© Marcel Burkhardt
Gerber, M. (2011)
Territory choice of the Wood Warbler Phylloscopus sibilatrix in Switzerland in relation to habitat structure and rodent density.
Master Thesis - Universität Zürich
For successful conservation of threatened species, it is crucial to know the main factors influencing population dynamics. Habitat loss and degradation still are the primary threats for most declining bird populations. The Wood Warbler Phylloscopus sibilatrix, a ground-nesting, long-distance mitratory passerine, has undergone severe population declines throughout Western Europe in the last decades, while populations in Eastern Europe remained stable but experienced strong annual fluctuations. Among the most frequently mentioned causes for Wood Warbler population declines are habitat loss and degradation in the breeding area due to changed forestry practices and increased nest predation. To better understand the process of habitat selection of this species, I evaluated predictions of the forest structure hypothesis and the nest predation hypothesis, but also addressed predictions relating to the disturbance hypothesis, which has been suggested to be important in habitat selection of Wood Warblers and other passerines. Wood Warbler habitat selection was examined at the territory level in 12 study areas throughout northern Switzerland in 2010. Corresponding to the hypotheses considered, I compared variables related to forest structure, nest predation and disturbance between occupied Wood Warbler territories, random control areas and abandoned territories. Occupied territories had higher numbers of trees and denser herb layer and were located on steeper slopes than nearby areas not occupied by Wood Warblers. Rodent density was lower in occupied territories than in control areas. Variables related to disturbance did not differ between territories, control areas and abandoned territories. According to this study, habitats preferred by Wood Warblers are forests with high tree numbers and open stem space and a dense herb layer. These habitat preferences suggest that widespread forestry practices involving the cutting of single trees to open up closed forests and the rejuvenation of forests might be unfavorable for Wood Warblers. Such practices lower the number of trees and allow growth of dense underbrush that will replace the herb layer. The results concerning rodent density are consistent with the hypothesis that Wood Warblers might avoid high rodent densities in order to avoid increased nest predation pressure by generalist predators feeding on the rodents. The absence of effects of disturbance might be due to methodical reasons and study area characteristics. A closer analysis of the influence of different forestry practices and disturbance on Wood Warbler habitat selection and their potential effects on population trends in future studies is desirable.