The Bell Vireo exhibits "classic" passerine territoriality. Within a specific area, a pair of this species carries out pair-formation, courtship activities, copulation, nesting, rearing the young, and foraging. With the cessation of reproductive activities, a pair continues to restrict its other daily activities to the same general area.

Establishment of Territory

In early May the segment of the total suitable habitat within which a Bell Vireo restricts its activities is not rigidly defined and the first male of the season ranges over an area too large to be maintained permanently—one that seems greatly to exceed the needs of breeding. Male 1 (1960), for instance, was first seen foraging over an area of approximately seven acres. With the influx of other males, portions of this large tract were usurped and the territory of the original male was gradually reduced to an area of little more than an acre.

In this initial period, a male becomes identified with a large area but is restricted to an area of nearly typical size by the encroachment of other males. Territorial disputes in this period often involve physical contact, as well as protracted sessions of high-intensity singing at rates exceeding three hundred song-phrases per hour.

Eventually the carrying capacity of the habitat is reached and no further partitioning occurs. The beginning of nestbuilding coincides with this relative stabilization of the territorial boundaries. Through the remainder of the cycle of behavior associated with any one nest, all activity is that of the occupant pair within its territory.

Size of Territories

The nine original territories established in 1960 varied in size from 0.26 acre to 3.1 acres (Table 2). Fitch (1958:270) found the territories of several pairs of Bell Vireos at the University of Kansas Natural History Reservation to vary from 0.4 to 1 acre. Hensley (1950:243) estimated the size of the territory of a pair of Bell Vireos observed in Piatt County, Illinois, at 3.1 acres. Nolan (1960:227) records home ranges of 2 to 3 acres. The pairs that he studied were sole occupants of fields several acres larger than the portions actually utilized. His description of the vegetation indicates that most of the second growth was not much taller than 7 feet. As indicated elsewhere, the second-growth in my tract averaged 15 feet tall. The smaller average size of territory (1.25 acres) that I found is probably a function both of this greater vertical range of available foraging area and the much higher gross density of birds (40 pairs per 100 acres).

Permanence of Territories

Most pairs remain in their original territories throughout the summer, although some shift certain territorial boundaries. In 1960 pairs 2 and 6, in the course of selecting a site for a replacement nest, annexed adjacent areas previously occupied by other pairs. Pair 2 relocated in a space that originally included territories 1 and 4, and pair 6 built a nest in an area formerly occupied by pair 7. Males 1 and 4 were sacrificed for specimens and pair 7 probably was destroyed by a predator. Owing to the presence of a nest, the annexed area becomes the focal point of the activities of a pair, but the original area is regularly visited and may be returned to in a later renesting.

Table 2. Size of the Nine Original Territories Occupied in 1960.

Maintenance of Territory

Except in the early stages of nesting, territory is maintained primarily by song. In the period of incubation a male regularly patrols his territory between sessions of sitting on the eggs. He sings several songs from each of several perches. A male follows a predictable path, rarely traveling more than 150 feet from the nest. Incipient patrolling is seen early in the breeding season when territorial boundaries are in a state of flux.

The male White-eyed Vireo travels a semi-predictable route, as does the Solitary Vireo (R. F. Johnston, MS). According to Lawrence (1953:50), the male Red-eyed Vireo has a distinct singing area completely divorced from the nest area dominated by the female. Southern (1958:109), working with this same species in Michigan, did not recognize separate areas, but found that the male wandered randomly over the territory. In a species so highly active as the Bell Vireo, the degrees of hostile action associated with an encounter overlap in such a fashion that no clearcut distinction can be drawn among the various displays. Nevertheless, certain generalized patterns are characteristic of all situations in which members of this species are in a state of anxiety. The threat displays described in the succeeding paragraphs may all be utilized within as little as two minutes; mutual agonism may be terminated at any stage by concerted attack of the dominant bird.

1. Vocal threat. When an intruder is discovered the resident male markedly increases his rate of singing. The alarm note, eh-eH-EH, is the first call uttered during the nestbuilding and egglaying periods.

2. Head-forward threat. If the intruder does not flee, the resident male adopts a specific threat posture. The head and neck are extended. The feathers of the crown are erected, but those of the body are sleeked. The bird crouches slightly and the tail is flicked laterally, but not fanned. The intensity of the singing increases and is supplemented by scolding, also delivered at a rapid rate. The intruder normally retreats at this juncture.

3. Wing-flicking and submaximal tail-fanning. If the interloper remains, the anxiety of the resident male increases. He slightly depresses the tail and, at the same time, rapidly fans and closes it. The tail is only partially fanned. The wings are held slightly away from the body and rapidly flicked above the back. This flicking should not be confused with quivering of the wings associated with begging and other solicitory postures. Song is now almost completely replaced by high-intensity scolding. Associated with this high degree of anxiety are displacement behaviorisms, including bill-wiping, reversal of direction on a single perch, and a nervous hopping from one perch to another.

4. Ruffling and maximum tail-fanning. This display is most often seen in conjunction with the harassment of predators, but occasionally it is observed in territorial disputes occurring at the boundary of adjacent territories where neither male is strictly dominant and in which there is much vacillation prior to attack. The feathers of the abdomen are ruffled. The term "ruffled" pertains to a full erection of the feathers, giving a ragged appearance to the body outline (Morris, 1956:80). Ruffling of the abdominal feathers emphasizes their yellow color and seemingly heightens the intimidatory effect. The tail is fully fanned, and so maintained, for a few seconds at a time; it is held at a 45° angle to the body. The scold becomes an extremely intense, stacatto buzz, ZZ-ZZ-ZZ-ZZ.

5. Supplanting attack. The attack directed against a trespassing male is initiated as a lunge that results in a collision with the opponent in mid-air or on his perch. The bird attacked is struck by his adversary's open beak or body.

Hinde (1952:71-72) indicates four courses of action followed by a Great Tit (Parus major) when attacked under similar circumstances. "(a) It flies away: The attacker usually flies after it and a chase ensues. (b) It shifts its perch a few inches: the attacker lands in its place, and both usually show head-up postures. (c) It remains where it is, but adopts a head-up posture: the attacker usually then shows upright flight. (d) It may fly up and meet the attacker in mid-air: in that case an actual combat may result, or both combatants 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

[B] Directed against a stuffed Bell Vireo.