Journal of Threatened Taxa | www.threatenedtaxa.org | 26 December 2025 | 17(12): 28131–28134

ISSN 0974-7907 (Online) | ISSN 0974-7893 (Print)

https://doi.org/10.11609/jott.9992.17.12.28131-28134

#9992 | Received 11 June 2025 | Final received 28 October 2025 | Finally accepted 11 November 2025

A new bird record of Oriental Darter Anhinga melanogaster (Suliformes: Anhingidae) in Bhutan: range expansion and plastic entanglement mortality

Kelzang Dorji ¹, Khandu Subba ², Pema Dorji ³, Kaye L. Gutiérrez ⁴ & R.J. Gutiérrez ⁵

^1,2 Bhutan Birding and Heritage Travels, House No. 12, Olkakha, Thimphu, 11001 Bhutan.

³ Phibsoo Wildlife Sanctuary, Nichula Range, Lhamoizhingkha, Dagana, Bhutan.

⁴ 539 Old Roundhouse Road, McKinleyville, CA 95519, USA.

⁵ Department of Fisheries, Wildlife, and Conservation Biology, University of Minnesota, St. Paul, MN 55108, USA.

¹ Bhutan Birdlife Society, Norzin Lam Wog, Thimphu, 11001 Bhutan.

² Taba Lam Wog, Thimphu, 11001 Bhutan.

¹ bhutanbirdlifesociety@gmail.com, ² khandusubba1982@gmail.com, ³ pemadorji997@gmail.com, ⁴ westcottkaye@gmail.com,

⁵ gutie012@umn.edu (corresponding author)

Editor: H. Byju, Coimbatore, Tamil Nadu, India. Date of publication: 26 December 2025 (online & print)

Citation: Dorji, K., K. Subba, P. Dorji, K.L. Gutiérrez & R.J. Gutiérrez (2025). A new bird record of Oriental Darter Anhinga melanogaster (Suliformes: Anhingidae) in Bhutan: range expansion and plastic entanglement mortality. Journal of Threatened Taxa 17(12): 28131–28134. https://doi.org/10.11609/jott.9992.17.12.28131-28134

Copyright: © Dorji et al. 2025. Creative Commons Attribution 4.0 International License. JoTT allows unrestricted use, reproduction, and distribution of this article in any medium by providing adequate credit to the author(s) and the source of publication.

Funding: Self-funded.

Competing interests: The authors declare no competing interests.

Acknowledgements: We thank Dr. Gavin M. Jones, USDA Forest Service, Rocky Mountain Research Station and William J. Berigan, Department of Forest and Wildlife Ecology, University of Wisconsin for helpful comments on the manuscript. William J. Berigan also produced Image 1. We also thank Hishey Tshering, Bhutan Birding and Heritage Travels, for providing logistical support for field work.

Abstract: The Oriental Darter Anhinga melanogaster, ranges from Pakistan to Indonesia and the Philippines. Although it occurs in adjacent India, it has not been recorded in Bhutan. The first observation was of a live bird in a novel habitat, and the second a carcass, which indicates a potential threat from plastic pollution, as the cause of mortality was entanglement in plastic trash. We recommend that biologists and others carefully observe the newly created lakes supporting hydroelectric production in Bhutan because they may be creating novel habitats for species in Bhutan, as well as fostering range expansion of species into Bhutan.

Keywords: Darter, new record, novel habitat, observation, plastic pollution, population decline, threat, wetland habitat.

The Oriental Darter Anhinga melanogaster is distributed across southern and southeastern Asia ranging from Pakistan to Indonesia and the Philippine Islands (Orta et al. 2014). It is classified as a species of ‘Least Concern’ with an increasing population trend (Birdlife International 2024). Its status is enigmatic because it has been classified in the past as ‘Near Threatened’ owing to potential threats from habitat loss, pollution, competition with humans for prey, and has shown unexplained regional population declines with only minor expansions (Orta et al. 2014). We document the first confirmed record of this species in Bhutan, comprising a live individual in a novel habitat, and a carcass demonstrating mortality from plastic entanglement. We also add possible alternative explanations for this potential incipient range expansion.

First record of an Oriental Darter in Bhutan

On the morning of 20 April 2025, four of us (KD, KS, KG, & RJG) stopped alongside the Wangdue-Tsirang Highway (27.322⁰ N, 89.941⁰ E), overlooking a lake newly created by a recently constructed PunaTsangChu Hydroelectric Project dam (PunaTsangChu II) on the Puna Tsang Chu River in central Bhutan (Image 1) (Note: this lake was not yet depicted on Google Earth as of 30 September 2025, although the downstream dam site was visible). There, we observed and photographed an adult Oriental Darter. The only other waterbird we observed was a Great Cormorant Phalacrocorax carbo. Two of the authors (KD & RJG) simultaneously identified the darter using a spotting scope (Leica APO-Televid variable power) and binoculars (Swarovski EL 10 x 42), while others used binoculars. The darter was 200–400 m away on the far side of the lake. It was initially perched on an exposed branch of a fallen dead tree at the edge of the lake (Image 2). The darter then flew about 10 m to a small live tree or large shrub that was partially submerged in the water, approximately 7–10 m from the shore (Image 3). Although it is not clear if the bird moved in response to us because we were on a skyline above it, albeit distantly, it appeared to be aware of us because it made frequent intentional movements and ducked its head under the live vegetation. This is the first documented sighting of a live Oriental Darter in Bhutan.

