The remote Chatham Islands lie way to the south of New Zealand and here, in isolation, two species seem to have developed from an ancestral stock that resembled the Buff-banded Rail. The less evolved of these is Dieffenbach’s Rail, and this form seems to have developed from a comparatively recent invasion of the ancestral birds.
Indeed there is some doubt as to whether or not this form had actually passed beyond the stage at which it could be regarded as just a subspecies. Apart from some minor plumage differences, it sports a sharply downturned beak and this is clearly an adaptation to the conditions prevailing on the island. Nothing is known of the bird in life. It is known from just a single specimen collected during the 1840’s and now in the collection of the Natural History Museum, London.
Trav. New Zealand 2: 197