Grendelmeier, A. (2011)

The enigmatic decline of the Wood Warbler Phylloscopus sibilatrix: nest predation and habitat characteristics.

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Master Thesis - Universität Bern



1. Among the greatest threats to biodiversity are habitat loss, degradation and fragmentation, but other factors such as predation may also impact populations. Many species of ground nesting birds are declining. They are especially exposed to nest predation. As mesopredators often have increasing population sizes, it remains difficult to disentangle the effects of habitat change vs. predation.
2. Using trail photographic cameras, we first investigated the effects of nest predation on the breeding success of the Wood Warbler, a woodland passerine that is declining in Central Europe. Second, we studied the links between breeding success and habitat characteristics. We monitored 45 natural nests and 32 artificial nests in 12 study areas in northern Switzerland.
3. Survival of Wood Warbler nests declined progressively during the approximately 32 days of nesting, resulting in an overall nest survival rate of 40.8% (n=49). Breeding success averaged 1.9 fledglings per initiated nest and 4.6 fledglings per successful (≥ 1 fledgling) nest (n=20). Predation was the primary cause of nest failure in 79% of 29 unsuccessful nests. Predators of real nests were Vulpes vulpes (n=9), Martes spp. (n=6), Meles meles (n=4), Garrulus glandarius (n=3) and Strix aluco (n=1). In constrast, artificial nests were predated by muroid rodents (n=26), V. vulpes (n=4) and Sciurus vulgaris (n=1).
4. There was a positive relationship between daily nest survival and nest concealment, as well as tree diameter. Daily nest survival showed a quadratic relationship with crown coverage, with a peak at 80-85%. It was negatively, but only weakly related to rodent density, 4 presumably because Wood Warblers settled in territories with low rodent densities.
5. Martens as well as jays left behind broken egg shells while V. vulpes destroyed or deformed nests. Apodemus mice (n=3) and S. vulgaris (n=2) inspected Wood Warbler nest content without causing depredation. Our findings contradict the claim that rodents may be major predators of Wood Warbler nests.
6. Synthesis and applications. Our study confirms that predation is a main factor of nest failure in the Wood Warbler, with meso-predators (carnivores, corvids) being the main agents. Although our results rule out a direct role of rodents as nest predators, further investigations are needed to see whether rodents abundance might favor meso-predators populations and thus influence nest predation risk. A preference of Wood Warblers for mature forest stands with a rather closed canopy provides initial guidance for forest management.
Keywords: artificial nests, experiment, GLMM, trail cameras, AIC,nest survival