Location data of four White-tailed Sea Eagles (Haliaeetus albicilla) (WTSE) from four pairs equipped with GPS data loggers or automated GPS–GSM units and visual behaviour observations were used to record their home range size and territoriality. Between 18 and 571 positions were received from the satellite devices. Extensive excursions were identified by defining the distance to the activity centre and have been removed before home ranges were computed. Home range size varied considerably among the individuals tracked. Minimum convex polygons, with outliers removed, ranged between 2.25 and 19.16 km2 and 95 % kernel contours (calculated in Ranges 8) varied from 2.37 to 12.04 km2 in the Mecklenburg Lake District. Occurrence of territorial behaviour as identified by visual observations changed significantly throughout the year with a maximum in May and a minimum in December. An optimal habitat quality characterized by shallow and fish-rich freshwater lakes with undisturbed nesting areas in surrounding forests is a prerequisite for small home ranges and for the high abundance of breeding birds found within the Mecklenburg Lake district. The year-round territoriality is attributed to a sufficient food supply throughout the seasons, either by mild winters allowing access to fish and/or waterfowl on open waters or, in hard winters, to the availability of carcasses and remains of larger animals from hunting activities when frozen lakes restricte access to prey.