Status and distribution of endemic and threatened birds of the Eastern Himalaya in Sikkim, India
Bhoj Kumar Acharya 1 & Lalitha Vijayan 2
1,2 Sŕlim Ali Centre for Ornithology and Natural History, Anaikatty, Coimbatore, Tamil Nadu 641108, India
Present address: 1 Department of Zoology, Sikkim Government College, Tadong, Gangtok, Sikkim 737102, India
Restricted range and threatened bird species need considerable attention from ecologists and conservationists compared to wide ranging and common species. These species are more sensitive to disturbance and invite immediate conservation concern (BirdLife International 2001; Lei et al. 2003, 2007; Wijesinghe & Brooke 2005; Pandit et al. 2007). Information on status and distribution of threatened and endemic birds, therefore, aids in prediction of disturbance level and execution of conservation measures at all potential sites of their occurrence (Stattersfield et al. 1998; Riley 2002; Robin & Sukumar 2002).
The Eastern Himalaya is represented by diverse habitats and has high variations in altitude and climate. This habitat supports a high diversity of birds and is one of the most diverse regions in the Orient (Crosby 1996). The Eastern Himalayan Mountain range is identified as an endemic bird area (BirdLife International 2001), which supports 22 restricted-range bird species of which 19 are endemic to the Eastern Himalayas (Stattersfield et al. 1998; Jathar & Rahmani 2006; IUCN 2009). The region also represents one of the largest concentrations of globally threatened birds in Asia. The principal threat to the avifauna of Himalaya is the loss of habitat; particularly tropical lowland, sub-tropical and temperate forests (Crosby 1996).
Sikkim lies within the Eastern Himalaya Endemic Bird Area (Islam & Rahmani 2004). Out of the 19 endemic bird species of the Eastern Himalaya, 10 are reported to occur in Sikkim. It also represents a relatively higher number of threatened bird species. Out of the 78 threatened birds of the Indian Sub-continent, 17 (1 Endangered, 3 Critically Endangered and 13 Vulnerable) occur in Sikkim (IUCN 2009). Similarly, 10 Near Threatened species of birds are also reported from Sikkim.
Baseline information on distribution of a few endemic and threatened birds of the Eastern Himalaya is available (Stattersfield et al. 1998; Mauro & Vercruysse 2000; Ali & Ripley 2001; BirdLife International 2001; Acharya & Vijayan 2007). No study has been conducted to asses the status of endemic and threatened birds in Sikkim. This paper discusses the status and distribution of the Eastern Himalayan endemic, threatened and near-threatened bird species in Sikkim. Conservation measures are also highlighted in brief.
Study area: Sikkim (27003’-28007’N & 88003’-88057’E), a small state of India, is situated at the western extremities of the Eastern Himalaya (Image 1). Within the 7096km2 area it has a varied climate, topography, vegetation and altitudinal ranges. As rainfall varies from 2000-5000mm, Sikkim is considered the wettest region in the entire Himalayan belt (Ali 1962; Mani 1974) with a relative humidity of 70-80% throughout the year. The climate varies from hot tropical at the lower elevations to cool temperate at the middle and arctic cold at the higher elevations. The altitude ranges from 300m to more than 8000m. Vegetation, which is chiefly determined by elevation and various physical and ecological factors, changes from tropical to sub-tropical, temperate broadleaved, mixed coniferous, sub-alpine and alpine. The alpine zone represents the typical trans-Himalayas.
Point count method (Bibby et al. 2000) was used along the predetermined transects for sampling birds. On encountering a bird, details such as species, number of individuals and the distance from the observer when first sighted were noted. Depending upon the vegetation and elevation of the study area, sampling sites were divided into five zones. These are - Zone I (<900m) - tropical semi-deciduous forests; Zone II (900-1800m) - tropical moist and broad-leaved forests; Zone III (1800-2800m) - temperate broad-leaved forests; Zone IV (2800-3800m) - temperate coniferous and broad-leaved forests; and Zone V (3800-4500m and above) – sub-alpine and alpine vegetation. Permanent points were established in each transect. Points were replicated 9-12 times covering different seasons during 2003 to 2006. In addition, surveys were also conducted covering various parts of the state.
