Aceros subrificollis occurs in south-east Myanmar, west, south-west and extreme southern Thailand, and northern Peninsular Malaysia. All previous records from northern Myanmar, India and Sumatra were the result of misidentifications. Historically described as relatively common to very abundant in Myanmar, there have been no recent records and it appears to have undergone a rapid decline, at least in the Sittang valley. In 1993, the Thai population was estimated at <1,000 individuals, although the recent discovery of a single roost of 900 birds in the extreme south suggests that this figure should be revised upwards. Several thousand individuals attend one or two roost-sites in northern Malaysia and this area clearly supports a key population. The Malaysian Nature Society has been monitoring seasonal migration movements since 2004 and has logged a maximum count of c.1,500 individuals at Belum Temengor IBA; this figure indicates a population reduction since surveys recorded >2,000 individuals two decades ago⁴. Recently, 150 individuals have been recorded flying to roost within Lampi Island Marine Park in southern Myanmar².