Wing morphology changes with habitat availability and elevation in an alpine-specialist bird

Abstract

Intraspecific morphological variation of organisms is known to be influenced by several factors, but the role of habitat availability has been scarcely investigated. Studying bird wing morphology is ideal to explore this topic, given the usually rapid response of birds to environmental changes, but other factors such as sexual dimorphism, habitat structure, climate and elevation need to be considered. Here, we investigated the effects of habitat availability, local climate and elevation on the wing morphology of a high-elevation specialist bird (Montifringilla nivalis), while accounting for sexual dimorphism. We hypothesized that birds relying on less extended suitable areas around their breeding sites show wing traits allowing a more energy-efficient flight, given their need of more frequent and longer movements to find foraging areas in the post-breeding period and the longer dispersal distances. We also expected that individuals breeding at higher elevations show wings traits allowing higher flight efficiency, given the higher hypoxia risk. We derived wing traits (isometric size, pointedness and concavity) by measuring primary feathers of individuals from 7 breeding sites in the European Alps, and we obtained habitat availability from detailed habitat suitability maps. Consistently with the need for a more energy-efficient flight, birds relying on less extended suitable habitat showed larger and more concave wings, and individuals breeding at higher elevations showed more concave wings. Local climate had a less clear effect. The observed patterns may result from local adaptations and could represent one of the ways mountain birds cope with the harsh and unpredictable environment they inhabit.