Natural History of the Bell Vireo, Vireo bellii Audubon

Chapter II: Part 2

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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 associated with courtship continue through the fourth or fifth day of incubation.

_Displays and Postures_

The principal courtship 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 appeasement display and it is seen in a variety of situations involving a dominant-subordinate relationship. 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 courtship activities. Nolan (1960:228-229) observed mutual displays between periods of violent sexual 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 courtship 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 sexual unreadiness. Instances of the male deliberately colliding with the female as she flies in the course of gathering nesting material are probably analogous to pouncing. In none of the above situations are females observed to fight back in any way. Nice (1943:174) believed pouncing to be analogous to sexual 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 associated 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 deg.; vertically, they traverse an arc slightly less than 180 deg.. At the low point of any one swing, the delivery of the courtship song begins. At the termination of the swing the two normal, ascending notes are emitted. This performance may last as long as three minutes.

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 sexes.

6. Post-copulatory display. On June 25, 1960, after a second attempt at copulation with a stuffed bird in which semen 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 passed 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 sexes throughout the period of courtship. Possibly all displays, except the post-copulatory one, occur in no particular order in the courtship period. However, each ritualized display seemingly strengthens the pair-bond.

Swaying has been recorded in a variety of situations of a sexual and semi-sexual nature for the Solitary Vireo (_V. solitarius_; Townsend, 1920:158) and the Red-eyed Vireo (Tyler, 1912:230; Bent, 1950:342). In every instance the body feathers of the swaying birds were sleeked. Courtship behavior in any species of North American vireo seems closely to resemble that of any other; pairing and nestbuilding of a female _V. solitarius_ and a male _V. flavifrons_ as reported by Hauser (1959:383) support the idea of close resemblance.

A marked similarity will be detected between certain basic elements of aggressive and epigamic displays. These basic elements are wing- and tail-flicking, tail-fanning, and high-intensity delivery of the _chee_. Pouncing and supplanting attacks are essentially similar. Such similarities suggest either a common origin for certain aggressive and epigamic displays or the derivation of one from the other.

High-intensity _cheeing_ is obviously a function of excitement, whether in conjunction with hostility or sexual behavior. According to Andrew (1956:179), flicking of wing and tail in passerines are intention movements of flight. These actions have been emancipated from incomplete take-offs and incorporated in ritualized courtship and agonistic behavior. In incipient courtship behavior the male is governed by three conflicting tendencies; to flee, to attack, or to behave sexually before his mate (Tinbergen and Hinde, 1958:256). When pairing, Bell Vireos interrupt sexual chase with "greeting ceremonies," the male's tendency to attack and the female's tendency to flee are momentarily reduced, and the forming bond is strengthened. Thus, the intention movements become an integral part of courtship.

In situations where attacking and fleeing are the two conflicting tendencies, wing-flicking and tail-flicking are incorporated into threat display, but do not lose all of their original function, for they facilitate attack. Tail-fanning, as a display element, increases the awesome aspect of the threatening bird and in courtship presumably makes the sexes more attractive to one another.

Courtship feeding has not been recorded for the Bell Vireo. In general, it is unknown in North American vireos, with the exception of the red-eye (Lawrence, 1953:53). It would serve no "practical" purpose in the Bell Vireo since the male regularly relieves the female during incubation, thus allowing her ample opportunity to forage. In the Red-eyed Vireo, only the female regularly incubates, and courtship feeding is definitely functional. Nolan (1960:228) described a brief pecking or pulling with their bills between pairing birds. This may be incipient "symbolic" courtship feeding, or perhaps mutual preening.

SELECTION OF NEST-SITE AND NESTBUILDING

As far as can be determined, the nest-site is selected by the female. Typically, the pair makes short, low-level flights from tree to tree with the female invariably in the lead. The birds usually forage within each tree; the female interrupts this activity to inspect small forks of low, pendant branches and the male occasionally pauses to sing. The singing is loud but not particularly regular, as it is later when the male accompanies the female during actual nestbuilding. Method of selection of site resembles that described by Lawrence (1953:53) for the Red-eyed Vireo.

Nests are suspended from lateral or terminal forks about 27 inches high in bushes and small trees that, in the study area, averaged 11 feet, four inches in height (Table 5). The height above ground of the nests does not vary appreciably as the season progresses as is the case with nests of Red-eyed Vireos, for which Lawrence (1953:54) noted that late nests were placed higher than those built earlier in the season.

Most nests are so situated that they are protected and concealed by the dense foliage of trees. Where nests are placed in low bushes, as coralberry or dogwood, the bush is invariably overhung by the foliage of a much taller shrub or tree.

The nest tree or shrub was in every instance situated at the edge of a thicket or isolated from adjacent trees by several feet. Preference for open situations is characteristic of the species. In contrast, the nest of the White-eyed Vireo (Bent, 1950:229) is placed toward the center of thickets.

In the choice of sites in the study area, the Bell Vireos were almost unopposed by other avian species, owing to the size of the fork utilized and the fact that the nests are located peripherally, rather than centrally, in the bush or tree. This lack of competition for a nest-site provides a Bell Vireo with an ample supply of nest-sites within any one territory.

