The behavioral mechanisms that produce territoriality require that a resident's agonistic tendencies vary with the locations of encounters with opponents or with the distance between the resident and an opponent. Variation in agonistic tendencies with location could result from increased aggressiveness in familiar locations. Only a few field studies have previously documented variation in aggressive tendencies either with the location of an encounter or the distance of an opponent. In this study, we measured the spatial distributions of advertising activities in the territories of male yellow-hooded blackbirds Agelaius icterocephalus and related these distributions to aggressive tendencies of the residents. Advertising and agonistic activities resembled those of other marsh-nesting blackbirds in the genera Agelaius and Xanthocephalus. To portray the spatial distributions of activities, we determined convex hulls for the 95 % and 68% of instances or seconds of an activity closest to the vector mean of the distribution. A territorial male's time perched conspicuously, instances of conspicuous perching, songs and song flights were concentrated near the center of his territory. In spite of this uneven distribution of a male's time and advertising activities within his territory, a male's aggressive tendencies toward intruders did not vary clearly either with the intruder's location or distance from the resident, provided only that the intruder was within the resident's boundaries. Interactions with neighbors suggested that use of an area is a prerequisite for its defense. Changes in boundaries and establishment of new territories revealed that a male's use of an area increased before aggression toward neighboring residents began. Changes in a resident's territorial boundaries often resulted from changes in the spatial distribution of residents' activities as a result of localizing activities around nests of egg-laying females or changes in the locations of nests under construction. These observations suggest that predominance in use of an area leads to dominance in defense of that area.