Present in the western European Alps but absent in the eastern part: Can habitat availability explain the differences in red‐billed chough occurrence?

Abstract

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.