TABLE 5. NEST-SITES UTILIZED IN 1960.

Selection of the first nest-site may take as long as two days, possibly owing to incomplete development of the nesting tendency, but more likely to a general lack of familiarity with the territory. Red-eyed Vireos require five to six days to choose the first nest-site (Lawrence, 1953:54). Later sites of the Bell Vireo are chosen in as little as three hours. Nest 1-c (1960) was abandoned at about 11:00 a.m. on May 14, 1960, when part of the thicket on the edge of which this nest was located was removed by brush-cutters clearing a power line right-of-way. By 2:00 p.m. this pair had begun construction of 1-d (1960) in an Osage orange 110 feet southwest of 1-c (1960).

This particular site is of further interest because it is the same one utilized for nest 1-a (1960). In all, four instances of utilization of a nest-site a second time were recorded. Two-a (1960) and 2-d (1960) were built in the same fork; 1-c (1960) and 1-h (1960) were in the same tree, but not the same fork. It should be mentioned that 1-a (1960) and 2-a (1960) were abortive attempts that did not progress beyond the suspension apparatus. Nice (1929:16) recorded a similar instance of the re-use of a nest tree, but different forks were used.

Re-use of an exact nest-site would ordinarily be impossible if the initial attempt were not abortive, because the presence of a completed nest would pose problems in construction with which the birds would probably be unable to cope. (A report by Morse in Bent, 1950:256 of a double nest indicates that this may not always be true. At the time of discovery one nest contained two eggs and the other nest contained young.) Since nests are used only once there would be no tendency to adopt the old nest. However, abortive nests, usually little more than a few strands of nesting material secured to the fork, might stimulate the birds to continue building. Re-use of a single nest-site in 15.8 per cent of 38 nests built in 1960 seems to be more than fortuitous circumstance. This re-use may have physiological benefits in conjunction with apportionment of energy for other nesting activities, because rapid location of a nest-site would mean that energy normally expended in searching and selecting could be rechanneled for actual construction. In each of the instances of rebuilding, the new nest was begun on the same day that the previous nest was abandoned.

The re-nesting of pair 9 (1960) is worthy of note. These birds were established in the elm thicket on Clark land. Elm was by far the most abundant tree, with dogwood, Osage orange and honey locust also relatively common. There were only six boxelders in the territory and yet the four nests built by this pair were placed in them. This is the only instance of seeming preference.

_Building_

Nestbuilding by Bell Vireos can be best discussed in terms of the phases of construction described for the Red-eyed Vireo, Lawrence (1953:57), which are: (1) construction of the suspension apparatus, (2) construction of the bag, (3) lining of the bag and smoothing and polishing of the exterior, and (4) adornment of the exterior. Red-eyes (Lawrence, 1953:59) may continue adornment far into the period of incubation. Both the male and female Bell Vireo have been observed to add spider egg sacs and other silk to the exterior of the nest as late as the sixth day of incubation.

Nice (1929:16) recorded only the female Bell Vireo building, but she did recall, from previous studies, having seen males aiding somewhat. Pitelka and Koestner (1942:102) wrongly concluded that the female Bell Vireo builds unaided, but Hensley (1950:243) observed that both sexes participated in nestbuilding, and Mumford (1952:229) reported two instances of building by both adults. His description of the activities viewed in mid-May suggest that they were of the transitional period between the first and second phases. On the second occasion he recorded both adults building during the second phase. Since no details accompany this second observation I assume that it pertained to activity not necessarily typical of this phase of construction. Whereas both sexes of the Bell Vireo cooperate in building the nest, only the female Red-eyed Vireo builds according to Lawrence (1953:56). But Common (1934:242) saw both Red-eyed Vireos building a nest.

The suspension apparatus is constructed by only the male on the first day. He punctuates each trip to the nest with song. The single song phrase is given from three to eight times when the male, carrying nesting material in his bill, arrives in the tree. Typically, he alights on several perches within the nest tree before flying to the nest. He often interrupts his work with several songs; when he has finished adding a load of material he sings from several perches within the nest tree before departing. The male periodically stops building to court the female.

In eight hours (494 minutes) of observing the first phase of construction at five different nests, I saw the female come to the nest 28 times; the male made 95 trips. The female came alone only once, and brought nesting material ten times, but did not build; on the other 18 occasions her visits were brief and she usually confined her activities to an inspection of the nest. Twenty of the visits by the female were made late in the first phase, marking a gradual transition to her assumption of building responsibility. (The delay by the female in beginning to build is puzzling; because all evidence indicates that she helps select the nest-site, I would expect her to help with the initial building. There seems to be no clear advantage in her delay in beginning to build.) The courtship and building activities of the male plus the presence of a partly completed nest seem to stimulate the female to commence building. Her visits become more frequent as construction of the suspension apparatus nears completion. At a time early in the second day the transition has taken place, and the female becomes the sole worker.

On May 7, 1960, male 2 (1960), at the time unmated, was observed as he came upon a nest of the previous year. The nest, after a year's weathering, suggested in appearance perhaps an early second-day nest. The bird flew to the nest and tugged and wove loose strands of grass for three minutes. Before leaving the site, the bird sang twice from different perches. This observation suggests that a partly constructed nest can elicit nestbuilding behavior, even in an unmated male.

