Natural History of the Bell Vireo, Vireo bellii Audubon Part 3

Maximum tail-fanning prior to attack also appears as an element of aggressive behavior in White-eyed Vireos. A brief skirmish between a male of this species and a small, greenish pa.s.serine was observed at the Natural History Reservation on May 25, 1960. The White-eyed Vireo was singing from a perch 30 feet high in a dead elm, when the unidentified pa.s.serine landed 10 feet distant. The white-eye ceased regular song and uttered several catbirdlike calls, and at the same time slightly depressed and fully fanned the tail. After 10 seconds, the white-eye lunged at the intruder, striking it in mid-air. A brief looping flight ensued through the branches of the elm before the intruder was able effectively to retreat.

_Aggressive Behavior of the Female_

The female Bell Vireo is concerned primarily with the defense of the nest and the young and she rarely a.s.sists the male in defense of distant parts of the territory. She employs the same threat displays as the male.

_Interspecific Relations.h.i.+ps_

A number of meetings between Bell Vireos and other species were observed in the course of the study (Table 4). Resident pairs of this species exhibited different degrees of tolerance toward other species.

Many birds, including Cardinals, Field Sparrows, Painted Buntings and Mourning Doves were ignored completely. Chickadees evoked responses characterized by slight increase in song and some anxiety; this was perhaps owing to similarity in size, motion and call notes. Warblers, when met with, were invariably chased. They may be momentarily mistaken for rival vireos.

TABLE 4. INTERSPECIFIC CONFLICT OBSERVED IN 1959 AND 1960.

================================================================== | Number | Phase of | Behavior of | | | Bell Vireos Species | of | breeding +------+---+----+--- |conflicts| cycle |HFT[C]| S | TF | A ---------------------+---------+--------------+------+---+----+--- _Coccyzus | 1 | Nestling | | | | america.n.u.s_ | | period | -- | x | | | | | | | | _Cyanocitta | 3[D] | Nestling and | | | | cristata_ | | incubation | | | | | | period | x | x | x | x | | | | | | _Parus atricapillus_ | 1 | Prenesting | -- | x | | | | | | | | _Molothrus ater_ | 1 | Nestling | | | | | | period | -- | x | -- | x | | | | | | _Dendroica petechia_ | 1 | Prenesting | -- | x | -- | x | | | | | | _Geothlypis trichas_ | 1 | Nestbuilding | -- | x | -- | x | | | | | | _Pituophis | | | | | | catenifer_[E] | 1 | Post-fledging| -- | x | -- | x ---------------------+---------+--------------+------+---+----+---

[C] HFT = head-forward threat; S = scolding; TF = tail-fanning; A = attack.

[D] Includes attack against a dummy Blue Jay.

[E] The Bull Snake is here included because the vireos directed typical aggressive displays towards it.

Blue Jays were vigorously attacked, especially late in incubation and throughout the nestling period of the Bell Vireo. I did not see a jay struck, but a vireo would circle one closely as it perched and pursue it when it flew, following as far as 100 yards beyond territorial bounds. The buzz, ZZ-ZZ-ZZ-ZZ, was uttered in conjunction with this hara.s.sment.

A stuffed jay placed eight feet from a nest elicited threat display and displacement behavior from the owners of the nest, but no attack.

Incubation 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 hara.s.s 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 cla.s.sic "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 s.h.i.+fted. 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 t.i.t, 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 pa.s.serines, 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 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 relations.h.i.+p. 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.

COURTs.h.i.+P 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 pa.s.sing 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.

Early courts.h.i.+p activities of the Bell Vireo are characteristically violent affairs, with the male directing strong aggressive attacks toward the female. Rapid, looping flights through the thickets occur, the female leading the male. Occasionally he deliberately collides with her in mid-air, but the pair quickly separate. This violent s.e.xual chasing is manifest prior to the inception of nestbuilding.

With commencement of this activity, s.e.xual chases through the territory subside.

Absence of s.e.xual dimorphism in the Bell Vireo obviously suggests that behavioral criteria are used by the birds in s.e.x-recognition. The lack of aggression by the female upon initial aggression by the male is an essential component of recognition of s.e.x; she is clearly subordinate.

Such subordination is also the significant feature of continued s.e.x-recognition. Courts.h.i.+p display by a resident male, directed toward a stuffed male and a wounded male which sat motionless, supports the contention that a subordinate or submissive att.i.tude of the female is a key factor in s.e.x-determination.

Nestbuilding and courts.h.i.+p are intimately a.s.sociated in this species.

The male constructs the suspension apparatus of the nest, the completion of which coincides with the a.s.sumption of nestbuilding activity by the female. Roles of the s.e.xes in nestbuilding are described in the section on nestbuilding. The male frequently interrupts construction to court the female. This, in combination with perpetual song as he works, serves to strengthen the pair-bond and stimulate nestbuilding tendencies of the female.

It is doubtful that any attempts at copulation are successful up to this time. The female is singularly unresponsive to the advances of the male; a female retreats before most violent attacks and is seemingly oblivious to less vigorous behavior. After the female a.s.sumes the responsibility of building, the tempo of courts.h.i.+p activities increases.

