Song-count surveys and population estimates reveal the recovery of the endangered Amami Thrush Zoothera dauma major, which is endemic to Amami-Oshima Island in south-western Japan

摘要

The Amami Thrush, Zoothera dauma major, is an endemic subspecies of the Eurasian Scaly Thrush that is distributed only on Amami-Oshima Island in south-western Japan. This bird was formerly considered to be a distinct species (Z. major) and was listed on the IUCN Red List as ‘Critically Endangered’ based on the small population size estimated in the early 2000s. To re-evaluate the conservation status of this bird, we estimated the number of singing males from song-count surveys conducted by an NPO with public participation from 2007 to 2013. An estimation that applied a distance sampling method revealed the number of singing males to be 945–1,858 up to 2012. A sudden increase in song counts was recorded in 2013, and the estimate increased to 2,512 in 2013. Based on the assumption that the sex ratio does not deviate from 1:1, simply doubling the number was considered to produce the estimated population size (number of males and females that are capable of breeding). The present study also confirmed that the Amami Thrush was more abundant in older forest with less open habitat, suggesting that forest maturity is an important factor for thrush abundance. The relative density of the invasive small Indian mongoose Herpestes auropunctatus probably affected thrush abundance before the early 2010s prior to mongoose eradication efforts. Our results suggest that thrush recovery was likely associated with forest regeneration and mongoose eradication. However, it is important to continue population monitoring approaches including public participation to promote further conservation of the Amami Thrush.