Search

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.

The morphology of bird wings is subject to a variety of selective pressures, including migration, predation, habitat structure and sexual selection. Variation in wing morphology also occurs at the intraspecific and intrapopulation level, and can be related to sex, age, migration strategy and environmental factors. The relationship between environment and intraspecific variation in wing morphology is still poorly understood. In this work, we studied the relationship between wing morphology and breeding environment in a high‐elevation specialist bird, the water pipit Anthus spinoletta. We calculated wing isometric size, pointedness and convexity of 84 birds mist‐netted at breeding sites in year 2021 in the European Alps. We then searched for associations between these traits and potentially relevant breeding site characteristics (vegetation structure, elevation, latitude). For all wing traits, sex and one or more environmental factors best explained the variation, with environmental factors explaining between 3 and 8% of the variation. Wing size was negatively related to tree cover and wing convexity was negatively related to bush cover. Elevation contributed to explain variation in wing pointedness, but the direction of its effect was unclear. The negative relationship between wing size and tree cover could be due to intraspecific competition, i.e. to the relegation of smaller winged low‐quality individuals in marginal grassland areas. Higher wing convexity could improve predator escape ability in areas with scarce protecting vegetation, with possible effects on habitat choice. These findings represent one of the few demonstrated cases of wing morphology–environment relationships at the intraspecific level.

Unravelling the environmental factors driving species distribution and abundance is crucial in ecology and conservation. Both climatic and land cover factors are often used to describe species distribution/abundance, but their interrelations have been scarcely investigated. Climatic factors may indeed affect species both directly and indirectly, e.g., by influencing vegetation structure and composition. We aimed to disentangle the direct and indirect effects (via vegetation) of local temperature on bird abundance across a wide elevational gradient in the European Alps, ranging from montane forests to high-elevation open areas. In 2018, we surveyed birds by using point counts and collected fine-scale land cover and temperature data from 109 sampling points. We used structural equation modelling to estimate direct and indirect effects of local climate on bird abundance. We obtained a sufficient sample for 15 species, characterized by a broad variety of ecological requirements. For all species we found a significant indirect effect of local temperatures via vegetation on bird abundance. Direct effects of temperature were less common and were observed in seven woodland/shrubland species, including only mountain generalists; in these cases, local temperatures showed a positive effect, suggesting that on average our study area is likely colder than the thermal optimum of those species. The generalized occurrence of indirect temperature effects within our species set demonstrates the importance of considering both climate and land cover changes to obtain more reliable predictions of future species distribution/abundance. In fact, many species may be largely tracking suitable habitat rather than thermal niches, especially among homeotherm organisms like birds.