Research
Research
Research at the intersection of avian ecology, global change biology, and applied conservation.
My research interest sits at the intersection of avian ecology, freshwater biology, global change science, and applied conservation. I combine long-term field monitoring, GIS, technology-assisted detection, population modelling, and conservation breeding to generate science that is both rigorous and directly actionable for species recovery. My work is currently centred on the critically endangered White-bellied Heron (Ardea insignis) — the world's rarest heron — across its range in the eastern Himalayas, with a framework designed to scale across threatened waterbird systems globally.
Current research on the White-bellied Heron is focused on the following areas:
Reproductive Ecology and Population Dynamics
Understanding the demographic mechanisms driving population decline is the foundation of any science-based recovery effort. This research addresses how threatened waterbirds such as the White-bellied Heron reproduce, survive, and respond to environmental and management interventions — generating the quantitative evidence base needed to evaluate extinction risk and guide conservation decisions.
With fewer than 60 individuals remaining, it represents one of the most data-deficient critically endangered waterbirds in the world. My research investigates breeding biology, nest habitat selection, chick provisioning and growth, reproductive success, adult survival, and long-term population trends. Central to this work is the deployment of long-range remote cameras at active nest sites to continuously record breeding behaviour — including incubation rhythms, parental attendance, nest disturbance events, and chick development — without disturbing the birds. This non-invasive approach has yielded the first detailed behavioural data ever recorded for this species. Combined with multi-year population survey data, I use integrated population models to estimate vital rates, assess extinction probability, and evaluate the demographic impact of alternative conservation scenarios.
Although this research is currently centred on the White-bellied Heron, the majority of large waterbirds across tropical and subtropical river systems are undergoing similar demographic declines. This work therefore contributes broadly to the population ecology of long-lived, low-fecundity birds facing accelerating habitat loss — a research agenda with implications well beyond any single species.
- Acharja et al. (2026). Increasing variability and declining breeding success: A major challenge in the conservation of the critically endangered Ardea insignis. Ornithology, ukag017.
- Acharja et al. (2025). Investigation of effective conservation measures for the critically endangered White-bellied Heron using an integrated population model and Bayesian PVA. Avian Conservation and Ecology, 20(1):19.
- Acharja, I. (2019). Evaluation of nest habitat, site preferences, and architecture of the White-bellied Heron in Bhutan. Bird Conservation International, 30(4):599–617.
- Acharja, I. (2019). Current population, distribution, and conservation status of the White-bellied Heron in Bhutan. Tropical Resources, 38:1–10.
- Khandu et al. (2020). First record of successful breeding of the White-bellied Heron in broadleaved trees. Journal of Animal and Plant Science, 30(2).
- Acharja et al. (2021). First observation of sexual conflict and parental infanticide in the White-bellied Heron. BirdingASIA, 35:38–42.
Phenological Shifts and Reproductive Failures
Climate change is reorganising the timing of biological events across ecosystems — altering when species breed, when prey becomes available, and whether these critical windows remain synchronised. For long-lived waterbirds with narrow reproductive margins, even modest mismatches between breeding phenology and environmental conditions can translate into population-level reproductive failure. This research investigates how climate variability and long-term climate trends disrupt breeding phenology, generate ecological mismatch, and drive declining reproductive success in threatened waterbird populations.
The White-bellied Heron serves as a sentinel species in Himalayan river systems — a climate-sensitive landscape where glacial melt, monsoon variability, and shifting temperature regimes are altering river hydrology and prey dynamics. Analysis of long-term nest monitoring data reveals increasing variability in breeding initiation dates and a measurable decline in reproductive success, consistent with climate-driven phenological disruption. Ongoing work integrates satellite-derived environmental data — river flow regimes, land surface temperature, and vegetation phenology — with nest-level reproductive outcome data to identify the environmental drivers of breeding success.
- Acharja et al. (2026). Increasing variability and declining breeding success: A major challenge in the conservation of the critically endangered Ardea insignis. Ornithology, ukag017.
- Acharja et al. (2025). Investigation of effective conservation measures using an integrated population model and Bayesian PVA. Avian Conservation and Ecology, 20(1):19.
