerable".

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 while 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.

What is thought to have been the first bird species on Guam?
unanswerable