An annotated avian checklist of Bornadi Wildlife Sanctuary, Assam, India

. The Bornadi Wildlife Sanctuary (BWS) boasts a variety of habitats, including forest, riparian, degraded forest, and transitional zones located near human settlements and cropland. From 2019 to 2022, a thorough avian survey was conducted using the point count method, covering all habitats and seasons. The study documented 227 avian species from 65 families and 18 orders. Of the bird species observed, 72.3% were resident, and 27.7% were migratory. Over the study period, one Endangered species, three Vulnerable, and six Near Threatened species were recorded based on their IUCN Red List status. The avian diversity pattern in BWS is influenced by a variety of factors, including forest contiguity and heterogeneous habitat. Proper evaluation and management of habitats are essential for the recovery and preservation of fragile environments susceptible to decline in the region.


INTRODUCTION
Birds play a vital role in our ecosystem, with their presence, absence, or behaviour offering valuable insights into the health and biodiversity of an area.Bird surveys have become a standard tool for assessing the health of ecosystems, especially in areas where data is scarce or limited (Bibby et al. 2000).By monitoring bird populations, scientists and conservationists can glean valuable information on climate change impacts, land use practices, and habitat degradation (Gregory and Strien 2010).In less-known areas, birds can be crucial in identifying regions with biodiversity and conservation value (Martínez-Núñez et al. 2023).For example, in the Eastern Himalayas, where there is a high level of bird endemism, bird surveys have been used to pinpoint areas that require conservation attention (BirdLife International 2023).
The state of Assam state Northeast India is a biodiversity-rich region where two global biodiversity hotspots meet, namely, the Himalayan and the Indo-Burma regions (Myers et al. 2000).These areas have high species richness and contain endemic, migratory, and threatened species of various flora and fauna, including birds (Jenkins et al. 2013).Some 820 bird species are known to occur in Assam (Choudhury plantations, mainly tea gardens, are located along the southern boundary.BWS was declared a wildlife sanctuary in 1980 due to the presence of the endangered Pygmy Hog (Porcula salvania Hodgson, 1847) and Hispid Hare (Caprolagus hispidus Pearson, 1839).BWS is also home to several important Indian species, including Panthera tigris tigris (Linnaeus, 1758), Panthera pardus fusca (Meyer, 1794), Neofelis nebulosa (Griffith, 1821), Elephas maximus indicus (Cuvier, 1798), and Bos gaurus (C. H. Smith, 1827).
BWS has a subtropical, monsoon-type climate, with an average annual temperature of 22.5 °C and relative humidity ranging from 42% in February, to 96% in July.The average yearly rainfall measures 2,621 mm (Climate-Data.org2023).BWS is categorised within the North-East-Brahmaputra Valley (9A) province of the Northeast India biogeographic zone (MoeF 2009).
Our study area can be categorised into forest, degraded forest, riparian, and forest edge habitats.The forest is the primary habitat type and covers almost half (45.61%) of the area of BWS (Sarma and Sarma 2008).The forest type is classified as Eastern Himalayan Moist Deciduous Forest (Champion and Seth 1968).A significant portion of BWS is covered by degraded forest, primarily in the western and southwestern parts.Surrounding much of the degraded forest are settlements and tea plantations.Riparian habitat lines BWS's eastern and western boundaries adjacent to the rivers.These riparian habitats host small grassy patches, mainly consisting of Kans grass Saccharum spontaneum L. and cogon grass Imperata cylindrica (L.) Raeusch.Additionally, BWS encompasses a limited expanse of scrubland and grassland.In our study, we combined scrubland areas with the degraded forest category, while grassland areas were classified with the riparian habitat group due to their proximity and similar characteristics.