Anthropogenic Threats, Ecosystem Integrity, and Conservation Risk
Freshwater ecosystems are among the most threatened on Earth, yet the cumulative impact of multiple simultaneous human pressures on freshwater-dependent species remains poorly quantified. This research develops frameworks for mapping, quantifying, and prioritising anthropogenic threats at the landscape scale — integrating freshwater biodiversity assessment, remote sensing, and multi-threat modelling to generate actionable conservation risk profiles for policy and management.
Current work is grounded in the Himalayan river systems of Bhutan — particularly the Punatsangchhu and Mangdechhu basins — where the White-bellied Heron serves as an ecological indicator of freshwater ecosystem health. Key threats under investigation include hydropower development and flow alteration, illegal and unsustainable fishing, sedimentation from land-use change, riparian forest degradation, and direct anthropogenic disturbance at breeding and foraging sites.
With ever-increasing pressure on freshwater ecosystems and waterbird habitats globally, this research is designed to scale into a transferable landscape-scale risk assessment framework applicable to freshwater-dependent threatened species across tropical and montane river systems — directly relevant to international freshwater biodiversity conservation priorities and IUCN Red List threat assessment standards.
- Maheswaran, Acharja et al. (2021). Save the white-bellied heron from extinction. Science, 373(6561):1317.
- Lungten, Acharja et al. (2023). Diversity of benthic macroinvertebrates in White-bellied Heron landscape, Bhutan. Int. J. Fauna and Biological Studies, 10(4):09–17.
- Dorji, Acharja et al. (2021). Biomonitoring of health of Chubachu stream using macroinvertebrate diversity. Bhutan Journal of Research and Development, 10(2).
- Tobgay, Acharja et al. (2022). White-bellied Heron Conservation Action Plan 2022–2031. RSPN & DoFPS, Bhutan.
- RSPN (2022). Ecosystem and Socio-economic Resilience Analysis and Mapping: White-bellied Heron habitats, Bhutan. BMUV, Germany.
- RSPN (2022). Developing Ecosystem-based Solutions for Managing Biodiversity Landscapes in Bhutan.
Conservation Breeding and Species Reintroduction
For species reduced to critically small wild populations, ex-situ conservation is no longer a last resort — it is an integral component of a science-based recovery strategy. This research addresses fundamental and applied questions in conservation breeding, genetic diversity management, and reintroduction biology, with the goal of developing replicable frameworks that bridge ex-situ programs and wild population restoration.
The White-bellied Heron Conservation Center in Bhutan — home to the world's first and only captive population of this species — serves as the living laboratory for this research. With no prior captive breeding history for this species anywhere in the world, this program is pioneering the development of husbandry protocols, reproductive management strategies, and genetic monitoring frameworks from the ground up. Alongside this, we are building the scientific foundation for a future reintroduction program: identifying and assessing candidate release sites, modelling habitat suitability under current and future conditions, developing soft-release and post-release monitoring protocols, and evaluating reintroduction success against demographic benchmarks.
- Acharja et al. (2021). White-bellied Heron Conservation Center Establishment and Management Plan. RSPN, Bhutan.
- Acharja, I. (2021). The White-bellied Heron Conservation Center: The center for conservation breeding, research, and information. RSPN, Bhutan.
- Acharja, I. (2021 & 2020). White-bellied Heron Annual Population Survey. RSPN, Bhutan.
- Acharja, I. (2023). Conservation of White-bellied Herons. Rewilding Success — Ecological Citizen.
Field Sites & Study Systems
Primary field sites span the White-bellied Heron's range in Bhutan — particularly the Punatsangchhu and Mangdechhu river basins, which together support the majority of the global population. These north–south-oriented river corridors are characterised by swift currents, rugged terrain, and narrow floodplains that provide critical ecological resources: shallow foraging areas, sand and gravel bars, and intact riparian forest. At the same time, these valleys concentrate human settlement, agriculture, road infrastructure, and resource extraction precisely because they offer the most accessible terrain in an otherwise mountainous landscape. Wildlife habitat and human land use are therefore in direct and intensifying competition along the same linear corridors — exposed to disturbance from settlement expansion, quarrying, riverbed dredging, road development, and hydropower infrastructure. This spatial overlap between ecological value and anthropogenic pressure makes these systems a priority for research that can identify and evaluate practical, targeted conservation interventions.