© Marcel Burkhardt
Huber, N. (2013)
Structural characteristics of Wood Warbler habitats in Switzerland: an analysis with remote sensing methods.
Master Thesis - ETH Zürich
Information on the distribution and abundance of endangered species is integral for wildlife conservation and land use planning. The Wood Warbler (Phylloscopus sibilatrix) is a groundnesting, long-distance migratory passerine with a distinctly European range. In Western Europe, Wood Warbler populations have declined in the last three decades. In Switzerland, the Wood Warbler has been classified as vulnerable in the red list of the breeding birds. Furthermore, the species is one of the 50 priority species of the Swiss species recovery program.
Remote Sensing (RS) methods were used to achieve an increased understanding of the factors that may influence the territory choice of Wood Warblers in Switzerland and to identify potentially suitable habitats in the Swiss Jura Mountains and the Swiss Plateau. The structural habitat needs were analyzed at the scale of the nesting area and at the scale of the territory. First, lidar metrics were correlated with structural habitat variables collected in the course of the research project Settlement behavior, population fluctuations and population structure of Wood Warbler of the Swiss Ornithological Institute, Sempach. Second, the following question was addressed: Is it possible to distinguish Wood Warbler territories from control areas without Wood Warblers using lidar data or other RS information? In a third step, predictive models were generated to model the current potential range of the Wood Warbler in the Swiss Jura Mountains and the Swiss Plateau.
The analyses at the two spatial scales, ‘nesting area’ and ‘territory’, suggest that Wood Warblers prefer rather uniform forests stands of intermediate age. Stands of these stages of development are characterized by a closed canopy, low canopy height diversity, an open stem space and a sparse herb and shrub layer, features promoting the occurrence of the Wood Warbler. The analyses further showed that Wood Warbler occurrence is positively related to inclination and solar radiation during March. Since the Wood Warbler is a groundnesting bird, the species may benefit from small-scale variation of snow melting and vegetation development. Alternatively, reduced disturbance due to recreational activity or low forest management intensity in steep areas may explain the observed effect. Solar radiation may positively influence food availability, and higher food availability on south-facing slopes than on north-facing slopes could attract Wood Warblers.
According to the predictive models, the current potential range of the Wood Warbler is predominantly located in the Swiss Jura Mountains. This finding corresponds to the abundance map of the Swiss Breeding Bird Atlas 1993-1996.
Locally, forest management may contribute to the deterioration of suitable areas, for example when relatively closed forests are opened up due to harvesting. Therefore, the focus of forest management at a regional scale should be on sustainable regeneration so that suitable stands are always present and new suitable stands are steadily developing. In consideration of the Wood Warbler’s habitat needs, the femel harvesting system (Femelschlag), leading to a relatively homogeneous age structure, appears to be most promising to maintain structurally suitable stands for Wood Warblers. Selection forestry (Plenterwald/Dauerwald), leading to a heterogeneous age structure and many gaps at a local scale, is rather unsuitable for the Wood Warbler.
Overall, this study suggests that RS variables derived from lidar data or other sources are suitable for distinguishing structural characteristics of Wood Warbler habitat from non-habitat. Additionally, lidar metrics and other RS variables convey additional information not captured by variables gathered in the field, and therefore have the potential to contribute to understanding the ecological niche of species.