Two of the authors (RG & KG) were most familiar with identifying Oriental Darters in the field, having seen them in India, Malaysia (Sabah), Thailand, and the Philippines, as well as observing the similar African Darter Anhinga rufa, Anhinga Anhinga anhinga, and Australasian Anhinga Anhinga novaehollandiae. This reservoir represents a newly available lacustrine habitat.

Past observation of a dead Oriental Darter in Bhutan

Subsequent to our sightings, KD alerted birders of the Oriental Darter observation in Bhutan through one of their social media networks. We were informed of a dead Oriental Darter found on 13 May 2021 at Lhamoizhingkha, Dagana, Bhutan (26.741⁰ N, 89.867⁰ E), which is approximately 0.5 km from the border with Assam, a state of India adjacent to Bhutan’s southern border (Images 1 & 4). This dead bird with plastic entangling its beak was found by PD floating in the Sunkosh River while he was on a forest patrol. Although the proximate cause of death (e.g., starvation, drowning, suffocation) cannot be determined retrospectively, the ultimate cause was likely the plastic entanglement as it would have impeded foraging, caused stress, and impacted the bird in other ways (e.g., Sigler 2014; Sazima & D’Angelo 2015; Ryan 2018).

The discovery of this dead bird does not prove it arrived alive in Bhutan on its own, especially because it was so close to India where it naturally occurs (Inskipp et al. 2012; Orta et al. 2014), but a natural arrival is far more likely than it being brought into Bhutan (alive or dead) illegally or by a predator given the plastic entanglement. Hence, this bird found dead may represent the first record of an Oriental Darter in Bhutan.

Discussion

Extralimital observations of birds are a topic of interest among birders and scientists – amongst the former due to the excitement of finding ‘vagrants’, and the latter because the phenomenon of vagrancy is not fully understood; new location records could portend something more important – range expansion (Veit et al. 2022). In our case, there are three potential alternatives to explain the live darter’s appearance in Bhutan: 1. ‘traditional’ explanations (e.g., genetic maladaptation, behavioural incompetence, or stochastic events (like storms)) (Byju & Raveendran 2022; Veit et al. 2022); 2. climate change/anthropogenic change resulting in development of suitable climate in a new location (or the opposite – climate changes driving dispersal), changes in food supply, and human induced habitat change (e.g., after visiting Bhutan, RJG and KG visited Krabi, Thailand, where they observed an Oriental Darter in a mangrove forest, and were told by a local long-time bird guide (Surasit Khueawan, pers. comm. 12 May 2025) that Oriental Darters had only appeared in the area in the past two years, which may be related either to food supply or anthropogenic changes); and 3. Population dynamics (increases in population leading to population expansion via normal dispersal), driving range expansion and colonization of new areas or novel habitats – such as this newly created lake (Zawadzki 2019; Veit et al. 2022).

In this case, the construction of new hydroelectric dams in the foothills of Bhutan may be creating novel habitats for darters and other birds to colonize that were previously not, or not widely, available in Bhutan. The dammed river sustains a fish population that supports birds that prey on fish. The adjacent forest provides potential nest sites for fish-eating birds such as darters and herons (Ardeidae). The rising waters of an impoundment lake will likely inundate adjacent trees, kill them, and those dead trees will provide suitable perching and roosting sites for darters, as shown in Images 2 and 3. Thus, a suitable prey base, suitable nest sites, and roost sites, coupled with the change from a flowing stream to a placid lake, provide key features associated with Oriental Darters (Orta et al. 2014). Of course, determining the response of Oriental Darters to anthropogenic change and population dynamics in terms of dispersal into Bhutan requires more information than we have, but many non-migratory vagrant birds are found near the edge of their ranges, which may be the case here.

Hydroelectric development often has deleterious conservation impacts owing to the potential environmental damage it causes (e.g., loss of terrestrial habitat, disruption to fish migrations, disruption of natural hydrological cycles (Baxter 1977)). That said, depending on how lakes develop behind dams, they may provide future habitat for some waterbirds, and as in this case, for bird species that occur in adjacent India that are also impacted by climate change or other stressors. Therefore, we recommend that biologists, conservationists, and birders consider newly created impoundments in the Himalaya Mountains as potential novel lake habitats that could be colonized by both resident and extralimital bird species rather than assuming they are ‘bird-sterile environments’. In our case, this particular Puna Tsang Chu River system may be an important area for colonization by Oriental Darters in Bhutan, given that it flows into India, which has a known population of Oriental Darters, both darters we reported here were found on this river, and the river now has novel habitats owing to hydroelectric development.

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