Nesting success and habitat choice of the Wood Warbler (Phylloscopus sibilatrix) in face of population decline in the Lake Constance area

During the breeding season of 2003, vegetation parameters of the herb, shrub and tree-layer were recorded in 29 occupied territories as well as 13 formerly occupied territories (deserted territories) of the wood warbler (Phylloscopus sibilatrix) and the breeding success of ten pairs was monitored. In ten territories of paired males and ten deserted territories, food availability was assessed and Ellenbergian indicator values were calculated. For the first time, a comparison between occupied and deserted territories was made utilising the obtained data to gain insights into the suitability of the actual natural habitats as breeding habitat for the wood warbler.The results of this study should contribute to answering the following questions:1. How is the breeding success of the wood warbler in comparison to the data obtained from the literature and does it present conclusive evidence of it s population decline?2. Is it possible to identify a cause and effect relationship of the wood warbler population decline in connection with the breeding grounds from the comparison of occupied and deserted territories?The breeding success of 50% was similar to data from the literature. The high number of unpaired males of 63% is noticeable and might indicate that the habitats may be fragmented for the wood warbler in the area under investigation. There were no apparent indications that increased predation is the source of the observed population decline. (Question 1)No differences were found between occupied and deserted territories in food availability and microclimatic conditions surrounding the nesting site. This indicates that these factors are not involved in the observed decline in the area under investigation. However, differences were found in structural parameters of the tree-layer. This data indicates that the trees in deserted territories tend to be older and that important structural elements are less available there. The reason for the significance of the structural elements is not currently understood and should be evaluated by comprehensive studies of the habitat preferences of the wood warbler. The forest stands in Central Europe are generally growing older and it is perhaps likely that the habitat quality for the wood warbler is declining due to the older forest stands. In contrast, local forestry data may suggest that the suitability as habitat for the wood warbler will continue despite alterations in the age group of the forest stands. In territories supporting paired males, continuous stands of grasses were more abundant, and more than 50 single tussocks were recorded more often than in territories of unpaired males. The prevalence of grasses appear to be an important factor for the location of the wood warbler nests and it is possible that changes in availability of suitable nest sites occurs as the result of the availability of the grasses. (Question 2)The decline in the population of the wood warbler is not exclusively a regional development but seems to be occurring throughout the western part of Central Europe. The magnitude and extend of the decline during the last 20 years (a population reduction of 87%) within the area of investigation strongly suggests that issues of environmental change outside of the breeding grounds may be considered as main cause for the decline. Locally, habitat fragmentation and the subsequent deterioration of the warbler s breeding grounds are the most important factors, but changes in and the availability of the resting and wintering grounds of this migrant species as well as possibly massive changes in the species main range, seem to be more significant