Accuracy and efficiency of mapping territorial willow warblers Phylloscopus trochilus: a case study

The number and distribution of willow warbler territories were established in a study plot of 15 ha in a subalpine birch forest by trapping and colour-ringing the male birds. The size and form of the territories were estimated by identifying and plotting the ringed males and their movements. The bird community was independently censused by application of the mapping method according to international recommendations. Fifteen males were ringed and found to be territorial and fifteen males were stationary according to the mapping census. The distribution of the stationary males (the mapped clusters) conformed largely with that of the territories, although the correspondence was not complete. The mapping census showed a slight tendency to produce two clusters within territories. The "mapping efficiency" was 71 ? 20% (mean number of observations in the clusters, as a percentage ? S.D., of the maximum possible).