The absence of a species in apparently suitable regions is often attributed to habitat deterioration, which, according to the IUCN-guidelines, would preclude reintroduction unless the habitat is sufficiently restored. The crux is therefore to determine species’ key habitat requisites and to localize potentially restorable sites based on the habitat selection of thriving populations in similar environments. The distribution of the Red-billed chough (Pyrrhocorax pyrrhocorax) in the Alpine arch is currently restricted to its western side. The eastern Alps have only been occupied sporadically during past centuries, which triggered a discussion around reintroduction. The fact that the last confirmed pairs bred at middle elevation, in derelict buildings instead of alpine cliffs, suggested a lack of habitat suitability in the uplands. To test this hypothesis, we modelled seasonal foraging habitat (during winter, breeding and dispersal) and nest site-selection in the western Swiss Alps using long-term observation data together with a wide palette of environmental predictors. The models were extrapolated to eastern Switzerland to estimate the quality and extent of the available habitat. Both foraging and nesting habitats were predicted with a high level of accuracy (AUC > 0.8). Despite variation between seasons, south-exposed dry meadows and extensively-grazed pastures were always preferred as foraging habitat, while forested and snow-covered areas were avoided. Availability of, and distance to suitable foraging habitats were the main determinants of nest-site selection, probably reflecting strong energetic constraints during reproduction. However, the extrapolation to eastern Switzerland revealed an even higher overall amount and relative percentage of all habitat types. One explanation could be that our predictors were too coarse to encapsulate qualitative, structural or compositional differences of the grasslands. However, the results could also point to an alternative hypothesis, namely that post-glacial recolonization patterns, in particular the absence of nearby source populations, precluded the occupation of the eastern Alps.

Species data:

Red-billed chough foraging locations for the years 2000 to 2014 were extracted from the database of the Swiss Ornithological Institute (www.ornitho.ch), which contains opportunistically collected observations from amateur bird watchers and professional ornithologists. Only locations with a minimum accuracy of 100m and ascertained foraging activity were retained (Fig. 1). The dataset was subdivided into three seasons: winter (November-April, N = 140), breeding (May-July, N = 193) and post-breeding dispersal (August-October, N = 78). Nest site locations (N = 76) were based on long-term, annual field surveys conducted from 1974 to 2016, mostly by one of the co-authors (P.-A.O.). Due to conservation reasons these sensitive data are only available upon request.

Environmental variables:

The environmental variables included in at least one of the models encompassed inormation on topography, climate, snow condition and land cover, including human infrastructure. Topography was described by slope and exposition (i.e. northness and eastness, defined as cosine and sine of aspect) as derived from the digital elevation model of Switzerland. Climate information included the precipitation in summer and winter, as obtained from the worldclim-dataset (www.worldclim.org), downscaled to a 100m resolution based on the SRTM-V4 digital elevation model (DEM) and the method described in Zimmermann & Roberts (2001). Snow cover data for the years 2006-2011 were generated by the WSL Institute for Snow and Avalanche Research SLF according to (Bavay et al. 2013). This model, based on the underlying alpine surface process model Alpine3D (Lehning et al. 2006) and the SNOWPACK model (Fierz and Lehning 2001) provided daily snow cover data at a spatial resolution of 200x200m. Monthly values of snow cover were calculated by averaging the values of four days randomly selected from each of the four weeks per study month, to account for within-month snow cover heterogeneity. We then calculated the average percentage of area covered by snow within 1 km2 (radius = 564m) for each of the study months separately.

Information on land cover, i.e. the percentage of forest, grassland, waterbodies, glaciers, scree and rock was obtained from the Vector 25 map (SWISSTOPO 2009). In addition, we calculated the distance to steep rock (>45°). Dry meadows and pastures were adopted from the mapping of the Swiss Federal Administration of the Environment (https://www.bafu.admin.ch). In addition, we calculated the mean number of sheep and goat per community in the years 2004-2014, as obtained from the Swiss Federal Administration for Statistic Switzerland (BfS) (https://www.bfs.admin.ch) and related them to the amount of pastureland per community in order to obtain a rough estimate of the density of livestock per hectare of pastureland. Human infrastructure was included as the distance to transportation infrastructure (roads and railways), ski-lifts and cableways as obtained from the Vector 25 map.

All predictor variables were prepared as raster maps (cell size: 25x25m). In order to capture the environmental conditions prevailing around the foraging locations, but at the same time to both account for sampling accuracy and avoid pseudo-accuracy given the original resolution of some of the predictor variables (see Read_Me.docx), we calculated means (continuous variables), percentages (boolean and categorical variables) or densities (for point and linear features) within a circular moving window with a radius 100m. Snow cover heterogeneity was considered within a 1km2 area (radius = 564m). For the nest site model, for which we had accurate nest locations, we used the data of rocks and slope at the original 25m resolution.

In addition, to test whether the amount of, and distance to suitable foraging habitat affected nest site selection, we generated additional variables, directly drawn from the breeding-foraging habitat model. First, we calculated the average suitability of foraging habitat (i.e. Maxent logistic output) within a 3-km radius around the nest site, which roughly corresponds to the distance birds use for foraging. Second, we converted this continuous variable into a binary map of nest site presence and absence (as specified below) and calculated the distance of the nest to the next foraging patch.

References:

BAVAY, M., GRUENEWALD, T. and LEHNING, M. 2013. Response of snow cover and runoff to climate change in high Alpine catchments of Eastern Switzerland. -Advances in Water Resources, 55: 4-16.

FIERZ, C. and LEHNING, M. 2001. Assessment of the micro structure-based snow-cover model SNOWPACK: thermal and mechanical properties. -Cold Regions Science and Technology, 33: 123-131.

LEHNING, M., VOLKSCH, I., GUSTAFSSON, D., NGUYEN, T. A., STAHLI, M. and ZAPPA, M. 2006. ALPINE3D: a detailed model of mountain surface processes and its application to snow hydrology. -Hydrological Processes, 20: 2111-2128.

ZIMMERMANN, N. E. and ROBERTS, D. W. 2001. Final Report of the MLP climate and biophysical mapping project. Birmensdorf, pp. 18.