The techniques of building by the male consist primarily of laying pieces of grass or bark across the fork, or along one of its branches, and then fastening them in place with pieces of animal silk. Once a "racket" has been formed, spider egg cases and plant down are emplaced among the fibers. The male employs weaving, twisting, and pecking motions of the head to emplace material.

As previously indicated, the female is the principal worker in the second and third phases of construction. The male infrequently visits the nest, but regularly visits the nest tree. The molding of the bag is accomplished by piling leaves, grasses and plant down onto the suspension apparatus. This material is also bound in with animal silk. As the amount of material accumulates, the female begins to trample it and gradually the bag takes shape. When trampling is first attempted, the nest often fails to support the female and she falls through the bottom of the nest. Such an occurrence was observed on May 23, 1960, on three consecutive trips by female 1 (1960), in constructing nest 1-e (1960). As the bag deepens, additional strands of grass are added to the wall and woven into place.

The male is extremely attentive during this and the following phase. He follows the female as she gathers nest-material accompanying both this activity and her building with rapid song; he may give an average of seven song phrases per minute. The male brings to the nest a strand of grass, or some other material, about every twentieth trip. He frequently inspects the nest and the activities of the female from perches near the nest. Construction of the bag is ordinarily completed in the third day.

The third phase, the lining of the interior and the smoothing of the exterior, involves an additional one and one-half to two days. Smoothing of the exterior refers to tightening of the grasses woven into the bag and addition of more animal silk. In lining the nest, the female stands on one of the branches of the fork and emplaces one end of a long, thin strand of some relatively stiff piece of grass or strip of bark. She then jumps into the bag and, while slowly turning around, pecks it into place, thus coiling the strand neatly around the interior of the bag.

As previously mentioned, the fourth phase overlaps the periods of lining, smoothing, egglaying, and incubation. The principal activity is the addition of white spider egg sacs to the exterior. The trips are infrequent; but, occasionally, birds will interrupt an hour of incubation with three or four minutes of active adornment, during which several trips may be made. Both sexes participate in this phase.

_Gathering of Nesting Material_

Nesting materials were gathered anywhere within the territory. Occasionally materials were collected from within the nest tree, but usually they were obtained 20 to 200 feet from the nest-site. On several occasions I observed birds inspecting stems or branches where bark was frayed. Loose ends are grasped in the beak and torn free with an upward jerk of the head. Possibly the notch near the distal end of the upper mandible aids in grasping these strands. Plant down is first extracted and then rolled into a ball by means of the beak while held with the feet before being transported to the nest.

_Length and Hours of Nestbuilding_

As indicated by Nolan (1960:230), accurate determination of the length of nestbuilding is difficult because of continued adornment and polishing after the nest is functionally complete. Most of the early nests for which I have records took from four and one-half to five days to construct. A four-to five-day period of building is reported by other observers (Nice, 1929:16; Pitelka and Koestner, 1942:99; Hensley, 1950:242; Nolan, 1960:230).

One instance of protracted building was recorded. Nest 6-d (1960) was begun on May 29, 1960, and not completed until nine days later on June 6, 1960. In contrast nest 1-g (1960) begun on May 31, 1960, was finished three days later on June 2, 1960. Nestbuilding occurs between the hours of 6:00 a.m. and 5:30 p.m. Heavy rain in the early morning may delay building.

_Abortive Nestbuilding Efforts_

Eight of 38 nests started in 1960 were never completed (Table 6). Six of these abortive attempts were abandoned during, or shortly after, the completion of the suspension apparatus. Five of these nests were abandoned because the female did not begin building following the end of work by the male. The early abandonment of the other three nests 1-a (1960), 2-c (1960) and 6-e (1960) was attributable to the interruption of building by the male because of heavy rain and protracted territorial conflicts. The occurrence of these abortive nests at any time within the nesting efforts of a single pair indicates that such attempts are not examples of "false nestbuilding."

_Renesting_

Renesting after desertion or successful fledging occurs within two to thirty-six hours. Young were fledged from 1-a (1959) on June 19, 1959, and nest 1-b (1959) was discovered when late in the second phase of construction on June 22. If the nest was started on June 20, then renesting took place within 15 hours after fledging.

_The Nest_

Several authors have described various aspects of the nest of the Bell Vireo, notably Goss (1891:535); Simmons (_in_ Bent, 1950:256), Nice (1929:13) and Nolan (1960:230-231). I can add but little to these descriptions.

The nest itself is a compact structure composed of strips of bark and strands of grasses that are interwoven and tightly bound with animal silk. The floor of the cup is first lined with a layer of small leaves and then the entire interior is lined with fine stems or strips of bark. Feathers are occasionally used to pad the bottom prior to lining, as are pieces of wool and milkweed down. Nest 2-e (1960) had been packed with small pieces of soil bearing moss prior to lining.

TABLE 6. ABORTIVE NESTING ATTEMPTS IN MAY AND JUNE OF 1960.

