Abstract
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.