© Marcel Burkhardt
Kühn, B.M. (2015)
The role of caterpillar abundance and phenology for breeding success, territory choice and population size of the declining wood warbler (Phylloscopus sibilatrix)
University of Zurich
Insufficient resources during the reproductive period are a threat for the continued existence of all species. The seasonal phenologies of predators and preys are often fine-tuned in that the predator’s reproductive period is synchronised with its prey’s highest abundance. A disruption of this synchrony, for example by global warming, has been frequently suggested as a possible threat for insectivorous bird species. Especially long-distance migrants may not be able to sufficiently advance their arrival to keep up with the advanced insect spring phenology at their breeding sites. The wood warbler (Phylloscopus sibilatrix) is such a long-distant migrant. It shows a decline all across its western distribution range. In addition to as yet unknown factors that may act at the wintering grounds and during migration, it is important to evaluate factors that may affect individuals and populations at the breeding sites.
This thesis had three aims: i) to examine the role of caterpillar abundance and phenology, respectively, for the wood warbler’s breeding success, ii) to assess the relationship between breeding territory selection and caterpillar abundance, and iii) to address the link between population density and caterpillar abundance. In order to minimise the expected negative effect of declining caterpillar abundance with increasing temporal asynchrony to its peak, the prediction is made that wood warblers settle in territories with higher caterpillar abundance relatively to the control. Moreover, local population sizes are expected to be higher in areas and years with increased caterpillar abundance. Relative caterpillar abundance was measured in several study areas in spring from 2012 to 2014 by means of caterpillar frass collectors. The vast amount of birds bred too early to fully profit from the peak of caterpillar abundance in 2013, whereas in 2014 most of them bred too late. At the same time, mismatch and caterpillar abundance had no effect on breeding performance. Previous research in this project already highlighted the role of various habitat variables in the wood warbler’s selection of its breeding territory. Caterpillar abundance does not seem to play a role in this small-scale selection process. On the large scale though, forests with high caterpillar abundance appear to be attractive habitats for the study species.
The negative effects of mismatch on breeding success frequently proposed as a possible cause of decline in long-distance migrants are not present in the wood warbler populations in Switzerland. Wood warblers local population sizes are higher in years with increased caterpillar abundance. Conservation measures in favour of the wood warbler should thus include the protection of breeding habitats based on previous findings and the enhancement of overall forest insect abundance.