The majority of the state falls within this ecoregion, which is formed by Ganga and the Himalayan rivers that feed into it. This agrarian zone is thickly populated but has several large and small wetlands, both natural and man-made, such as the Farakka Barrage (24.799°N, 87.946°E), an Important Bird Area (IBA) here.

Almost the entire western portion of the state falls under the ecoregion of the Chhota-Nagpur dry deciduous forests. These arid regions, falling mostly in the districts of Purulia, Bankura, and Birbhum, have contrasting habitats, attracting species that are resilient to dry conditions but are comparatively less diverse than those in the Himalayan zone. Another ecoregion, the East Deccan moist deciduous forests, almost reaches the south-western extremities of the state and hence shares some biodiversity.

The largest deltaic formation in the world, the Ganga-Brahmaputra delta or Sundarbans, is shared between Bangladesh and WB and is a UNESCO World Heritage Site. Two important ecoregions are Sundarbans freshwater swamp forests and Sundarbans mangroves. Although low in diversity, the relatively undisturbed nature of the ecosystem forms a refuge for several wetland-dependent biodiversity including those of intertidal wetlands, mudflats, and islands.

The systematic study of avifauna in WB finds its roots in the British colonial period of the late 18th century. British naturalists and sportsmen, driven by a keen interest in the natural world, diligently recorded their observations in diaries and journals (Rahmani 1999). However, in terms of growth in ornithological knowledge, we can sub-classify the three centuries of ornithology into five distinct eras.

The year 1832 marked a significant milestone with the commencement of the Journal of the Asiatic Society of Bengal in Calcutta, providing a platform for scholarly discourse on the region’s biodiversity. Notably, Edward Blyth (1810–1873) served as the curator of Zoology at the museum of the Asiatic Society of Bengal in Calcutta. He himself collected specimens around Kolkata and received specimens from across the country from other workers, some of whom were also from WB. We have used his catalogue (Blyth 1852) extensively for our research.

The year 1832 marked a significant milestone with the commencement of the Journal of the Asiatic Society of Bengal in Calcutta, providing a platform for scholarly discourse on the region’s biodiversity. Notably, Edward Blyth (1810–1873) served as the curator of Zoology at the museum of the Asiatic Society of Bengal in Calcutta. He himself collected specimens around Kolkata and received specimens from across the country from other workers, some of whom were also from WB. We have used his catalogue (Blyth 1852) extensively for our research.

Allan Octavian Hume (1829– 1912) played a pivotal role in advancing ornithological knowledge through his editorship of the journal Stray Feathers. Although Hume was not stationed in WB for a long time, some of his work, such as the specimens he obtained from the Kolkata [=Calcutta] market (Hume 1879), served as an important resource, at times substantiating the previous literature from Blyth. This period also included early time collectors from northern Bengal (hereinafter, N. Bengal), such as Louis Mandelli, William Edwin Brooks, William Ruxton Davison, and William Thomas Blanford, who contributed information to Stray Feathers through Hume. As Hume’s collection is considered the single largest collection of South Asian birds; almost entirely housed in the Natural History Museum, London (NHMUK), it is a major source of historical information.

C.M. Inglis (1870–1954), over an extended period of contributing to the ornithology of eastern India, was also the curator of the Darjeeling Museum, operated by the Bengal Natural History Society. His specimens, mostly in NHMUK and Darjeeling Museum, still serve as the only evidence of several species found in the terai and lowland forests of N. Bengal (Inglis et al. 1920a; b; Inglis 1951a; b).

Biswamoy Biswas (1923– 1994), probably the first Indian avian taxonomist, served as an ornithologist at the Zoological Survey of India (ZSI) in Kolkata. During the period spanning from 1964 to 1970, he spearheaded a comprehensive bird survey focusing on migratory species in the vicinity of Kolkata. He also collected numerous specimens from various regions of the state, and almost all of them are in the collections of ZSI. In later years, he was actively involved in advocacy and conservation.

