Guild Organisation and Morphology of High-Altitude Granivorous and Insectivorous Birds: Convergent Evolution in an Extreme Environment

Abstract

This study was conducted in high-altitude alpine habitats of the Central Himalayas of Nepal. We studied altitudinal distribution, species diversity, niche segregation and morphology of members of two different guilds: granivorous and insectivorous song-birds, belonging to two phylogenetically distant tribes: cardueline finches and turdid chats. We test for convergences in general guild organisation, in niche differentiation patterns and in patterns of morphological change related to habitat, i.e. we tried to assess evolutionary convergence by comparing patterns of radiation within genera or similarly closely-related taxa of different groups within the same environment. Our data suggest convergence in guild properties at different levels from species diversity over niche segregation patterns to morphological traits. Firstly, species of both guilds were similarly arrayed with similar species diversity along the major habitat gradients from the lower alpine zone to the nival zone. Secondly, there were clear directional patterns of morphological change among unrelated genera in insectivorous and granivorous species. Species which occupied the highest altitude open areas were heavier, had longer and more pointed wings with lower wing loading, and longer claws than their lower altitude equivalents. In contrast, shrub-dwelling and ground-dwelling species utilising boulder fields and the understorey of alpine dwarf shrubs, tended to have longer tarsi and longer toes. Thirdly, convergence in general phenotype was expressed by overall morphological Euclidean distances (20 characters used), which, in several cases, were lowest between species pairs with distinct phylogenetic histories but with similar microhabitats. In particular, high-altitude species were less distant to other high-altitude species of different and similar ancestry and more distant to shrub species and vice versa. The among-guild convergences can be functionally linked to specific constraints imposed by habitat configurations and resource situations of the single tractions of the alpine habitat gradient. We therefore conclude that under the extreme alpine environmental conditions only a limited number of adaptive solutions are possible which fostered the observed patterns of convergence