METHODS
Bird surveys were carried out between March 2019 and.V.2022, in all four seasons: winter, pre-monsoon, monsoon, and retreating monsoon.Sampling throughout these seasons ensured that migratory bird species were also included.During the nationwide COVID-19 lockdown, field visits were severely limited from May to August 2020 and April to July 2021.For the bird survey, 24 sampling stations were established, covering all four habitat types within our study area (Table 1).These sampling points were randomly selected from six grids per habitat, with a grid size of 500 meters on the map.We conducted point transect surveys with a fixed radius of 30 m (Bibby et al. 2000).Birds were observed in the morning (0630 hours) and evening (1600 hours).Unfavourable weather conditions such as fog, rain, and high winds were avoided due to their known impact on bird activity and detection (Bibby et al. 2000).All birds seen or heard within a 30 m radius were identified and recorded.Each count lasted for 15 minutes.Birds were observed using Olympus 8×40 binoculars, and photographs were taken of most observed species.No specimens were collected during our study.
For species identification, Birds of the Indian Subcontinent (Grimmett et al. 2011) was used in the field.The Book of Indian Birds by Ali (1979) and other resources, such as Birds of the World (2022), were consulted to confirm certain species.The nomenclature and taxonomy were followed as per the eBird/Clements checklist of Birds of the World (Clements et al. 2022).Birds were categorised as resident (R), local migrant (LM), summer visitor (SV), or winter visitor (WV) based on our field observation and description from Grimmett et al. (2011).The conservation and national protection status of species were determined following the IUCN Red List of Threatened Species (2022) and the Wildlife Protection Act of 1972, respectively.
Diversity was measured for each habitat type using the Shannon Diversity Index (Shannon and Weaver 1963).Shannon Diversity Index (H′) assumes that all species are evenly distributed in a sample and that the sample is randomly obtained.Shannon Diversity Index: Here, pi is the proportion of individuals found of the i th species, ln is the natural logarithm, and s represents the number of species.

RESULTS
During the survey period, 2,582 individuals from 227 avian species across 65 families and 18 orders were documented (Table 2).Passerines made up 53.6% (120 species) of the total number of species.The Accipitridae family had the highest diversity among non-Passeriformes with 15 species, while the Muscicapidae family was the most diverse family within the Passeriformes with 17 species.Of the 227 species recorded, 72.3% were resident, while 27.7% were migratory.Among the migratory species, 52 were winter visitors, 9 were summer visitors, and certain species, like Dendrocygna javanica (Horsfield, 1821), were classified as local migratory (Table 3).We observed highest species richness in the Forest habitat (117 species), followed by Edge (112 species), Degraded Forest (94 species), and Riparian habitat (81 species) (Table 2; Figure 3).In all, 43 species were exclusive to Riparian habitat, while Forest had 37, Edge had 27, and Degraded Forest had 19.Additionally, 20 species were present across all habitat types.Among specific abundances, Pycnonotus cafer (Linnaeus, 1766) was most abundant in degraded habitat, Acridotheres tristis (Linnaeus, 1766) in riparian habitat, and Treron phoenicopterus (Latham, 1790) in forest and edge habitats.The Shannon diversity index was highest for the Edge habitat (H′ = 3.95) and lowest for the Riparian habitat (H′ = 3.34).The rarefaction curve for our  sampling effort across the entire study area did not reach a plateau at its peak, suggesting the possibility of finding a few more species with additional survey effort (Figure 2).The rank abundance plot indicated that a few species were highly abundant compared to others in each habitat type (Figure 4).Regarding the IUCN Red List status, of the observed species, one species (Aquila nipalensis Hodgson, 1833) was Endangered, three species were Vulnerable, and six species belonged to the Near Threatened category (Table 3).Additionally, six species are listed as Schedule I species under the Wildlife Protection Act of 1972, ensuring high protection.The observation notes of some species recorded from BWS are described below.

Family Phasianidae
Gallus gallus (Linnaeus, 1758) Red Junglefowl Figure 5A Observations.Identification: Brown body with streaked pattern, white wings, bill yellow with black tip.

