We examined the vertical stratification of birds in relation to foliage in different vegetation types along an elevation gradient in Sikkim, Eastern Himalaya, India. We used variable-width point count methods for sampling birds spread across 20 transects along an elevation gradient from 300 m to 3,800 m above mean sea level. We estimated species richness, abundance and Shannon-Weiner diversity (H′) of birds in seven height categories (0 m, 0–5 m, 5–10 m, 10–15 m, 15–20 m, 20–25 m and >25 m). Foliage structure and complexity of vegetation was assessed along all transects following Erdelen (1984) and Jayson and Mathew (2003). Birds displayed distinct vertical stratification in terms of species richness, abundance and diversity in Sikkim. Overall, maximum species richness (231) was observed at 0–5 m height followed by 5–10 m, 10–15 m and the ground layer (0 m). There was no significant difference in stratification pattern among elevation zones. Each height class harboured distinct species composition of birds with low similarity among height categories. We observed maximum foliage concentration within 10 m height from the ground, and the trend was consistent in all of the zones. Correlation analysis revealed significant positive relations between foliage abundance and species richness, abundance and diversity of birds. Results of this study have highlighted the significance of under-storey or sub-canopy vegetation in maintaining and conserving avifaunal diversity in the Eastern Himalaya.
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Vol. 16 • No. 2