Does interspecific competition affect territorial distribution of birds?: A long-term study on Siberian Phylloscopus warblers

Seven species of Phylloscopus warblers breed in the middle taiga subzone of Central Siberia. We studied their territorial distribution over 18 breeding seasons between 1978 and 1996. Phyloscopus inornatus, the most abundant breeding bird species in the study area. breeds in dense clusters of territories. In some habitats, these clusters are very unstable from year to year, providing an excellent opportunity to study the influence of P. inornatus on the territorial distribution of other species. We present a method for quantifying decreases in a species' breeding density caused by the presence of a competing congener. Analysing all possible combinations of species, we found reciprocal territorial avoidance between P. inornatus and P. proregulus as well as between P. inornatus and P. collybita. This is consistent with the hypothesis that territorial avoidance occurs between the species of greatest morphological similarity. The occurrence of territorial avoidance is not explained by habitat characteristics such as vegetation cover and productivity. However low productivity may be necessary for territorial avoidance to occur. An alternative explanation for the observed territorial separation of some species. derived from the Fretwell-Lucas model of habitat occupancy, is examined and rejected. Causes and implications of clustering behaviour of P. inorntus are also discussed.