The female becomes increasingly more receptive and her work is often interrupted by advances of the male. Copulation occurs frequently from about the third day of nestbuilding through the first day of egglaying, a period of four to six days. Male displays and vocalizations a.s.sociated with courts.h.i.+p continue through the fourth or fifth day of incubation.

_Displays and Postures_

The princ.i.p.al courts.h.i.+p displays and postures that were seen throughout the nestbuilding phase are as follows:

1. Greeting ceremonies. Both birds are crouched from one to five inches apart. The feathers on one (the male?) are sleeked, and on the other are fluffed. Fluffing (Morris, 1956:80) denotes partial erection of the body feathers producing a rounded, unbroken body line and is not to be confused with ruffling, mentioned in the sections pertaining to territoriality and pre- and post-copulatory display. Fluffing is generally considered to be an appeas.e.m.e.nt display and it is seen in a variety of situations involving a dominant-subordinate relations.h.i.+p.

Both birds flick wings and tails rapidly and reverse directions on their perches frequently. A low, rapid _chee_ is uttered during this performance. This ceremony is repeated often in the first three days of nestbuilding, but less frequently thereafter. It usually occurs after building by one or both partners and prior to another trip in search of nesting material. It lasts from 10 to 50 seconds and is not immediately followed by any additional courts.h.i.+p activities. Nolan (1960:228-229) observed mutual displays between periods of violent s.e.xual chase that suggest that the greeting ceremonies that I have described are an integral part of pair-formation as well as a component of continued maintenance of the bond.

2. "Pouncing." The female rapidly quarter-fans and partially depresses her tail. She utters a high pitched scold (_chee_). The male, from a perch within two feet of the female, fans the tail fully and depresses it vertically, and, with mouth open, lunges at the female; or, with similar tail mannerisms, the abdominal feathers ruffled, the wings held horizontally, and the primaries spread, he sways from side to side from four to six times, and then lunges at the female. The male is silent when he pounces; the _chee_ or the courts.h.i.+p song is emitted when swaying precedes pouncing. The male strikes the female with his breast or with his open beak. The female rarely flees although she is usually displaced several inches along the branch upon which she is sitting. However, the female may fly several inches to a new perch.

The failure of the female to adopt a solicitation posture presumably indicates s.e.xual unreadiness. Instances of the male deliberately colliding with the female as she flies in the course of gathering nesting material are probably a.n.a.logous to pouncing. In none of the above situations are females observed to fight back in any way. Nice (1943:174) believed pouncing to be a.n.a.logous to s.e.xual chasing found in such species as the Red-winged Blackbird. In the Song Sparrow, pouncing is observed most often in the first and second days of nestbuilding.

3. "Leap-flutter." The male, in the course of displaying with the tail fanned before the female, suddenly leaps eight inches to ten inches vertically and flutters in mid-air several seconds, before dropping to the original perch. This display occurs in full view of the female. It is often a.s.sociated with pouncing and is also seen prior to copulation. In the latter instance it is probably pragmatically functional, for it permits the male to orient above the female before dropping to her back to copulate. No vocalization is uttered during the leap-flutter.

4. Pre-copulatory display (Fig. 3). The male faces the female. The tail is fanned fully and depressed at a sharp vertical angle to the body. Body feathers, both dorsal and ventral, are ruffled, almost tripling the apparent volume of the thorax. The head is withdrawn and slightly thrown back. Feathers of the head are not erected. The mouth is opened wide. The legs are slightly flexed and the body is swayed laterally. Horizontally, the head and body traverse an arc of about 100; vertically, they traverse an arc slightly less than 180. At the low point of any one swing, the delivery of the courts.h.i.+p song begins.

At the termination of the swing the two normal, ascending notes are emitted. This performance may last as long as three minutes.

[Ill.u.s.tration: FIG. 3. A single male Bell Vireo in the pre-copulatory display. Note the ruffled dorsal and ventral body feathers. The male on the left has reached the zenith of a single swing. The male on the right has nearly reached the low point of a swing.]

The pre-copulatory display of the male elicits receptive behavior in the female. She crouches in a solicitous manner, with the body feathers fluffed and the tail raised slightly, and utters a muted _chee_.

5. Copulation. The male abruptly terminates his swaying display with a leap-flutter that positions him above the female's back. He then descends and copulation occurs. The male continues to flutter his wings to maintain balance throughout the two seconds of cloacal contact. Following an unsuccessful copulation on June 23, 1960, displacement preening and bill wiping were performed by both s.e.xes.

6. Post-copulatory display. On June 25, 1960, after a second attempt at copulation with a stuffed bird in which s.e.m.e.n was actually deposited on the dummy's back, male 10 (1960) performed a swaying display. In this instance, however, instead of addressing the dummy from the front, the male alighted one inch to the right of the stuffed bird. When swaying to the left (toward the dummy) the head of the displaying male actually pa.s.sed above the neck of the stuffed bird.

This ritualized behavior could conceivably be derived from hetero-preening.

_Discussion_

Within the scope of my research it was difficult to detect the over-all sequence of epigamic displays that result in synchronization of the physiological states of the s.e.xes throughout the period of courts.h.i.+p. Possibly all displays, except the post-copulatory one, occur in no particular order in the courts.h.i.+p period. However, each ritualized display seemingly strengthens the pair-bond.