Although volcanic mountains and islands usually form where two crustal plates meet and crush against one another they also form over 'hot spots' on the earth's crust - areas lying above intense activity deep in the earth's mantle. Directly over the hot spot a volcano is formed. When the crust passes away from the center of activity the volcano becomes extinct and a fresh one then erupts alongside it, producing in time a chain of progressively older volcanic islands in the middle of the ocean. During the Neogene to Quaternary (starting around 5 million BC), a hot spot was responsible for producing the Hawaiian island chain, and in the Pacific during the Posthomic a hot spot is in the process of generating the Batavian Islands.
Birds are usually the first terrestrial vertebrates to reach and settle on new islands, but in the case of Batavia the first vertebrates to arrive were their mammalian equivalents, the bats. By the time that the birds did arrive, the bats were so well established that there were few unoccupied evolutionary niches left and the birds have only colonized the islands to a minimal extent. The presence of suitable food on the ground, and the absence of predators enabled many bats to take up a terrestrial existence and to fill a large number of ecological niches.
Once other vertebrates had established themselves on the islands (such as rodents and reptiles), a family of ground-dwelling predatory bats arose: the hand-walkers.