Early nests tend to be bulkier, having thicker walls and bottoms than later efforts. However, nests in May were found to have 16 per cent thicker bottoms and 41 per cent thicker walls than nests in June (Table 7). Standard nest measurements do not show this to be so, for the exterior and interior diameters at the rim are governed by the angle between the two branches of the fork.

TABLE 7. DIMENSIONS OF NESTS IN MAY (1960) AND JUNE (1960).

========================================================
Measurements | May (N 10) | June (N 8)
------------------------+---------------+---------------
External depth | 61.6 mm. | 59.3 mm.
Depth of cup | 45.5 mm. | 46.3 mm.
Outside diameter | 57.3/55.5 mm. | 54.3/53.5 mm.
Inside diameter | 43.4/42.2 mm. | 45.5/43.9 mm.
Thickness of forward | |
wall 1 inch below rim | 13.8 mm. | 7.6 mm.
Thickness of bottom | 11.3 mm. | 4.6 mm.
------------------------+---------------+---------------

EGGLAYING AND INCUBATION

_Egglaying_

Egglaying begins the first or second day after completion of the nest. The female sits in the nest occasionally for periods of five to twenty-five minutes on the day the nest is completed. This is interrupted by periods of nest-adornment and foraging; such activities sometimes keep the female off the nest for several hours. Prior to the laying of the first egg, only the female is seen on the nest, although the male is often seen sitting quietly within the nest tree a few feet from the female. The infrequency of the "congested" song and the alarm (_eh-eH-EH_) after the inception of "broodiness" indicates the waning of courtship behavior. As later in incubation only the "normal" song and the scold are heard.

Eggs are laid early in the morning prior to 5:30 a. m. according to Nolan (1960:232). The nest is usually left unoccupied for considerable periods after the first egg is laid, but, on the first day of laying, both sexes have been observed sitting for brief periods averaging ten minutes in length. Eggs are laid at one-day intervals until completion of the clutch. I found incubation to begin with the second egg.

_Clutch-size_

The average clutch-size of the Bell Vireo in Kansas, based on thirty-three records, is 3.39 eggs (Table 8). Seasonally, the largest average clutches are produced in the middle of the breeding season, that is, in June. Lack (1947:308-309) indicates that in European passerines the highest seasonal average clutch-sizes likewise occur in June. The largest average clutch-size in the Bell Vireo is presumably related to some aspect of the availability of food.

TABLE 8. AVERAGE NUMBERS OF EGGS PER NEST (NUMBER OF RECORDS
IN PARENTHESES)[F].

========================================================
| | | | Mean
Year | May | June | July | annual
| | | | clutch-size
----------+---------+----------+---------+--------------
1959 | 3.0 (7) | 3.2 (12) | 3.0 (1) | 3.06
1960 | 3.3 (6) | 3.83 (5) | 4.0 (2) | 3.72
----------+---------+----------+---------+--------------
1959-1960 | 3.17 | 3.52 | 3.5 | 3.39
----------+---------+----------+---------+--------------

[F] These data have been supplemented from the literature
pertinent to Kansas.

Caution is necessary in determining mean clutch-size in the Bell Vireo. Eggs occasionally disappear from the nest prior to or during incubation, without subsequent addition of cowbird eggs. Unfamiliarity with the history of such a nest on the part of the observer would lead to an inaccurate determination of clutch-size.

Complete clutches are not replaced with the same regularity as are nests. I have recorded intervals of six to thirty days between successive clutches. Successful replacement of clutches is determined by a number of factors: nest-site, completion of a nest, weather, predation, and parasitism by the cowbird. The difference between the number of renesting attempts and the successful replacement of clutches seems to indicate that different physiological processes are responsible for these two phenomena and that there is lack of synchrony between them. The development of the ovarian follicle requires a specific number of days that is not always coincident with the building of replacement nests. If, in the Bell Vireo, replacing a nest were solely a responsibility of the female, instead of involving the male to a considerable extent, it would seem likely that replacement of nests and the replacement of clutches would be more closely coordinated.

_Incubation_

Nice (1954:173) considers the incubation period to be the elapsed time between the laying of the last egg in a clutch and the hatching of that egg, when all eggs hatch. My data indicate that, normally, intensive incubation begins when the second egg is laid and lasts fourteen days in the Bell Vireo. Nice (1929:99) also considered the incubation period in this species to be fourteen days but believed it to commence when the third egg was laid. Pitelka and Koestner (1942:99) noted that the first and second eggs hatched fourteen days after laying of the second egg. However, they thought incubation began with the first egg. This would mean a fifteen-day period for this egg. All the eggs that Nolan (1960:234) marked hatched in approximately fourteen days. Eight eggs artificially incubated by Graber (1955:103) required an average of 15.01 days to hatch. As Van Tyne and Berger (1959:293) indicate, periods of sitting on the nest, even all night, do not necessarily mean that incubation has begun, for it has been demonstrated in several species that birds may sit on an egg without actually applying heat. My own observations demonstrate that the first egg may be left unattended for several hours at a time on the day that it is laid.

