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

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

One. Introduction

Intraspecific morphological variation of organisms is known to be subject to a variety of selective pressures such as predation,

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, 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 seven 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.

climate, sexual selection, and habitat structure. A poorly investigated and still unclear aspect is the relationship between morphological variation and habitat availability. Morphological changes may potentially help species to adapt to environmental changes such as anthropogenic land use and land cover modifications, but evidence for this is still scarce. As an example, several North American songbird species showed changes in wing morphology across a short time frame (one hundred years) according to anthropogenic changes in the amount of available habitat, evolving towards a higher flight efficiency in regions where they experienced habitat loss and fragmentation, consistently with the need for more frequent and longer movements and crossing of larger habitat gaps. Other studies have further supported the occurrence of changes in avian wing morphology also over relatively limited time frames and spatial scales, according to environmental stressors or habitat configuration. This is consistent with the strong selective pressures acting on wing morphology (also due to the high energetic costs of flight), their frequently rapid changes in space and time, and the fast response of many birds to environmental changes. Investigating bird wing morphology is therefore ideal for exploring the relationships between habitat availability and morphological variation.

However, a variety of other factors can affect such a relationship; wing morphology of birds can indeed vary at the intraspecific level according to sex, age, differences in migration habits, habitat structure or elevation. For example, migration selects for longer, more pointed, and more concave wings, as these characteristics allow a more energy-efficient and sustained flight. In contrast, dense vegetation implies the need for higher flight manoeuvrability, selecting for more rounded wings. Differences in wing morphology among sexes are widespread among birds and can be connected, e.g., to the specific requirements derived from males' song flights, to intersexual differences in migration distances, or to higher exposure to predation of females during incubation, which could select for wing characteristics allowing higher escape abilities.

Within this context, another poorly understood potential pressure is the influence of elevation on avian wing morphology, as several studies dealing with this topic provided partly contradictory results, including both a total lack of effects, a positive relationship with wing length, or both of these patterns, depending on the species or the geographic area considered. The reported cases of positive relationships could be explained by the need for a more energy-efficient flight due to the lower air pressure and higher hypoxia risk associated with higher elevations. As a further limit to the current knowledge, as most studies considered wing length only, the effects of elevation on wing shape have been very rarely investigated. In addition, only a few studies on this topic have accounted for the potential influence of other environmental factors, such as vegetation structure. Therefore, to better understand the effects of elevation on wing morphology, studies considering both wing shape and size and accounting for other potentially relevant environmental factors are necessary.

In this study, we investigated the effects of habitat availability, local climate, and elevation on avian wing morphology, using as a model a high-elevation specialist, the white-winged snowfinch. Based on current knowledge of the relationships between the environment (i.e., habitat availability, climate and elevation) and avian wing morphology, we formulated the following, non-alternative, hypotheses:

One. Habitat isolation: Birds can show wing traits allowing a more energy-efficient flight (longer and more pointed/concave wings) where the extension of suitable habitat is lower, because of the need to reach other and more distant areas for foraging opportunities after reproduction, as well as the longer natal and breeding dispersal distances required to find settlement opportunities;

Two. Winter climate: Longer and more pointed/concave wings can be expected in areas with harsher winter climates (lower temperature, higher precipitation). For a species facing the challenging conditions of winters at high elevations (scarce food, harsh climate), local winter climate may also affect the length and frequency of erratic movements: birds breeding at sites that become colder and have stronger precipitation in winter may more likely be forced to move towards mountain sectors with milder climates. A high snow cover makes access to food resources (mainly seeds in our model species) more difficult, and low temperatures imply a high energy demand for thermoregulation;

Three. Elevation: Birds can have longer and more pointed/concave wings at higher elevations, given the need for a more energy-efficient flight.

These three hypotheses are not necessarily mutually alternative; in the case of co-occurrence of effects, we aimed to assess which is the prevailing one, especially by comparing the effects of habitat availability and winter climate while taking into account the effect of elevation. To our best knowledge, while some highly valuable studies cited above investigated the effects of elevation or (in very few cases) habitat availability on avian wing morphology, the potential effects of these factors have never before been simultaneously tested, for wings of birds or of any other taxa.