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Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	Analysis of the substantial population decline of the Wood Warbler (Phylloscopus sibilatrix) at Lake Constance, south-western Germany (2009)
Auteurs :	Arno Reinhardt, Auteur ; Hans-Guenther Bauer, Auteur
Type de document :	Article
Dans :	Vogelwarte (47(1) FEB 2009)
Article en page(s) :	23-39
Langues:	Allemand
Résumé :	During the breeding season of 2003, vegetation parameters of 29 occupied territories and of 13 abandoned territories of the Wood Warbler (Phylloscopus sibilatrix) were recorded and the breeding success of 10 pairs was monitored. In ten territories of paired males and ten abandoned territories, food availability was assessed and Ellenbergian indicator values were calculated.The breeding success of 50 % was similar to data from the literature. The high percentage of unpaired males (63 %) was striking and implies that the Wood Warbler habitat may be fragmented in the area under investigation. There is no evidence that increased predation may be involved in the observed population decline.No differences were found between occupied and abandoned territories with regard to food availability and microclimatic conditions surrounding the nesting site. This indicates that these factors are not involved in the observed population decline. However, differences were found in structural parameters of the tree layer. In deserted territories the trees tended to be older and important structural elements were less common. In territories of paired males, grass cover was higher, and more than 50 single tussocks were recorded more often than in territories of unpaired males. The prevalence of grasses appears to be an important factor for the location of Wood Warbler nests and it is possible that the amount of suitable nest sites depends on the availability of extensive grass layers. The significance of some structural elements is currently little understood and should be evaluated in more comprehensive studies of the habitat preferences of the Wood Warbler. The forest stands in Central Europe are generally growing older and it is likely that habitat quality for the Wood Warbler is declining due to the maturation of forest stands. In contrast, local forestry data suggest that habitat Suitability for the Wood Warbler is not reduced despite alterations in the age group of the forest stands.The population decline of the Wood Warbler, amounting to a reduction of 87 % in the study area at Lake Constance, is not only a regional development but seems to be occurring throughout the western part of Central Europe and beyond. The magnitude and extent of the decline during the last 20 years strongly suggests that environmental changes outside the breeding grounds may be the main cause. Locally, habitat fragmentation and subsequent deterioration of the Wood Warbler's breeding grounds are the most important factors. However, changes in and the availability of resting and wintering grounds of this migratory species, as well as possibly massive changes in the species' main range, seem to be more significant.

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