The number of species and density of species in each zone were estimated. Density was calculated based on Reynolds et al. (1980); D = n*10000/ pr2C, where D = bird density (numbers/ha), n = total number of birds observed in all counts within the specific radius, r = specific radius (m), C = total number of counts conducted and p = 3.14. The information on altitudinal distribution was obtained from literature (Ali 1962; Stattersfield et al. 1998; Ali & Ripley 2001; BirdLife International 2001; Islam & Rahmani 2004) as well as field observations.
Five endemic species, namely Chestnut-breasted Partridge Arborophila mandellii, Wedge-billed Wrenbabbler Sphenocichla humei, Rufous-throated Wrenbabbler Spelaeornis caudatus, Ward’s Trogon Harpactes wardii and Giant Babax Babax waddelli were not observed during the course of this study. Among these species, Chestnut-breasted Partridge was reported to occur in habitats similar to zones I, II and III, Rufous-throated Wrenbabbler to zone IV and Wedge-billed Wrenbabbler to zone V (Ali 1962; Ali & Ripley 2001; Islam & Rahmani 2004) but the distribution of the rest two is not known.
Similarly, out of the 17 threatened and ten near-threatened birds of Sikkim (IUCN 2009), only three near-threatened species (Rusty-bellied Shortwing, Firethroat and Satyr Tragopan) were observed in Sikkim during this study. Species such as Palla’s Fish Eagle Haliaeetus leucoryphus, Black-breasted Parrotbill Paradoxornis flavirostris, and Beautiful Nuthatch Sitta formosa are reported to have a wider distribution range but the other species such as White-rumped Vulture Gyps bengalensis, Long-billed Vulture Gyps indicus, Red-headed Vulture Sarcogyps calvus, Baer’s Pochard, Aytha baeri, Greater Spotted Eagle Aquila clanga, Lesser Kestrel Falco naumanni, Blyth’s Tragopan Tragopan blythii, Black-necked Crane Grus nigricolis, Slender-billed Babbler Turdoides longirostris, Rufous-necked Hornbill, Aceros nipalensis and Wood Snipe Gallinago nemoricola have merely presence-absence records in Sikkim (Ali 1962; Ganguli-Lachungpa 1998; Ali & Ripley 2001; Islam & Rahmani 2004).
The result indicates that the population of these birds in Sikkim is very low. Habitat loss and fragmentation is considered the major factor for decline in population of threatened and endemic birds in the Himalaya and elsewhere (Crosby 1996; Mauro & Vercruysse 2000; BirdLife International 2001; Chettri et al. 2001, 2005; Riley 2002; Robin & Sukumar 2002; Pandit et al. 2007). It is reported that endemic birds are unable to utilize non-forest habitats resulting from deforestation and conversion of land to other uses (Lei et al. 2003; Wijesinghe & Brooke 2005; Vijayan & Gokula 2006). Fragmentation and loss of habitat of the restricted range species threatens their very survival (Lei et al. 2003). The study by Pandit et al. (2007) predicts that by the end of this century, a quarter of the endemic species of the Himalaya will be extinct due to habitat loss. Extinction of endemic species is primarily driven by the loss of potential breeding habitat (Mauro & Vercruysse 2000; BirdLife International 2001).
Maximum species of endemics and threatened birds in Sikkim are found in tropical broadleaved forests (zone II) and temperate broadleaved forests (zone III). The diversity of birds is also high in these zones (Acharya 2008). Tropical and temperate broadleaved forests are breeding ground for most of these species but human population pressures and various ongoing developmental projects have posed serious threats to these habitats. The forests in zone II is outside the purview of current protected area network (Acharya et al. 2008). Hence, conservation through community participation would be effective for conservation of birds in these habitats.
This study was undertaken as a part of the bird community study in Sikkim which was more focused to understand the distribution pattern of birds along elevation gradient (Acharya 2008). Hence, some species might have been missed or densities underestimated though special attention was given to endemics and threatened birds. Long-term specific study on these species provides better understanding of population status, distribution and threat. Hence, detailed study on the ecological aspects and habitat requirements is necessary for the execution of species-specific conservation measures.
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