 Bird species with a restricted range are especially vulnerable to human activities , and eight of the 26 bird species occurring only in the Northern Maluku Endemic 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 . 
 Bird species with a restricted range <a href='drop_aux' title='1'><mark>are</mark></a>especially vulnerable to human activities , and eight of the 26 bird species occurring only in the Northern Maluku Endemic Bird Area are threatened , including the invisible rail . Almost a quarter of all rail species <a href='got' title='2'><mark>have</mark></a> conservation concerns , and flightless island species are particularly at risk , at least 15 species <a href='been_done' title='3'><mark>having</mark></a> become extinct since 1600 . The estimated population of the invisible rail <a href='drop_aux' title='4'><mark>is</mark></a>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 . 
Bird species with a restricted range especially vulnerable to human activities , and eight of the 26 bird species occurring only in da Northern Maluku Endemic Bird Area are threatened , including tdainvisible rail . Almost a quarter of alll rail species got conservation concerns , and flightless island species r particularly at risk , at least 15 species done become extinct since 1600 . Thdastimated population of the invisible rail 3 @,@ 500 – 15 @,@ 000 birds , and its restricted range and small population mean that thedaecies is classified as vulnerable by the daernational Union for Conservation of Nature ( IUCN ) , although this rail is so poorly known dat it may be more common than the estimates suggest .