tion had just begun at this nest. A dummy Bell Vireo placed
close to another nest only momentarily disturbed the male, and the
female completely ignored it. Incubation had also recently begun at
this nest. At this same general stage, moreover, nesting pairs showed
little inclination to harass me.

_Discussion_

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