Study Area
This particular study focussed on river-dependent birds along snow-fed and perennial headstreams in the western Himalaya of India, specifically the Bhagirathi and Amrutganga rivers in Uttarakhand, and the Tirthan river in Himachal Pradesh (Fig. S1). Sites were selected over an altitudinal gradient from 330 to 3100 and represented a range of habitat types from near-natural environments in protected areas to river stretches affected by human activities such as farming or urbanisation at lower altitude. Climatological conditions vary from sub-tropical to temperate (Mathur and Naithani 1999; Sinha, 2021), with drainage varying from glacial meltwater, rainwater and underground springs. Areas above 1500m in the northwest Himalaya are highly seasonal with cold winters and mild summers (Barve, 2017).
Of the specific study locations, the Tirthan river (N 31.6396° E 77.401°) is a major tributary of the river Beas in the Indus system. Here, survey locations encompassed river reaches between 1400- 2300 m in the Great Himalayan National Park Conservation Area in the Kullu district where the natural terrain is characterized by numerous high ridges, deep gorges, and narrow channels (Mathur and Naithani 1999). The buffer zone of the protected area has hamlets with orchards and other small scale agricultural practices and game fishing for introduced Brown Trout (Salmo trutta) . Inside the protected area, the river flow is uninterrupted while the riparian zone is relatively pristine with conifers and broadleaf woodland. In the Gangetic headwaters, river reaches were sampled along the Bhagirathi (N 30.7564°E 78.5781°) and its associated low-order tributaries covering an elevation gradient between 300-3300 m. Riparian vegetation consists of conifers at higher elevations and subtropical vegetation at the foothills. In this non-protected area within the administrative districts of Uttarkashi, Tehri and Dehradun, habitats have been modified by a range of anthropogenic pressures from agriculture and settlements (Sinha et al., 2019). Sites in the Amrutganga valley (N 30.466°E079.269°) are part of the Kedarnath Wildlife Division in Chamoli district where riparian land-use ranges from well vegetated river reaches to small villages with traditional agriculture (Barve 2017) between elevations of 1400 m to 2650 m.