Population Variability and Apparent Recent Decline of River Birds in the Indian Himalaya

Counting birds along a mountain river looks simple until you ask what the count actually means. Some species sit in the open on a boulder; others slip past under the noise of the water. Raw counts therefore mix real abundance with how easy a bird is to see — and that problem is acute in the steep, fast, logistically difficult rivers of the Himalaya, where almost no long-term monitoring exists.

We addressed it in the Bhagirathi basin of Uttarakhand, western India, surveying 43 river reaches of 500 m between 330 and 3,100 m above sea level, three times each summer for five years (2014–2018). Using N-mixture models, we estimated detection probability and true abundance for 12 species — six obligate river birds and six that use rivers less exclusively — and modelled abundance against elevation, flow and river width.

Detection probability was highest in flycatching insectivores tied closely to the channel (above 0.5) and lowest in the two fish-eating kingfishers (0.15). River passerines increased with elevation, while kingfishers and wagtails were concentrated below about 1,600 m. Brown Dipper was the one species whose numbers tracked faster flow most closely (p ≈ 0.05).

Most importantly, the models suggest that five species declined by 5–10% over the five years — Brown Dipper, Plumbeous Water Redstart, White Wagtail, Common Kingfisher and White-throated Kingfisher — while Blue Whistling Thrush increased. For three of the five, the direction matches national trends reported from citizen science. Five years is a short window and we are careful not to over-interpret it, but the method works where data are sparse, and the signal is consistent with the pressures these rivers face.

Read it in plain words: what we found on the river birds page.

Recommended citation: Sinha, A., Chatterjee, N., Ormerod, S. J., & Krishnamurthy, R. (2025). "Population Variability and Apparent Recent Decline of River Birds in the Indian Himalaya." Biotropica, 57, e70017.
Download Paper