The Critically Endangered white-bellied heron Ardea insignis is a large-bodied, range-restricted, piscivorous bird of the Himalayan mountains. With fewer than 60 individuals remaining globally, regular monitoring is required for this rare species. We present the results of surveys during 2017 and 2023–2024 in Namdapha Tiger Reserve, Arunachal Pradesh, India. The encounter rate of white-bellied herons in 2017 was 0.55 individuals per km of river, but we did not detect the bird during intensive surveys in the same location during 2023–2024. The surveyed area within the Reserve was once a stronghold for the species, but its population appears to be declining. Monitoring the population has helped reveal this decline, but identifying and conserving stretches of river still utilized by the remaining individuals is now critical.

The White-bellied Heron Ardea insignis is currently known to be restricted to Bhutan, Myanmar, and north-east India. This ‘Critically Endangered’ species is under threat from the ever-changing river systems, primarily due to anthropogenic pressures including the large number of proposed hydropower projects. We report results of the first systematic, large-scale river survey for this species in the states of Arunachal Pradesh and Assam in north-east India with the aim of establishing its evidence-based distribution and assessing the severity of threats on the river systems in the region. We conducted river surveys along all seven major river basins in Arunachal Pradesh (October 2017–March 2018) and one in Assam (January 2019) along with 200 key informant surveys from 23 localities across the region. We encountered the White-bellied Heron six times in only three of 81 sites surveyed. Three interviewees reported observing the herons in three new localities, but we did not have a direct observation of the birds in any new sites. From field observations during the river surveys as well as the interview results, we found that certain fishing methods, garbage, and sand/gravel mining could be potential reasons for restricted occurrence of the White-bellied Heron in the region. Hunting is also a threat that prevails across the region. Our study calls for governmental commitment for the protection of the species and its vulnerable ecosystem and focussed research on understanding the anthropogenic impacts on the heron.