FOPPEN R & RIEN R (1996) The Willow Warbler Phylloscopus trochilus in trouble; Africa involved?. LIMOSA 69 (2): 51-56.

Willow Warbler Phylloscopus trochilus In recent decades, the Willow Warbler was one of the species with a remarkably stable population trend. However, in The Netherlands, the United Kingdom and some other countries a sharp decrease was reported in 1991. Data collected during an intensive population study in 1988-92 may provide a link to demographical features. The study area (150-200 ha), consisting of low willow- grounds, was situated in the middle of the country (between the cities of Den Bosch and Utrecht). Some 50-60 hectares were studied anually. Males and nestlings were colour-ringed, whereas mating and breeding success was scored qualitatively. Dispersal was measured by systematic and thorough searching for Willow Warblers in a radius of 2 km from the (rather isolated) study plots. In one year a much larger area (radius 5 km) was carefully checked, but in only one case dispersal over more than 2 km was recorded (900 territorial birds were checked). Return rates were used as a measure of annual mortality. The density of Willow Warbler in the study area decreased markedly in 1990-91 and 1991-92 (table 1). Survival rates of colour-ringed birds in 1991 and 1992 were lower than in preceding years, but breeding success was rather normal (table 2, 3. Elsewhere in The Netherlands, a decrease was recorded in the samen year (table 4). From the results in the study area, it is concluded that poor survival was responsible for the decrease, rather than poor breeding performance (figure 1,2). The Willow Warbler was not the only species showing a decline. Many migrant species showed a remarkable decrease in 1991. In 1992 the picture was more diverse, with some species declining further, but others showing a recovery. The paper concludes with a plea to expand systematic monitoring activites in The Netherlands. Not only breedmg bIrd numbers should be monitored, but productivity and survival of breeding birds as well. Systematic collection of demographical parameters will provide more insight into the causes of population changes. The first stept towards such a study have been taken recently.

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