After a lull period in general ornithology, birdwatching and photography as a hobby emerged in the present century, with an active interest in filling the gaps in our knowledge. Some of the first attempts to compile a state checklist were also initiated by Sumit Sen in 2002, who documented a total of 865 species in his now defunct website. In 2004, Apurba Chakraborty and Kushal Mookherjee published a list of 803 species with both English and Bengali names (Chakraborty & Mookherjee 2004). Citizen science and social media platforms are abuzz with several fantastic discoveries of birds from the state. We feel that it is the right time to update the prior efforts of all past workers by publishing a complete state checklist. While ornithological knowledge advanced, there were also serious efforts to communicate knowledge in Bengali. Satya Churn Law (1888–1984) authored three Bengali books on birds, namely, ‘Pakhir Katha’ (1921), ‘Kalidaser Pakhi’ (1934) and ‘Jalachari’ (1935), contributing significantly to the local ornithological literature. Ajoy Home (1913–1992) added more to Bengali literature through his books, namely, ‘Banglar Pakhi’(1973) and ‘Chena Achena Pakhi’ (1995), which encapsulated four decades of exploration in the wilderness of eastern India. Pradyot Kumar Sen Gupta (1898–1979) documented birds in and around Santiniketan and Kolkata. His personal observations and discoveries on birds were published by him in Bengali daily, Visva Bharati, and his writings were posthumously published in 2019 as ‘Birds around Santiniketan’.

After a lull period in general ornithology, birdwatching and photography as a hobby emerged in the present century, with an active interest in filling the gaps in our knowledge. Some of the first attempts to compile a state checklist were also initiated by Sumit Sen in 2002, who documented a total of 865 species in his now defunct website. In 2004, Apurba Chakraborty and Kushal Mookherjee published a list of 803 species with both English and Bengali names (Chakraborty & Mookherjee 2004). Citizen science and social media platforms are abuzz with several fantastic discoveries of birds from the state. We feel that it is the right time to update the prior efforts of all past workers by publishing a complete state checklist. While ornithological knowledge advanced, there were also serious efforts to communicate knowledge in Bengali. Satya Churn Law (1888–1984) authored three Bengali books on birds, namely, ‘Pakhir Katha’ (1921), ‘Kalidaser Pakhi’ (1934) and ‘Jalachari’ (1935), contributing significantly to the local ornithological literature. Ajoy Home (1913–1992) added more to Bengali literature through his books, namely, ‘Banglar Pakhi’(1973) and ‘Chena Achena Pakhi’ (1995), which encapsulated four decades of exploration in the wilderness of eastern India. Pradyot Kumar Sen Gupta (1898–1979) documented birds in and around Santiniketan and Kolkata. His personal observations and discoveries on birds were published by him in Bengali daily, Visva Bharati, and his writings were posthumously published in 2019 as ‘Birds around Santiniketan’.

The Global Biodiversity Information Facility (GBIF, www.gbif.org) is an excellent database for accessing specimen catalogues of ornithology collections. There are records of 4,455 preserved skins in this database, with the state/province being ‘West Bengal’. However, some of the skins collected within the political boundary of WB do not have the state/province entered into the database but have localities mentioned, typically ‘Calcutta’, ‘Darjeeling’, etc. These were included in a separate query (full details of the GBIF query in the supplementary information). Some of the catalogues have not been ingested into GBIF (see Table 1); in such cases, we resort to published catalogues (Blyth 1849, 1852; Horsfield & Moore 1854; Sharpe 1877, 1879, 1883, 1888; Seebohm 1881; Sclater 1888, 1892; Reid 1890; Sharpe & Ogilvie-Grant 1892, 1898; Ogilvie-Grant 1893; Salvadori 1893, 1895; Abdulali 1968, 1969a; b; c, 1976, 1983; Mukherjee & Dasgupta 1986; Saha et al. 1997; Zarrin 2023) and, at times, personal contacts with curators or researchers (see Acknowledgements). Very rarely, the reference to a specimen would be from a research publication, mostly a treatise on a specific species. If a particular specimen is unusual, we always contacted the respective curators to obtain photographs of the specimen and the label to help assess the record. We usually received excellent support from museum curators within India and abroad, except for rare instances (e.g., ZSI collection) where support has not been forthcoming. We have taken a cautious stand while evaluating important specimens that we were not able to verify to our satisfaction. All additional specimen details we collected during this verification exercise are included in the supplementary information. Despite the existence of several egg collection records and a few skeletons from WB, their inclusion was deemed uncertain and therefore not accepted as evidence due to potential inaccuracies. While assessing historical specimen records, we were particularly challenged with records that had localities marked as the ‘Calcutta market’, ‘Darjeeling’ or ‘Duars’ (or variants such as ‘Bhootan Duars’). While the identification of such specimens was beyond doubt, on the basis of photographs from the respective museums, the locality itself did not have sufficient provenance to consider the species for the WB checklist. The specimen tags of such specimens or associated literature often do not offer reasonable clarity to assess the locality unequivocally. Hence, we developed our own criteria to address these three instances.

