Rivers are affected by changes in catchment land-use and other modifications to their channel, floodplains and riparian zones. Such changes can affect biodiversity downstream, and specialist river birds might indicate the effects across multiple scales and through different ecological pathways. The risks of catchment-scale effects on rivers are especially acute in the Himalayan mountains, where the world's greatest diversity of river birds occupies one of the most rapidly changing riverine environments on Earth. Here, we use multivariate analysis on data collected over two years to investigate the distribution of this group of birds in relation to natural and anthropogenic variations in riverine habitats along one of the major headwaters of the Ganges. River bird distribution was linked to channel character, bank morphology, aspects of river flow and land use. Riverine specialists were associated significantly with the least modified reaches characterised by faster flows, exposed bedrocks, banks with pebbles, boulders with more intact riverine forests. Our data provide evidence from which to develop specialist river birds as cost-effective indicators of human impacts on river ecosystems, but further work is needed to separate the effects of natural and anthropogenic influences. Such work could also guide conservation action to help balance the exploitation of catchment ecosystem services with the protection of river biodiversity.

Natural riverine areas mark ecotonal habitats harbouring a characteristically diverse faunal assemblage, especially birds that also use these habitats as pathways crucial for their movement. Increasingly, riverine systems are subjected to large-scale habitat alterations due to climatic fluctuations and anthropogenic changes. Therefore, it is important to understand broad-scale community patterns for conservation planning and prioritisation for these ecotone habitats. The Bhagirathi river is one of the major headwaters of the river Ganges; despite its rich and diverse fauna, little is known about the bird species that inhabit this montane region. This study presents an extensive list of 281 bird species from 59 families, their seasonal distribution and habitat associations as recorded from field surveys along the riverine areas between April 2013 and May 2018. The present communication simultaneously discusses a few noteworthy sightings for the region and provides a baseline for future research on the distribution of birds in the Western Himalaya.

Abiotic and biotic factors drive compositional differences among local species assemblages. Determining the influence of different drivers on beta diversity patterns can provide insights into processes governing community organization. Examining beta diversity patterns along taxonomic, phylogenetic and functional dimensions enables a nuanced understanding of underlying processes that govern community assembly and dynamics. The dynamic and complex riparian habitats in the Himalaya, and the hyper-diverse riverine bird community offer a fascinating setup to examine the role of environmental factors in influencing community structuring. Using a large dataset on river bird communities from field census across multiple drainages in the Indian Himalaya, we aimed at discerning processes that structure these communities through an understanding of pair-wise dissimilarities in species composition across sites. We determined the relative contributions of turnover and nestedness in taxonomic, phylogenetic, and functional beta diversity patterns in the Eastern and Western Himalaya that differ in species richness. Generalized Dissimilarity Modeling was used to examine the relative contributions of climatic, geographic, and anthropogenic factors toward explaining different metrics of beta diversity. The riverine bird communities in the drier and seasonal Western Himalaya were poorer in species richness, more phylogenetically and functionally clustered than that in the Eastern Himalaya. The contribution of the turnover component to the overall beta diversity was higher than the nestedness component in river bird communities, particularly in the Eastern Himalaya. Habitat and climatic factors differentially influenced the beta diversity patterns in both Eastern and Western Himalaya, with river width consistently explaining a large variation in beta diversity in the east and the west. The results show that environmental filtering plays a crucial role in structuring riverine bird communities in the Himalayan headwaters, highlighting the need to ameliorate the threats posed by the slew of hydroelectric projects and forest loss in the region.

Heterogeneity in riverine habitats acts as a template for species evolution that influences river communities at different spatio‐temporal scales. Although birds are conspicuous elements of these communities, the roles of phylogeny, functional traits, and habitat character in their niche use or species' assembly have seldom been investigated. We explored these themes by surveying multiple headwaters over 3000 m of elevation in the Himalayan Mountains of India where the specialist birds of montane rivers reach their greatest diversity on Earth. After ordinating community composition, species traits, and habitat character, we investigated whether river bird traits varied with elevation in ways that were constrained or independent of phylogeny, hypothesizing that trait patterns reflect environmental filtering. Community composition and trait representation varied strongly with increasing elevation and river naturalness as species that foraged in the river/riparian ecotone gave way to small insectivores with direct trophic dependence on the river or its immediate channel. These trends were influenced strongly by phylogeny as communities became more clustered by functional traits at a higher elevation. Phylogenetic signals varied among traits, however, and were reflected in body mass, bill size, and tarsus length more than in body size, tail length, and breeding strategy. These variations imply that community assembly in high‐altitude river birds reflects a blend of phylogenetic constraint and habitat filtering coupled with some proximate niche‐based moulding of trait character. We suggest that the regional co‐existence of river birds in the Himalaya is facilitated by this same array of factors that together reflect the highly heterogeneous template of river habitats provided by these mountain headwaters.

Heterogeneity in riverine habitats acts as a template for species evolution that influences river communities at different spatio-temporal scales. Although birds are conspicuous elements of these communities, the roles of phylogeny, functional traits and habitat character in their niche-use or species’ assembly have seldom been investigated. We explored these themes by surveying multiple headwaters over 3000 m of elevation in the Himalayan Mountains of India where specialist river birds reach their greatest diversity on Earth. After ordinating community composition, species traits and habitat character, we investigated whether river-bird traits varied with elevation in ways that were constrained or independent of phylogeny, hypothesising that trait patterns reflect environmental filtering. Community composition and trait representation varied strongly with elevation and river naturalness as species that foraged in the river/riparian ecotone gave way to small insectivores with obligate links to the river channel. These trends were influenced strongly by phylogeny as communities became more clustered by functional traits at higher elevation. Phylogenetic signals varied among traits, however, and were reflected in body mass, bill size and tarsus length more than in body size, tail length and breeding strategy. These variations imply that community assembly in high altitude river birds reflects a blend of phylogenetic constraint and habitat filtering coupled with some proximate niche-based moulding of trait character. We suggest that the regional co-existence of river birds in the Himalaya is facilitated by the same array of factors that together reflect the highly heterogeneous template of river habitats provided by these mountain headwaters.