batants
may show upright flight."

Head-up posturing and upright flight are not presently recognized
components of the behavior of the Bell Vireo. The behavior of the
attacked Bell Vireo is similar to that described in (a), (b), and (d)
above, and is clearly dictated by the proximity of his own "home
base."

Eleven disputes among occupants of adjacent territories were witnessed
between May 6 and June 3, 1960, in which some or all of the described
threat displays were manifest (Table 3). In each instance, patrolling
males were gradually attracted to each other. As they approached,
their rates of song increased from an average of six repetitions per
minute to 15 per minute. Eight of the disputes involved physical
combat.

On May 6, 1960, when male 2 (1960) was in the process of usurping an
eastern segment of the original territory of male 1 (1960), a violent,
protracted dispute was observed. By this date male 1 (1960) had
obtained a mate and had begun construction of nest 1-a (1960); male 2
(1960) had not yet acquired a mate. At first the two males were
singing vigorously, from one to 10 feet apart. Female 1 (1960)
followed her mate closely and scolded, at the same time partially
fanning her tail. In the course of vocal dueling the males had
traveled to within 50 feet of nest 1-a (1960), when male 1 (1960)
suddenly lunged at 2 (1960). The males plunged to the ground, locking
bills and clutching at each other with their feet as they fell. As
soon as they touched the ground they separated. Male 2 flew east with
male 1 in pursuit. This conflict lasted three minutes.

Additional physical combat was witnessed several minutes later. This
again involved striking with the bill, wings and feet. A high pitched
squeaky _chee_ was uttered by both combatants. The female scolded from
a nearby perch. Upon separating, the males engaged in a wild, looping
flight. At about 350 feet from nest 1-a (1960), the chase abruptly
ended. For ten minutes thereafter, both males sang at a high rate from
perches about 10 feet apart. This terminated the physical combat, but
three additional protracted, vocal duels occurred in the remainder of
the morning.

TABLE 3. INTRASPECIFIC DISPUTES IN MAINTENANCE OF TERRITORY.

Behavior

==============================================================
| Number | | | Average
| of | Vocal | | length of
| conflicts |