The ‘Calcutta market’ was not a single site but rather a term encompassing various markets situated near wetlands, both north and south of the city. During winter, migratory birds were captured using nets and sold primarily in these markets for their flesh. Blyth and Hume frequented these markets, as they served as convenient locations for acquiring bird specimens. The specimens here were procured within a 25 mile [=40 km] radius, with the majority originating within a 10 mile [=16 km] radius of the stalls where they were sold (Hume 1879:480). However, he also stated that some ducks and geese were brought from other places (Hume 1879:498), and this is supported by other notes, e.g., Finn recounted purchasing three Lesser White-fronted Geese A. erythropus from ‘a bird-dealer in the Provision Bazaar who constantly imports birds from up country…these came from somewhere in the direction of Rawal Pindi’ (Finn 1898a:6), in Pakistan. In all the cases, the specific identity of the specimens was beyond doubt. While authorities (Ali & Ripley 1987; Grimmett et al. 2011; Rasmussen & Anderton 2012a) consider most of these records valid, their scope at the national or South Asia level does not necessitate them to validate whether the birds were actually collected within the borders of WB, which, however, is a matter of concern for us. We accepted specimens from the market in cases where the rough geography from where the specimens were collected was also known, usually through the writings of Blyth or Hume. We presume that these workers would have interacted with the vendors to gather the provenance before documenting the localities. We have been extra careful with Anatids, and if they remain the lone evidence for its presence in WB, they are treated with greater caution. In each case, the evidence details and deliberation before a decision is made are documented in the results, while more details are available in the supplementary material. In addition, Blyth also referenced some specimens as being from ‘near Calcutta’, ‘vicinity of Calcutta’, and ‘neighbourhood of Calcutta’. From the perspective of the habitats and species involved, we believe that these locations are likely to include Saltwater Lake [=Salt Lake], Kolkata [=Calcutta] Botanical Garden, Hooghly, and Budge Budge areas. These sites are close to Kolkata and fall within a 47 km radius; the majority of the specimens with these annotations appear to have come from sites within an 11 km radius of the city. Hence, we feel safe in concluding that such sites can be treated as within WB. While there is no way to verify whether any of these specimens have originated in the market, our assumption is that, Blyth, a methodical worker, he was, must have been careful in assessing the provenance from his sources before ascribing these sites in the literature.

Darjeeling, a hill station in N. Bengal, was the seat of the British administration for a long period of time. Initially, an area of 350 sq. km, Darjeeling was gifted to the British East India Company by the Raja of Sikkim in 1835. In 1864, the Bhutan Dooars, with passes leading into the hills of Kalimpong, were also ceded to the British, and the Darjeeling District assumed its present shape in 1866, with 3,196 sq. km. Although technically, this would make any specimen collected after 1866 from Darjeeling a part of the WB checklist, its historical ties with Sikkim would have created a more lasting memory among people who sometimes refer to these areas as Sikkim ([https://darjeeling.dcourts.gov.in/about-department/history/](https://darjeeling.dcourts.gov.in/about-department/history/)); hence, some of the 19th C Sikkim specimens could have originated from present-day Darjeeling. Conversely, some early time collectors, such as Mandelli (see Prys-Jones & Rasmussen 2018), are well known to have used the services of local collectors to obtain specimens across the hills of Bengal and Sikkim and yet labelled them ‘Darjeeling’. This is particularly concerning, as a nontrivial number of trans-Himalayan species have been reported with ‘Darjeeling’ as the locality. The highest peaks around Darjeeling, Sandakphu and Phalut are at elevations of less than 3,700 m and are extensions of continuous ridges from the Kanchenjunga massif, beyond which lies the trans-Himalaya. While there are no alpine-meadows or highaltitude rivers such as those of Sikkim today, there are still sizeable areas in the Darjeeling hills that are snowclad in the winter months. However, we also do not have evidence to justify the existence of such alpine habitats in these ranges 150–200 years ago. We do not have a straightforward solution that can address all these cases; although we approached them case by case, we also formulated some general rules. All collections that have a more precise collection locality than just ‘Darjeeling’, either by virtue of specimen labels or supporting literature, were accepted. The Darjeeling District has a wide range of altitudes (c.300–3,700 m), and species that are likely to occur at low and middle elevations until approximately c.3,000 m were readily accepted. In contrast, birds that are otherwise strictly trans-Himalayan and have low dispersal capacities, such as Pheasants, were excluded. The presence of appropriate habitat, present or historical, was another criterion that was considered along with other evidence. The distribution of these species in Sikkim and the nearness of their present range to the higher ranges of Darjeeling were also factors. Several standard works (Ali & Ripley 1987; Rasmussen & Anderton 2012a) have accessed the collections from Darjeeling and the assessments followed by them, specifically if they mention ‘Darjeeling’, were also given weight. Certain species are well known to move to lower altitudes during inclement weather in the Himalaya and such species were favourably considered; this is particularly true if a sister species has occurred as a climatedriven vagrant in Sandakphu or adjacent ranges. Multiple lines of evidence, where available, were carefully considered, specifically to avoid multiple references pointing to the same record.

