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.

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governing maintenance of territory seemingly are no longer active and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 mech