Hinde (1956:341-342) indicates that territory has been defined in a number of ways by many workers. All of the definitions involve modification of Howard's classic "defended area." Pitelka (1959:253) has reacted against this behaviorally-oriented concept. He thinks that the concept of territory should be based on exclusive use of an area by its occupants, and not so much the defense by which they maintain it. Methods of treating territoriality in the Bell Vireo seemingly incorporate features of both schools of thought. The area used exclusively for all biological needs by a single pair of Bell Vireos is vigorously defended both physically and vocally early in the breeding season and vocally as the season progresses. In the period of territorial establishment a relatively large area is actively defended. The building of a nest establishes a focal point of activity in a somewhat more restricted area than that originally occupied. After the success or failure of a nest, a new site is selected to which the focal point of activity is shifted. If suitable habitat adjacent to the extant territory is unoccupied by other Bell Vireos the unoccupied area may be annexed in the course of searching for a new site. Such annexation occurs only when pairs formerly occupying adjacent suitable habitat disappear from this territory; possibly the size of the territory of any one pair is dictated by the density of population of the species as well as by the presence of suitable habitat. This may not always be true as indicated by Kliujver (1951:40), who in studying the Great Tit, found no appreciable difference in the size of territory in two different habitats even though there was a marked difference in population density of the birds. Fluctuation of territorial boundaries is not uncommon in passerines, especially when no rivals exist to contest movement. Hinde (1956:351) indicates that fluctuations in size of territory are to be expected although the territories of different species of birds have different mean sizes. Once nesting activities commence there is a marked reduction in the amount of territory utilized and a distinct decrease in the aggressive tendencies of the male; it would seem that energy previously utilized in regular fighting is rechanneled for nestbuilding, incubation and care of the young. Further, contraction of the area of activity obviates high-intensity territorial defense, as adjacent males, even in regions of high population density, are isolated from one another by an area no longer regularly traversed.