The Duars or Dooars, a variant of “Doors”, implying a gateway, is a low-altitude Terai region situated just south of the Eastern Himalayas in WB and Assam. The historical references to Sikkim Duars, Bhutan Duars, Buxa Duars, and Eastern & Western Duars in species habitat descriptions have caused significant confusion (see Fig. 2). The ‘Bengal Duars’ was established after the Bhutan War of 1865 and merged with the Jalpaiguri District in 1869, making it entirely within WB. The ‘Western Duars’ extends from the Teesta River in WB to the Goalpara District in Assam, whereas the ‘Eastern Duars’ lies entirely within Assam. The ‘Buxa Duars’, stretching from the Torsa River to the Sankos River, and the ‘Sikkim Duars’, west of the Jaldhaka River, are both fully within WB. There are no Duars areas in Sikkim; they are all now part of WB. The ‘Bhootan Duars’ [= ‘Bhutan Duars’] lies east of the Jaldhaka River, encompassing the Jalpaiguri District and parts of Assam. All specimens that were tagged as ‘Bengal Duars’, ‘Sikkim Duars’, and ‘Buxa Duars’ were accepted as from WB. However, specimens from ‘Western Duars’ and ‘Bhutan Duars’ were accepted only if they were tagged as ‘Bhutan Duars, Bengal’ or ‘Western Duars Bengal’. On a related note, ‘Sikkim Terai’ lies to the western side of the Jaldhaka River, and ‘Darjeeling Terai’ comprises the western part of the Teesta River, both of which are fully in WB; Sikkim does not have a terai zone.

All photographs and audio recordings in the Macaulay Library of eBird (www.ebird.org) were compiled to create a list of species supported with media evidence. We also actively searched social media and other citizen science platforms, such as iNaturalist (https://www.inaturalist.org/) and XenoCanto (https://xenocanto.org/), and encouraged observers to upload them into a single database, which is eBird. We also used a group account of our society in eBird to upload the scans of photographs published in newspapers or journals. While most of the identifications are straightforward with data quality measures of eBird, there are still some cases where a commentary is required for their inclusion. We provide annotations in select cases where identification or provenance is involved.

As in other state checklists, these form the last stream of evidence and are the most difficult to evaluate. Our evaluation process involved meticulous scrutiny of the level of detail and supporting notes provided by original observers. All of these records were also uploaded in eBird, either through observers or through our group account. We have provided annotations for each of them and provided our assessment.

757 have support of museum specimens that were examined by a competent authority, 868 have support of photographs, and seven others are included solely on the basis of well-documented field descriptions (Table 2, Fig. 3). Among the specimen records, 603 were directly accessed from the GBIF, while 154 were collated from other sources. Among the 154, 38 have now been lost and most relate to Blyth’s specimens at the Indian museum (see Sclater 1892). There are 55 species that are included solely on the basis of specimen evidence. Seventy species are listed as unconfirmed records, where the evidence available was equivocal for inclusion.