 Bird species with a restricted range are especially vulnerable to human activities , and eight of the 26 bird species occurring only in the Northern <unk> <unk> Bird Area are threatened , including the invisible rail . Almost a quarter of all rail species have conservation concerns , and flightless island species are particularly at risk , at least 15 species having become extinct since 1600 . The estimated population of the invisible rail is 3 @,@ 500 – 15 @,@ 000 birds , and its restricted range and small population mean that the species is classified as vulnerable by the International Union for Conservation of Nature ( IUCN ) , although this rail is so poorly known that it may be more common than the estimates suggest . 
