Known in the cage bird trade as the Japanese Hill-Robin, Peking Robin, or Peking Nightingale, the Red-billed Leiothrix was first imported into the Hawaiian Islands in 1911 ( Fisher and Baldwin 1947 ), with intentional releases to the wild occurring after 1918 ( Caum 1933 ). A native of Southeast Asia, southern China, and the Himalayan regions of India, this species is a medium-sized, green and yellow babbler with a conspicuous red bill and strongly notched tail. The species is extremely active, but individuals are somewhat secretive and difficult to see as they flit around in the understory, often in small groups. The Red-billed Leiothrix is found in a wide variety of habitats in the Hawaiian Islands, including both native and exotic forests from sea level to near mountain summits exceeding 4,000 m elevation.
Leiothrix use a wide variety of native and introduced plants for foraging and nesting, and they feed on fruit and on invertebrates gleaned from foliage and dead wood. They forage and nest mostly among lower branches of dense vegetation, and rarely use canopy trees. The species is more gregarious and nomadic outside of the breeding season, when flocks of up to 100 birds have been observed. It sings most persistently during the breeding season, but also throughout the year, particularly when going to roost. Both sexes give a harsh, repetitive Chatter Call in response to human and other animal intruders.
Leiothrix populations have fluctuated widely on different islands. On O'ahu Island, this species was one of the most common birds seen on Christmas Bird Counts (CBCs) in the 1950s, but it declined dramatically in the 1960s. The O'ahu population persisted at barely detectable levels until the mid-1980s, but it has been increasing since then. In the 1940s, this may have been the most common bird species on Hawai'i Island, found from sea level in towns to the summits of Mauna Loa and Mauna Kea Volcanoes. Today, it is no longer common in many low-elevation sites and is rare in some districts of the island. The Red-billed Leiothrix was abundant on Kaua'i Island in the 1930s, persisting until the 1970s, but the breeding population may have disappeared since then (Male and Snetsinger in press). The cause or causes of these dramatic population fluctuations remain unknown.