_The Roles of the Sexes in Incubation_

Both the male and female sit on the eggs in the daytime. My study of histological sections of ventral epidermis indicates that the male does not possess a brood patch; the increased vascularization typical of the brood patch in females is not evident in males. But, the male loses most of the down feathers of the ventral apterium. Also, according to Bailey (1952:128), the male Warbling Vireo that sits on the eggs lacks a brood patch.

Bailey (1952:128) suggests that male passerines lacking brood patches that habitually sit on eggs do not heat the eggs. Thus it cannot be considered true incubation since no increase of temperature in the eggs is effected by such means. He further notes that it is at night when eggs are likely to experience a drop in temperature that embryonic development will be impaired. I have no data directly pertaining to which sex sits at night, but it is presumably the female, because she is always seen on the nest early in the morning and late in the evening.

If a highly-vascularized brood patch is essential for true incubation, then it is surprising that males take regular turns on the nest in cold, rainy weather. On May 20, 1960, male 3 (1960) sat on the eggs longer than did the female (fig. 4). The temperature during this hour and a half of incubation was 54 deg. F. One solution to this problem is supplied by Skutch (1957:74). He indicates that, "the type of the incubation is determined largely by innate factors, so that it persists through fairly wide fluctuations in weather, although it may break down in extreme conditions." Obviously then, in the example described above, the weather conditions do not qualify as "extreme." Sitting by the male is certainly functional to some extent for it relieves the female to forage; furthermore, the eggs are sheltered from inclement weather and protected from predators. Nolan (1960:232) suggests similar reasons for incubating by the male and adds the "conservation of heat supplied to the eggs by the female."

My data, based on incubation beginning with the second egg, indicate that the female incubates more often daily than the male (fig. 5). The male sits on the eggs only occasionally in the morning, but almost as often as the female in the afternoon. Nolan (1960:233) found that 95.5 per cent of the male's time on the nest and only 40 per cent of the female's time were attributable to the early hours of the day. Although I lack data on the critical hours of 5:00 a.m. to 6:59 a.m., I have enough observations (20) from 7:00 a.m. to 9:00 a.m. to indicate that the males sit on the eggs infrequently (3 of 20 instances) in those hours. The discrepancy in the two sets of data, which may be merely an artifact of sampling techniques, does suggest two possible alternatives: (1) the male sits on the eggs in the morning and gives the female, who sits on the eggs throughout the night, an extended rest and an opportunity to forage; (2) the female continues to sit throughout the morning, especially during the early hours of daylight, a time of day when the temperature may still be low enough to impair development of the embryo.

_Relief of Partners in Incubation_

Relief of partners involves some ceremony. When the female is incubating, the male sings several times as he approaches the nest tree; the female responds with several _chees_, but otherwise remains immobile. The male sings several more times upon alighting in the nest tree whereupon the female _chees_ again and flies directly from the nest. A few seconds later the male appears at the edge of the nest and, after inspecting the eggs, hops in and settles upon them. When the male is sitting he is notably anxious prior to an exchange with the female, often arising and craning his neck as he surveys the surrounding vegetation, seemingly searching for his mate. The singing of the male and the calling of the female serve as signals, coordinating the exchange.

NESTLING PERIOD

_Hatching Sequence_

As indicated earlier, hatching normally occurs fourteen days after the second egg is laid. Hatching of the young was staggered at three nests under observation. In nest 2-b (1959) the first young hatched on June 8, 1959, the second on June 10. In 3-b (1959) one young hatched each day from the 12th through the 14th of June. In 5-a (1959) two young hatched on June 15, the third on June 16, and the fourth on June 17. Size of the young differed notably for about three days as a result of staggered hatching, but after that day the younger birds tended to catch up in size with their older brood-mates. The fourth young in nest 5-a (1959) grew steadily weaker and was missing from the nest on June 23, 1959. Staggered hatching is usually thought to be related to the availability of food that will insure survival of at least some of the nestlings when a shortage of food exists. It is doubtful that staggered hatching has adaptive significance in the Bell Vireo, since there seems to be no shortage of food for the young. In small passerines such as the Bell Vireo the principal problem is to insure fledging as quickly as possible because of the danger from predators.

_Development of the Nestlings_

Young are pinkish at hatching and devoid of visible natal down. Du Bois (_in_ Wetherbee, 1957:380), inspected day-old nestlings by means of a magnifying glass and was unable to detect any down. Nolan (1960:236) also indicates that the young are naked at birth and that the "body color is between flesh and rufous except where folds of the straw yellow skin obscure the underlying colors." The Hutton Vireo (_Vireo huttoni_) is essentially naked at birth, save for sparse hairlike down on the head and back (Wetherbee, 1953:380). The Red-eyed Vireo, according to Lawrence (1953:67) is naked at birth save for a sparse covering of greyish natal down, on the head, shoulders, and back.

In the Bell Vireo the pterylae darken slightly on the second day and the color becomes more intense daily until the quills of the dorsal tracts, the wings, and the tail break from their sheaths on the sixth day. In Red-eyed Vireos the pterylae darken by the end of the first day and the quills break through the skin on the fifth day, erupting from the sheaths by the seventh day (Lawrence, 1953:67).