Bubulcus ibis (Linnaeus, 1758)
Cattle Egret Figure 5I Observations.INDIA  Identification.Small eagle with dark blackish hood and upperparts, throat and breast white, accompanied by rufous-coloured underparts.
Identification.Large-sized eagle with dark brown colour, primaries banded, iris brown.Pale rufous patch on the nape, light-coloured chin.
Identification.Large owl with dark brown upperparts and whitish buff underparts.Prominent ear tufts slant outward,tarsus fully feathered.

Upupa epops Linnaeus, 1758
Eurasian Hoopoe Figure 6G Observations.INDIA  Identification.Hornbill with distinctive black and white colouring, with ayellow bill and casque.Tail long and black, with white tips.Circumorbital skin blue.
Identification.Small blue-coloured kingfisher with barred blue-black crown and rufous ear coverts.Females similar except for a reddish lower mandible.
Identification.Large black and white coloured kingfisher with a prominent crest.Males with rufous patch on the breast, females have a rufous underwing patch.
Identification.Golden colouration on mantle and upper back.Black and white scale-like pattern on breast and underpart.Males have red foreheads and crests, females have black and white spotted foreheads and crests.
Identification.Small-sized falconet with black eye stripe, black upperparts and white underparts.
Identification.Green-coloured bird with long tail and red-coloured upper bill.Male has a pink line on the nape, joining a black chin stripe.Female lacks a pink line and chin stripe.
Identification.Brown-coloured bird with dark eye stripe, brown iris, and white supercilium.
Identification.Throat, lower back and rump white, tail greyish-brown.Male has a black mask, female has a pale brown mask.Identification.Yellowish-green males with black tail, females with green tail.Identification.Medium-sized warbler with yellow supercilium and crown stripe.Upperparts olive-green,underparts yellowish.
Identification.Grey-coloured bird with long, black, downcurved bill and red wing coverts.
Identification.Male has blackish-grey face, reddish-brown throat underparts, pale-brown and greyish upperparts.Female has brown upperparts.White iris and wing patch present in both sexes.Identification.Male has greyish-black upperparts with a white supercilium.Underparts mostly whitish, with slightly dark skin at the breast.Bill and legs black.Female lacks a greyish-black upperpart, but with rufous colouration and buff throat and supercilium.

