In a population of Little Owls (Athene noctua) at the Lower Rhine (FRG), the factors were studied that influence the size and shape of territories of this non-migrating, all-year territorial owl species. These studies were carried out between September 1984 and June 1987. The birds were individually marked with transmitter packages so that their locomotional movements could be monitored. Using a standard protable stimulus (a dummy with an attached loudspeaker) the territorial boundaries of 19 male Little Owls were surveilled during the different seasons of the year. Male Little Owls defended their territories in all seasons, but distinct seasonal changes of intraspecific aggressiveness and territory size were observed. The seasonal variations of territory size followed a common pattern in all studied Little Owls, in spite of individual differences in the size of the defended areas. The largest territories were defended in March/April during the courtship season [Formula: see text]. In May/June all male Little Owls reduced the size of their territories [Formula: see text]. This corresponds to the breeding and nestling period. The yearly minimum of territory size [Formula: see text] was reached in the summer months July and August when the fledglings were still fed in the parental territory. When the first-year birds started to disperse in autumn (September/October), the size of the defended areas of the adult tenants again increased [Formula: see text]. In winter (November to February), a further increase in territory size was observed for periods of warm weather [Formula: see text]. Male Little Owls, however, were less aggressive during winter than in the following courtship season. On cold winter days with a ground cover of snow territorial aggressiveness ceased. Pastures and meadows offered a continuous food supply almost throughout the year. In some seasons they were overproportionally represented in the territories of Little Owls compared to the general surroundings. In reaction to changing accessibility of food, the hunting ranges within the home ranges were shifted much faster than the boundaries of the defended territories. The significance of various factors contributing to the variability of territory size in Little Owls are discussed (e.g. availability of food, seasonal and individual differences in aggressiveness, experience in the occupied area, population density).