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.
1. Tropical deciduous forests show strong seasonal variations due to temporal dynamics of precipitation and temperature and therefore, resource availability for animals are also limited accordingly. Certain harsh environment even pushes animals to seasonal movements towards available resources. 2. We hypothesize that the density distribution of four sympatric ungulate species is structured by habitat covariates but more affected by seasonality. We then investigated density gradient of these species between contrasting season and correlated with environmental covariates. 3. We used distance-based density surface modelling with survey effort of 518 km in winter and 356 km in summer and with count data as a function of environmental variables in generalised additive modelling framework. We extrapolated seasonal abundance of each species and calculated coefficient of variation to ensure precision for the entire study area. 4. We observed a clear seasonal shift in the density distribution of all four species between summer (more abundant in valley) and winter (evenly distributed), significantly influenced by anthropogenic and topographic factors. Solitary species were congregated in larger groups during summer while group living species were in larger groups during winter. 5. Our study provides a clear understanding of species-habitat relationship as a function of seasonality in tropical forest and is useful in spatial prioritization of the habitats for relevant management inputs.