From the first day the young are able to squeak. Poking a young bird was sufficient to elicit this sound, phonetically a nasal _peek_. The only other vocalization noted throughout the nestling period was an abbreviated _chee_.

For the first three days tapping the nest or even movement of it caused by wind would elicit begging. By the fifth day at nest 2-a (1959) only vigorous agitation of the branch to which the nest was attached evoked any response. At this nest on June 16, 1959, one young begged while the other cowered. Cowering is correlated with opening of the eyes, as the young bird that begged had its eyes only partly open. Both young cowered on June 19, 1959. Table 9 summarizes the maturation of the nestling Bell Vireos.

TABLE 9. MATURATION OF NESTLING BELL VIREOS. THE FIRST DAY
THAT AN ACTIVITY WAS OBSERVED IS SHOWN.

==================================================================
| Day of nestling life
+---+---+---+---+---+---+---+---+---+----+----
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11
--------------------+---+---+---+---+---+---+---+---+---+----+----
| | | | | | | | | | |
Eyes open | | | | x | | | | | | |
Feathers erupt | | | | | x | | | | | |
Sound: Squeak | x | | | | | | | | | |
_Chee_ | | | | x | | | | | | |
Begging | x | | | | | | | | | |
Cowering | | | | | | | | x | | |
Head scratching and | | | | | | | | | | |
Preening | | | | | | | | | x | |
Hopping to rim of | | | | | | | | | | |
nest | | | | | | | | | x | |
Fledging | | | | | | | | | | |x[G]
--------------------+---+---+---+---+---+---+---+---+---+----+----

[G] This is the commonest fledging day.

_Parental Behavior_

No eggshells were found in nests on the days of hatching. Presumably they had been removed by the parents. Nolan (1960:234) indicates immediate disposition of the eggshell after hatching. Lawrence (1953:62) suggests that conspicuous removal of eggshells by the female Red-eyed Vireo informs the male that the young have hatched.

Both sexes brood and the exchange of partners resembles that described for the incubation period. Decrease in brooding in the daytime begins about the sixth day of nestling life. Nolan (1960:235) reports a sharp decrease in brooding when the oldest nestlings are seven days old. Brooding decreases notably on the sixth day of nestling life in the Red-eyed Vireo (Lawrence, 1953:62). Nice (1929:17), Hensley (1950:244), and Nolan (1960:235) report that the female Bell Vireo assumes a slightly greater role in brooding than the male.

Apparent sun-shading was noted at nest 3-b (1959) at 2:00 p.m. on June 17, 1959, on the fifth day of the nestling period. The nest contained three young. An adult flew to the nest; while standing on its rim the bird dipped its head into the nest six times, afterward appeared to be eating a fecal sac, than shifted position to the unattached portion of the rim, gaped three times, thereupon spread its wings, and sat motionless 35 minutes. In this attitude it formed an effective shield sheltering the young from direct sunlight penetrating the thin foliage of the honey locust in which the nest was situated. The temperature at this time was 95 deg. F., but the sky was partly cloudy. By 2:30 p.m. the sky had become overcast and the sun passed behind a cloud. Although sunlight no longer fell directly upon the nest, the bird remained in the shielding posture for another five minutes before flying from its perch. Sun-shading was not observed at either of the other nests containing young; dense overhead vegetation protected those nests. Sun-shading has been noted in other species where the nest was poorly protected from the sun. Lawrence (1953:62) observed this behavior at two Red-eyed Vireo nests in conifers. The "sun-shield" posture of the Bell Vireo does not correspond to any of the sunning postures described by Hauser (1957).

_Feeding of the Nestlings_

Both sexes fed the young, and presumably began shortly after the first nestling hatched. My data indicate that the female does more feeding than the male (Table 10); in about eight hours of observation a total of 67 morsels were brought, 43 by the female and 24 by the male, for an average of once every 7.6 minutes. Nice (1929:17), however, observed a male to bring food 53 times as compared to 21 visits by the female. In five and one-half hours of watching, meals were brought once every 4.9 minutes. Du Bois (_in_ Bent, 1950:257) recorded seven trips in an hour and forty minutes, or one every fourteen minutes.

At three nests containing young the adults were sometimes silent and sometimes vocal on their approach. The female often emitted a subdued _chee_ which, coupled with the vibration of the nest caused by her arrival, elicited begging behavior from the young. None of the males was heard to utter such a call, but I have the impression that they often did call although I failed to hear the sounds. The males did, on occasion, sing several songs as they approached, even with food held in their beaks. Such singing elicited begging from the nestlings. Once the eyes of the young were open they often began begging when a silent adult was within two or three feet of the nest; begging behavior probably is elicited by tactile, auditory or visual stimuli in that order, or, as the nestling period proceeds, by any combination of these stimuli.

TABLE 10. FEEDING OF THE NESTLINGS.

