Density fluctuations in an urban population of Tawny Owl Strix aluco: A long-term study in Rome, Italy

The population of Tawny Owls Strix aluco breeding in urban Rome, Italy, was studied from 1992 to 2001. Yearly density (range: 0.9-1.1 territories km-2) was rather stable compared with data from central Europe, likely due to mild weather conditions and abundant food supply. The rate of territory occupation, regarded as estimator of density fluctuations, was non-linearly correlated with density and proportion of wooded and farmland areas, and not correlated with the proportion of built-up areas. In plots with owl density ranging from 2-4 territories km-2 all the breeding territories were stable over years. At decreasing density the lower rate of territory occupation was attributable to the large proportion of unsuitable habitat. Competition with neighbours and the consequent reduction of mean territory size may account for the lower territory stability at high densities, where the proportion of wooded areas is high. The proportion of farmland areas, which is a poor habitat for breeding but represent a suitable hunting ground, explains additional variance of the density fluctuations.