With cessation of breeding activities physiological mechanisms governing maintenance of territory seemingly are no longer active and yet the pairs of Bell Vireos remain within a restricted area which they alone use. Earlier definitions of territory as a "defended area" do not adequately cover such situations and yet from the standpoint of Pitelka the area still retains the characteristics of true territory. In fact, territory as defined by Pitelka is clearly manifest at this time. Whether the birds remain in an area through "force of habit" is of little consequence. I have retained the term "territory" in preference to the term "home range" used by Nolan (1960:227). His failure to observe territorial defense is responsible for his terminology, although it is readily understandable that such defense would be lacking in a population of relatively low density in which pairs were isolated from one another by areas of unfavorable habitat. This isolation in itself would tend to preclude territorial conflict but territories were, in fact, maintained. The marked similarity in the essential features of aggressive behavior in North American vireos attests to their close relationship. Flicking and fanning of the tail are distinct components of the hostile behavior of the Bell Vireo, White-eyed Vireo, Red-eyed Vireo (Lawrence, 1953:69), and the Black-whiskered Vireo (_Vireo altiloquus_; Bent, 1950:319), and, presumably, of the remaining species of the genus. The occurrence of these same displays as intrinsic behavioral elements of interspecific hostility suggests a common derivation. Moynihan (1955:256) indicates that all intraspecific hostile displays, and probably most interspecific hostile displays, evolved originally as social signals having the same general function. Further, Hinde (1956:344) points out that there is a fundamental similarity in the motor patterns used in fighting in different contexts, including both interspecific and intraspecific fighting. COURTSHIP BEHAVIOR The precise mechanism of pair-formation in the Bell Vireo is not known. My experience has been to find a male one day and then one or two days later to discover that it has a mate. Lawrence (1953:53), tells of a male Red-eyed Vireo singling out a female from a flock of migrants passing through his territory and violently driving her to the ground. Shortly after this attack the pair was seen searching for a nest site. But such an incident has not been reported for other vireos, nor have I witnessed such behavior myself.