====================================================
Day of | Length of | Adult involved
nestling period | observation +---------+---------
| | Male | Female
-----------------+--------------+---------+---------
1 | 30 min. | 3 | 5
2 | 60 min. | 1 | 4
3 | 60 min. | 2 | 5
4 | 30 min. | 1 | 4
7 | 60 min. | 4 | 7
2 | 60 min. | 3 | 3
6 | 60 min. | 3 | 6
7 | 30 min. | 3 | 3
9 | 60 min. | 4 | 6
+--------------+---------+---------
Totals | 510 min. | 24 | 43
-----------------+--------------+---------+---------

Not all trips made by parents resulted in successful feeding of young; some visits seemed to be purely for inspecting the young. On other occasions the adults experienced difficulty in transferring food to the young, and, thus thwarted, would themselves eat the food. Nice (1929:17) estimated that from five to twelve of a total of seventy-five meals were eaten by adults.

_Nest Sanitation_

Both parents regularly removed fecal sacs from the nest, eating them for the first five days and thereafter carrying them off and presumably dropping them. It is doubtful that fecal sacs were actively removed in the last two days of nestling life as the bottoms of nests from which young flew away were invariably covered with excrement.

On several occasions a parent brought food to the nest and then remained perched on the rim alternately peering into the nest and then preening. Once bill swiping was observed and another time an adult male sang once. The adult remained at the nest from twenty seconds to a full minute.

_Fledging_

Eight young were fledged from the four nests in 1959. The nestling period lasted from nine to twelve days. Human interference may have been largely responsible for the fledging of the young at nine days. Pitelka and Koestner (1942:100) found nestling life to last eleven days. Nolan (1960:235) reports nestling periods varying from 10.5 to 12 days. The young Red-eyed Vireo is ready to leave the nest at ten days but often remains an additional day before departing (Lawrence, 1953:68).

The oldest nestling at nest 2-a (1959) hopped out on June 17, 1959, when I disturbed the parents. On this date the juvenal plumage was only partly developed and the young bird was incapable of flight. By the tenth day of nestling life the young in all the nests were observed to hop to the rim, flutter their wings, hop back into the nest and also to preen and scratch their heads. The young at fledging are usually completely feathered, but have notably short tails and relatively short, stubby wings. According to Ridgeway (1904:205) the juvenal plumage is much like that of the adult.

_Nest Parasites_

Pitelka and Koestner (1942:103) found that incubating adults and later the young suffered infestation of the northern fowl mite, _Ornithonyseus sylviarum_. Nolan (1960:241) reports a heavy infestation of this mite at four nests. Unidentified mites were noted at four nests in my study area in 1959. Incubating adults were observed to peck at their breasts and scapulars from the eleventh through the fourteenth day of incubation. Serious infestations were not noted at the nests until the ninth day of nestling life. At this time the young were observed to scratch their heads and peck at their breasts, scapulars, and the base of their tails. On the day of fledging the nests were a seething mass of crawling mites; the mites also extended well up the branches to which the nests were attached. Nest 1-a (1959), which was discovered on June 18, 1959, presumably on the day after fledging, was densely covered with mites. Some mites were still crawling on this nest on June 20, 1959.

FLEDGLING LIFE

On June 20, 1959 I located one young 80 feet northeast of nest 2-a (1959), about five hours after it had left the nest. One parent was observed to feed it once. No young were seen thereafter from this or any other nest. Extreme agitation on the part of one or both parents on several occasions shortly thereafter, however, suggested the proximity of the young. Search in the immediate vicinity on each of these occasions proved fruitless. Three days after fledging their young, pair 2 (1959) was primarily occupied with courtship activities. Pair 1 (1959) was involved in courtship and nestbuilding one and one-half days after the apparent fledging of their young. Nolan (1960:238) indicates that the young remain within the territory and perhaps are fed by the parents up until an age of about 40 days. Sutton (1949:25) and Lawrence (1953:68) present contradictory reports on fledgling-parent relationships in the Red-eyed Vireo. Sutton concluded that the young quickly took leave of their parents whereas Lawrence reported a young bird being fed 35 days after fledging.

_Second Broods_

The curve based on 66 nesting records of the Bell Vireo representing the breeding activity in northeastern Kansas demonstrates a tendency toward double-broodedness (fig. 6). The peak of the breeding season is from May 20 to June 20. The large number (20) of replacement nests built in late May of 1960 tends to distort the curve of the breeding data; a second peak about 35 days after the first is evident.

I am of the opinion that the vast majority of vireos are single-brooded solely by virtue of the limited success of early nesting efforts, and that in "good" years most pairs would be double-brooded. Each of the four pairs that successfully raised one brood in my study area in 1959 renested within a day or two after the fledging of the young. I do not know the fate of these nests. Nolan (1960:237) reports at least one instance of a second brood in the course of his study. Nolan (_op. cit._) notes that the literature, in general, indicates that vireos are double-brooded, but that his evidence, mentioned previously, is the only evidence based on banded birds.

REPRODUCTIVE SUCCESS

Only four nests were successful; all of these were observed in 1959. The principal external factors responsible for nesting failure were severe weather, predation, parasitism by Brown-headed Cowbirds (_Molothrus ater_) and human interference (Table 11).

In late winter and early spring of 1960 heavy snow, continuously at a depth of at least 10 inches, covered most of the Mid-west from February 20 through March 20. Consequently, the growing season was some two weeks behind that of 1959. Of all the species in the study area, the Bell Vireo is the most dependent on dense foliage for cover and concealment for its nests. Consequently the tardiness of the season seemingly negatively influenced reproductive success of this more than any other species of bird in the study area.

