Which of these is the smallest extant flightless bird in the world?

The Inaccessible Island rail (Atlantisia rogersi) is a small bird of the rail family, Rallidae. It is the only species in the genus Atlantisia. Endemic to Inaccessible Island in the Tristan Archipelago in the isolated south Atlantic, it is the smallest extant flightless bird in the world. The species was described by physician Percy Lowe in 1923 but had first come to the attention of scientists 50 years earlier. The Inaccessible Island rail's affinities were a long-standing mystery; in 2018 its closest relative was identified as the South American dot-winged crake (Porzana spiloptera), and it was proposed that both species should be nested within the genus Laterallus.

A small species, the Inaccessible Island rail has brown plumage, black bill and feet, and adults have a red eye. It occupies most habitats on Inaccessible Island, from the beaches to the central plateau, feeding on a variety of small invertebrates and also some plant matter. Pairs are territorial and monogamous, with both parents being responsible for incubating the eggs and raising the chicks. Its adaptations to living on a tiny island at high densities include low base metabolic rates, small clutch sizes, and flightlessness.