A multi-isotope and morphometric analysis to uncover ecological niche divergence in two endemic island birds from Madagascar: the Dark and Common Newtonia (Vangidae)

Abstract

The common ancestry of congeneric species implies that their morphology and ecology are similar, and thus that these closely related species may experience intensified levels of competition when sympatrically distributed. Under such circumstances, selective pressure may lead to niche partitioning between and within species, with segregation achieved through variation in morphology, ecology and life history. Examining the mechanisms underlying the coexistence or segregation of congeneric species requires detailed data on aspects of their ecology such as their feeding behaviour or habitat use. Endemic island birds, such as the vangas of Madagascar, are good candidates for studying processes of niche segregation. Vangas underwent rapid speciation following the initial colonization of the island by a shared ancestor and now provide a prime example of adaptive radiation, with considerable variation in body size and shape. Four small species of Newtonia are an exception to this variation, as they show morphological overlap and partial spatial range sympatry. Here, we describe the morphology of two Newtonia species, the Common Newtonia Newtonia brunneicauda and Dark Newtonia Newtonia amphichroa, with respect to their ecology and trophic niches using a multi-isotope approach (stable isotope ratios of carbon, nitrogen and sulphur). We report evidence for adaptations involving morphological feeding traits and provide data on contrasting trophic niches between two species with a close phylogenetic relationship. We document micro-habitat niche specialization that may be due to vertical stratification within the forest. Differences in feather isotopic signatures indicate different nutrient sources and point towards microhabitat segregation that is sufficient to maintain species integrity and permit coexistence.