_Behavior_

Several aspects of the behavior of the Bell Vireo tend to contribute to nesting failure. They include:

1. Nest-site. Nests are occasionally suspended from exposed branches. Occurrences of this sort suggest that the dimensions of the fork are more important in the choice of a site than availability of cover.

2. Song. The loud, continuous song of the male during nestbuilding alerts cowbirds and predators to the presence of a nest. The incongruous habits of the male of singing in the nest tree and while sitting on the nest may facilitate location by some enemies, particularly cowbirds.

TABLE 11. EGG MORTALITY IN BELL VIREOS.

======================================================
| | Eggs (N-29)| |
Mortality agents | N[H] | 1959 | N | 1960
| | Per cent | | Per cent
-----------------+------+------------+-----+----------
Predation | 4 | 13.8 | 5 | 10
Weather | 2 | 6.9 | 8 | 16
Cowbird | 14 | 48.3 | 37 | 74
+------+------------+-----+----------
Totals | 20 | 69[I] | 50 | 100
-----------------+------+------------+-----+----------

[H] Number of eggs out of the total number laid lost to
mortality agents.

[I] In 1959 nine eggs were successful (ultimately gave rise to
fledglings).

I am not fully convinced that song from the nest is simply a "foolish" habit, since snakes, the principal predators with which this species has to contend, are deaf. My own field observations and the circumstances of the innumerable instances recorded in the literature of male vireos singing from the nest suggest that this is a function of the proximity of the observer. As mentioned elsewhere, vocal threat is the initial as well as the primary means by which territory is maintained. Song from the nest evoked by an enemy also serves to alert the female to danger.

3. Flushing. The Bell Vireo normally relies upon cryptic behavior to avoid detection at the nest. Most sitting birds, especially the females, either flush silently when an enemy is about forty feet from the nest or remain sitting upon the nest tenaciously, refusing to flush even when touched or picked up. Some birds flushed at intermediate distances of from three to fifteen feet. In so doing they revealed the location of their nests. Since none of these "intermediate flushers" enjoyed nesting success there is possibly some correlation between these two factors.

_Predation_

Several complete clutches being incubated disappeared from nests that were unharmed. Absence of eggshells in the vicinity suggests predation by snakes.

On May 25, 1960, I found a _Peromyscus_ climbing toward nest 1-a (1960). The mouse moved to within two inches of the nest whereupon I removed the mouse. Such small rodents constitute another potential source of predation.

_Cowbird Parasitism_

In this study the failure of 12 of 35 nests can be directly attributed to cowbird interference. It is well established that the incidence of cowbird parasitism of Bell Vireo nests is high (Friedmann, 1929:237; Bent, 1950:260-261). Nolan (1960:240) found only one nest of eight studied to be parasitized by cowbirds. He indicates that this is surprising in view of the heavy molestation of the Prairie Warbler (_Dendroica discolor_) in the same region. A possible explanation of this phenomenon seems to lie in the much greater abundance of the Prairie Warbler in comparison to that of the Bell Vireo. In my study area the incidence of cowbird parasitism on Bell Vireos in 1959 and 1960 greatly exceeded that of all other nesting species that were parasitized (Table 12).

As indicated previously, the female Bell Vireo leaves the nest unoccupied several hours at a time in the transition period between completion of the nest and the start of egglaying. Such behavior early in the morning certainly would facilitate deposition of cowbird eggs. Early in the nesting period the mere presence of a cowbird egg in the nest prior to the laying of the host's first egg leads to abandonment of the nest. This seems to be correlated with the relative strength of the nesting tendency; anyhow cowbird eggs laid in later nests prior to the appearance of the host's own eggs did not cause the nesting birds to desert. The Bell Vireo does abandon the nest when all but one of its own eggs have been removed by the cowbird. Mumford (1952:232) records the removal of a cowbird egg by the host birds and I recorded a similar instance involving nest 2-b (1960). On May 14, 1960, I found one punctured cowbird egg on the ground about 10 feet west of this nest. Occasionally a cowbird egg is buried beneath the lining of a nest. Mumford (1952:23) observed this in mid-May in 1951 and I observed pair 8 (1960) actively covering with building material a cowbird egg on July 5, 1960. Covering a cowbird egg constitutes effective removal. Since the egg cannot be turned, an adhesion develops.

TABLE 12. INCIDENCE OF COWBIRD PARASITISM OF THE BELL VIREO
COMPARED WITH OTHER PASSERINES IN THE STUDY AREA IN 1959 AND
1960.

=========================================================
| Bell | Other
| Vireo |passerines
--------------------------------------+-------+----------
Total nests examined containing | |
at least one host egg | 35 | 43
Total nests parasitized | 24 | 14
Total number of cowbird eggs | 33 | 23
Per cent of nests parasitized | 68.6 | 32.6
Total number of cowbird eggs per nest | .94 | .54
--------------------------------------+-------+----------

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Natural History of the Bell Vireo, Vireo bellii AudubonChapter II: Part 2

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