DISCUSSION
BWS is a contiguous Transboundary Manas Conservation Area (TraMCA) forest spread over 6,500 km 2 in the eastern Himalayas (Ahmed et al. 2016).Due to its contiguity and diverse habitat types, high avian diversity is known in the study area.The high number of species that were found exclusively in riparian habitat, highlights the importance of habitat heterogeneity in promoting species richness.Such heterogeneity accommodates the specific requirements of various species, offering abundant food sources in forested, waterbody, and farmland areas, leading to enhanced bird richness (Tu et al. 2020).The present study demonstrates that the contiguity of the forested area of the sanctuary directly influences species richness in the forest habitat.Similar trends have been noted by Joshi et al. (2022) for specialist birds.Additionally, studies by Basile et al. (2021) and Asmare et al. (2023) have reported analogous findings, highlighting greater richness in forested habitats.Forest interiors provide enough food year round along with better nesting success.Conversely, open forests and edge habitats render birds more vulnerable due to challenges in finding consistent food sources and increased nest predation (Flaspohler et al. 2001).The BWS is situated in the foothill region of the eastern Himalayas, which might explain the high bird diversity in the sanctuary, particularly of the forest species, which is also evident from other studies (Bell 1984).
The avian species richness recorded here represents 27% of all the bird species known to occur in Assam (Choudhury 2000) and approximately 17% of bird species recorded from India (Grimmett et al. 2011).Despite its small size (26.22 km 2 ), BWS showcases remarkable avian species diversity.This study reveals that nearly 22% of bird species in BWS are winter visitors.Many warbler species found in BWS, including Phylloscopus trochiloides, P. reguloides, P. inornatus and P. humei, visit the region during winters, predominantly occupying forested areas and edges, thereby increasing species richness in these habitats.
Other migratory birds include water-dependent species such as Phoenicurus ochruros, Phoenicurus hodgsoni, Motacilla alba, and Motacilla cinerea.These species are commonly found near rivers with boulders and rocky banks.The rivers within BWS provide an ideal habitat for such species, given that they primarily consist of hill streams originating from high-altitude areas within the eastern Himalayan region of Bhutan before entering the sanctuary.Birds from higher altitudes in the Himalayas often migrate to lower elevations during winter.Certain species, such as the Spot-winged Starling (Saroglossa spilopterus), undertake longitudinal migrations along the Himalayas, descending to lower elevation areas as part of this process (Grimmett et al. 2011).Sighting of Ibidorhyncha struthersii, a winter visitor from high elevation river valleys in the Himalayas, is an interesting find, as it has previously only been reported from Manas National Park (eBird Basic Dataset 2022) and Nameri National Park in Assam (Barua and Sharma 2005).
The overall diversity pattern of BWS is influenced by several factors, including the contiguity of forests and the presence of heterogeneous habitats.The significance of the region is highlighted by the presence of globally threatened and nationally protected species.The absence of detection of Houbaropsis bengalensis in BWS during our study raises concerns about the quality of specific habitats.H. bengalensis requires grassland habitat, which is presently in degraded condition in the BWS (Collar et al. 2020).Protected areas sharing borders with human settlements and farmlands limit the movement and abundance of forest-dependent avian species while fostering generalist species' diversity and abundance in edge areas.Proper habitat evaluation and management practices are crucial for reviving and sustaining delicate habitats prone to degradation.These efforts will positively impact the avian community and further aid in the conservation of birds in the region.

Figure 2 .
Figure 2. Species rarefaction curve for the survey sampling effort.A. Among different habitat types B. Entire study area of BWS.

Figure 3 .
Figure 3. Overlap of bird species across different habitats in BWS, shown with a Venn diagram depicting the number of species observed.

Figure 4 .
Figure 4. Distribution of bird species abundance across the various habitat types of BWS.

Table 1 .
Geographical location of sampling stations in BWS.

Table 2 .
Summary of species richness, abundance and diversity in BWS.
*Numbers within parenthesis signify unique species observed only in the particular habitat

Table 3 .
Annotated list of birds from Bornadi Wildlife Sanctuary.Legend: IUCN: International Union for Conservation of Nature, LC: Least Concern, NT: Near Threatened, VU: Vulnerable, EN: Endangered, WPA: Indian Wildlife Protection Act, 1972, SC-I: Schedule List I species, SC-IV: Schedule List IV species, SC-V: Schedule List V species, MS: Migratory Status, R: Resident, LM: Local Migrant, SV: Summer Visitor, WV: Winter Visitor, De: Degraded habitat, Ed: Edge habitat, Fo: Forest habitat, Ri: Riparian Habitat, RA: Relative Abundance.
Identification.White-coloured, short-necked, stocky bird with a yellow bill.Breeding plumage has orangebuff on the head.
Large-sized hornbill with yellow bill, casque and neck.Body predominantly black, with a white tail bearing a black band.Male eyes have red iris, females have white.Large hornbill with black body, white tail, and whitish neck.Male distinguished by yellow gular pouch with black bar, female features a black neck and blue gular pouch.

Psilopogon virens (Boddaert, 1783)
; V. Chetry obs.; forest.Identification.Green-coloured bird with red forehead.Throat, upper breast, and auricular blue.Sexes similar.Largest barbet with a large yellow bill and dark bluish head.Wings, rump and tail green.Small bird with brown-greenish upper part and a very short black coloured tail.Dark eye stripe flanked by white-bordered stripes.Males have orange foreheads, females have uniform olive colouration on forehead and crown. Identification.