Clustering of territories in the Wood WarblerPhylloscopus sibilatrix

Territorial establishment of Wood Warblers was studied during 5 years in 1800 ha of forest in central Belgium. The total population size varied dramatically between years, with maximum of 167 and a minimum of 56 male territories. The spatial distribution of territories within the forest also differed greatly between years. Most territories were associated in clusters or chains, which were not located at the same sites every year. Females preferred males that were closer to other males, and re-productive output tended to be higher under this condition. These results could fit the optimal habitat choice hypothesis, but at variance with this concept, areas with huge infestations of caterpillars were not found to contain many Wood Warbler territories. A direct social advantage of clustering is suggested as an alternative hypothesis. Males outnumber females in some Western European populations of Wood Warblers, and males clearly improved their chances of attracting a female by aggregating in 'supersexy' communal units. Males that remain unpaired can only reproduce by promiscuous behaviour, which is also facilitated by closer territories.